What’s Better? Heat Pump Or Conventional Hot Water + Solar?

Heat pump vs electric element storage HWS

Evaluating the Cost-effectiveness of Heat Pumps vs. Solar and Resistive HWS

Mr Smith recently wrote to SolarQuotes about his experience at ‘Sustainable Home Day’ where he asked the panel, “whether we should use heat pumps, or spend the same money on resistive hot water systems + extra solar?”

This subject has been a burning issue for some time now and could get me in a lot of hot water if I don’t tread carefully. There are passionate advocates on either side of the argument who swear by their weapon of choice.

In this article, I’ll do my best to leave bias out of it and let the numbers do the talking. First, we’ll look at the basis for Mr Smith’s hypothesis, then I’ll break it down again from a different angle to try and determine the long-term cost-effectiveness of installing a resistive element electric storage hot water system (HWS) + additional solar vs a hot water heat pump.

Key Factors and Assumptions Influencing the Comparison

To do any comparison, assumptions have to be made to have a baseline to work from. These assumptions may or may not reflect every reader’s situation. However, it will hopefully provide a starting point to do your own analysis.

Some variables that could change the outcome would be location, electricity tariff, feed-in tariff, rooftop solar size, demand, upfront costs, degree of automation, lifespan, maintenance, etc.

I’ll leave a link to my simple spreadsheet at the end of the article. You can download it, and then plug in your variables to see what you can make out of it. I’d be happy to see your results.

Resistive element electric HWS + PV vs heat pump

Which is cheaper in the long term – a hot water heat pump or a resistive element electric storage HWS + additional solar?

Mr Smith’s Hyposesis

Mr Smith writes:

“A good heat pump that has a 6-year ‘parts only’ warranty on the pump, and the 500W of solar panels to run it, looks like costing about $5200 fitted (including a $945 rebate) when installed as part of a large PV system.”

“A restive element storage tank using 8kWh/day, and 2kW of panels to run it with a timer will cost $3500 when installed together with a large PV (SolarQuotes prices $1.11/watt).”

Additional Solar

So Mr Smith’s assumptions are that he would need 2kW of additional solar to cover his 8kWh per day load if he had a resistive element storage HWS, or 500W additional solar to cover the load from a good quality hot water heat pump.

That sounds about right, assuming a heat pump with a CoP (Coefficient of Performance) of 4 would theoretically use about 25% of the energy as a resistive HWS to heat the same amount of water. After that though, his logic doesn’t make much sense to me.

Coefficient of performance simple equation

CoP (Coefficient of Performance) is the ratio of electrical energy in – to heating energy out. The ratings should be shown on the datasheet. Remember that the measurement is at standard test conditions in the lab.

The next assumption is that Mr Smith has already decided to install a PV system, and the additional solar panels to cover his HWS load would hypothetically be installed on the same day. Another assumption is that the design of his system would allow this modification without upgrading the solar inverter.

Upfront Costs

Using Mr Smith’s numbers, 500W of additional panels for the heat pump load would add about $600 to the installation price at $1.11/watt. That leaves $4,600 for the heat pump (supplied and installed).

Woah, Mr Smith is going top shelf for his heat pump!

2kW of additional panels for the resistive element storage HWS would add about $2400 to the installation price at $1.11/watt. That leaves $1,100 left for the HWS, plus the timer (supplied and installed).

Hmm, I’m not sure; the HWS sounds too cheap if the timer is included.

Running Costs

Despite my misgivings, let’s run with it and finish Mr Smith’s comparison. He hasn’t given me his running costs, so I’ll work it out. That shouldn’t be too difficult. Before I do though, we should address the next major stumbling block.

I have an inkling that Mr Smith thinks because he’s added enough additional solar to cover his hot water heating load, he’ll have free power from the sun to do the job. That may be why I don’t have any of his forecast running costs.

“… We should objectively look at the issue without bias of ‘energy efficiency’, cos free is free no matter how efficient our machine is… ” says Mr Smith.

As most of you know, free isn’t actually free when it comes to energy. There’s always a cost – even for renewable energy. The only free energy is the energy you don’t use. That, Mr Smith, is why ‘energy efficiency’ matters.

Solar energy is not 'free energy'. It has a cost.

PV Self-Consumption: Is Free Really Free?

Among other things, in most cases of PV self-consumption, the cost of electricity used is equal to the amount you would have otherwise received as a feed-in tariff. But wait, there’s more.

Assuming that the HWS is set to run in the middle of the day, to take advantage of his ‘free’ solar power and off-peak tariff rates, there is no way he would be able to run off 100% solar power using a ‘dumb’ timer because the sun isn’t always shining, so the shortfall would be taken from the grid.

It would be possible to harness closer to 100% energy from the PV in a more sophisticated system with a hot water PV diverter or other smart energy management system. But this would add at least $1000, and there would still be a shortfall on some days. We’ll stick with the timer for now, at Mr Smith’s request.

A heat pump can get more PV self-consumption than a resistive element HWS because it draws less current, so there would be fewer occurrences where the grid would have to step in and supply the shortfall. Having a higher current draw, the resistive element would poke its head outside the solar yield curve more often, and the grid would have to take up that slack.

Solar yield + heat pump daily load profile

Hypothetical daily solar yield + heat pump load showing the grid supplying the shortfall when the sun isn’t shining.


Solar yield + electric element storage HWS load.

Hypothetical daily solar yield + resistive element storage HWS load showing the grid supplying a greater amount with the same solar yield as the first diagram.

A bit of guesswork is required next. I’ll assume that in the heat pump scenario, 70% will be covered by solar, and 30% by the grid. For the resistive element HWS, because of its higher current draw, let’s say 50% will come from solar and 50% from the grid.

I’ll also need to plug the tariff rates into my spreadsheet. Mr Smith hasn’t shared his own, so I’m picking a popular AGL plan with an off-peak (10 am-3 pm) rate of 31.63c and a feed-in tariff of 6 cents. Another thing – he says the warranty period for the heat pump in question is 6 years, so we’ll run the numbers over a 6-year period, with the recommended service every 5 years.

Opportunity Cost Or Lost?

Mr Smith says we should account for a lost opportunity cost, the interest otherwise earned from money going towards a more expensive heat pump rather than a good old-fashioned electric storage HWS.

“… So the free powered heat pump costs $1,700 more upfront than the free powered resistive HWS, and this $1,700 at 5% interest will cost an extra $85 every year… “

I’m going to knock that idea on the head straight away for two reasons:

  1. I don’t think his upfront costs are accurate.
  2. Even if they were, we would equally have to account for the opportunity cost of money going towards the running costs as well.

We’re splitting hairs here and could be creating a lot of unnecessary work for little change to the outcome, so we’ll keep it simple and move on. Also, his maintenance cost of $100 to replace the anode every 5 years looks too low, but won’t have much effect on the outcome either.

Let’s Crunch Some Numbers

6 Years

  • Resistive element storage heater + additional solar
    Storage HWS (installed): $1,180
    Timer (installed): $130
    Additional 2kW solar (installed): $2,220
    Electricity (8kWh/day): $3,308
    Maintenance (5yr anode): $100
  • Hot water heat pump + additional solar
    Heat pump (installed): $4,615
    Additional 500W solar: (installed) $555
    Electricity (2kWh/day): $599
    Maintenance (5yr anode): $100

The hot water heat pump has a $1,069 advantage after 6 years.

But The Old Fashioned HWS Will Last Longer!

Mr Smith doesn’t think a hot water heat pump will last as long as an electric storage HWS. Maybe he’s right. Let’s rerun the numbers over 12 years and assume the heat pump will be replaced after the 6-year warranty period.

12 Years

  • Resistive element storage heater + additional solar
    Storage HWS (installed): $1,180
    Timer (installed): $130
    Additional 2kW solar (installed): $2,220
    Electricity (8kWh/day): $6,616
    Maintenance (5yr anode) $200
    TOTAL COST OVER 12 YEARS: $10,346
  • Hot water heat pump + additional solar
    Heat pump (installed): $9,230
    Additional 500W solar (installed): $555
    Electricity (2kWh/day): $1,199
    Maintenance (5yr anode): $200
    TOTAL COST OVER 12 YEARS: $11,184

The old-fashioned HWS now has a $838 advantage after 12 years. The battle is on! Or is it?

The Verdict (Take 1)

Hold on, not so fast! Even though we have run some numbers through a spreadsheet, due to the vast amount of variables, it’s still only possible to make the best guess about the future based on information available today. As much as I am a proponent of the ‘keep it simple’ philosophy and by no means an early adopter, my gut feeling tells me the hot water heat pump will win by a mile. Here’s why:

1. Mr Smith’s upfront costs are unrealistic for the savvy buyer, so the above calculations are inaccurate, and we need to rerun the numbers.
2. Energy prices are skyrocketing and forecast to stay high, swinging the balance in favour of the heat pump as time passes.

Running The Numbers Again

So, one of the problems with Mr Smith’s hypothesis, apart from all the other problems, is that the prices quoted for both the heat pump and the electric storage HWS aren’t giving a like-for-like comparison.

His calculations are based on a top-end-of-the-market Reclaim Energy REHP-CO2 315L heat pump suitable for a large family ($4,615 installed ouch), compared to a mid-range 250L Rinnai Electric Hotflo electric storage HWS suitable for a medium-sized family ($1,180 installed – really?), and a timer ($130 installed – not by any sparky that I know!)

This time I’ll use an iStore 270L hot water heat pump ($3,200 installed), and keep the 250L Rinnai Electric Hotflo electric storage HWS ($1,180 installed) + $250 minimum for the sparky to throw a timer into the switchboard while he’s there. These systems are low/mid-priced and designed to suit a medium-sized family’s hot water needs.

6 Years

  • Resistive element storage heater + additional solar
    Storage HWS (installed): $1,180
    Timer (installed): $250
    Additional 2kW solar (installed): $2,220
    Electricity (8kWh/day): $3,308
    Maintenance (5yr anode): $100
  • Hot water heat pump + additional solar
    Heat pump (installed): $3,200
    Additional 500W solar (installed): $555
    Electricity (2kWh/day): $599
    Maintenance (5yr anode): $100

The hot water heat pump now has a massive $2,603 advantage after 6 years! Let’s crunch the numbers over 12 years and assume the heat pump dies halfway through as before.

12 Years

  • Resistive element storage heater + additional solar
    Storage HWS (installed): $1,180
    Timer (installed): $250
    Additional 2kW solar (installed): $2,220
    Electricity (8kWh/day): $6,616
    Maintenance (5yr anode): $200
    TOTAL COST OVER 12 YEARS: $10,466
  • Hot water heat pump + additional solar
    Heat pump (installed): $5,700
    Additional 500W solar (installed): $555
    Electricity (2kWh/day): $1,199
    Maintenance (5yr anode): $200
    TOTAL COST OVER 12 YEARS: $8,354

The hot water heat pump still has an advantage of $2,112 after 12 years.

Yeah But

Yeah but what if Mr Smith has a 10kW rooftop solar system?

Good point. A bigger system would allow him to utilize more solar self-consumption. It still won’t give him 100% solar unless he spends more money on smarts. Let’s say he can harvest twice as much solar as before to meet his hot water needs. That would be 75% for the electric HWS, and 85% for the heat pump.

My 6-year calculation comes up with:

Resistive element storage heater + additional solar – $5,931

Hot water heat pump + additional solar – $4,286

Still a cost saving of $1,645 for the heat pump over 6 years.

I could go on changing variables to suit different scenarios, but I won’t. That’s why I’m letting you play with the spreadsheet. You can try to swing the outcome in favour of the old-fashioned HWS by using less hot water or allowing for a larger PV system. Your electricity tariff may be less than the one I used. Perhaps you could include a solar diverter in your calculations to see if it might be justified. Maybe the CoP of 4 is too optimistic for a heat pump?

The Final Verdict: Why Heat Pumps Hold the Advantage

By my calculations and estimations, I think it would be unlikely that a resistive element electric storage HWS + additional solar would be cheaper over the system’s lifetime than a hot water heat pump. Energy prices dictate consumer behaviour, and there is an unstoppable groundswell of appetite for sustainable, energy-efficient products. In today’s reality, energy efficiency is important, Mr Smith, and the only ‘free energy’ I know about is the energy you don’t use.

Please download the spreadsheet I used for the calculations in this article and prove me wrong.

Heat pump vs electric storage HWS calculator

About Kim Wainwright

A solar installer and electrician in a previous life, Kim has been blogging for SolarQuotes since 2022. He enjoys translating complex aspects of the solar industry into content that the layperson can understand and digest. He spends his time reading about renewable energy and sustainability, while simultaneously juggling teaching and performing guitar music around various parts of Australia. Read Kim's full bio.


  1. I have heat pumps for all heating and cooling in my house, so love the technology. My only hesitation with heat pump hot water is that the people I know who’ve gone down that path have sometimes had terrible experiences with the reliability of the systems. And I also live in a very cold town, well into the sub-zeros in winter, and the same people have said their heat pump hot water system really struggled in winter.

    • Geoff Miell says

      And I also live in a very cold town, well into the sub-zeros in winter, and the same people have said their heat pump hot water system really struggled in winter.

      What brand(s)/model(s), Nick? I’m aware of a cheap unit that the specifications indicate isn’t rated for operating in ambient air temperatures below -7 °C (and above +40 °C).

      If a heat pump system “struggles” in ambient air temperatures down to -10 °C then I’d suggest it’s not adequate for most cold winter locations in Australia. It pays to check the specification.

      See my comments at:

      • Tim Chirgwin says

        I noticed in your other comments that the high COP of the heat pump was achieved for when the air temperature was 32.5Deg C.

        I’m not sure I’d be using much hot water at all then,… so now back to when I do want hot water.

        Unless you fork out double money or more for the most expensive brands of Heat Pump the energy savings are considerably less, with some units containing back up resistive element to help them out,…which draws the same energy to do the same job as the standard resistive without the cost or complexity.

        • Geoff Miell says

          Tim Chirgwin,
          I’m not sure I’d be using much hot water at all then,… so now back to when I do want hot water.

          Reiterating my linked comment, for the Sanden Eco® Plus CoP:
          * 5.96 (at 32.45 ºC ambient / 18.74 ºC cold water inlet)
          * circa 2.76 (at 0 ºC ambient air)

          An electric resistive element hot water system is no better than 1.
          A gas hot water system is significantly less than 1.

          So even at freezing ambient air temperature, the Sanden Eco® Plus is at least 2¾ times (in mid-winter) and near 6 times (in high summer) more energy efficient than an electric resistive element system, and even higher compared with gas.

          My system is set to recharge the tank from 10am, where the ambient air temperature has warmed above freezing.

          Unless you fork out double money or more for the most expensive brands of Heat Pump the energy savings are considerably less, with some units containing back up resistive element to help them out…

          You get what you pay for – cheaper upfront costs, but higher operating costs, noisier, and probably with a shorter operating life.

          I’d suggest if you are in an area that regularly experiences minimum ambient air temperatures in winter near freezing (below 3 ºC) then go with a system that doesn’t require a back up resistive element.

          • Tim Chirgwin says

            Geoff Miell

            I agree with your statement that the expensive heat pumps are more efficient and work at lower temperatures compared to lower quality heat pumps, and indeed you do get what you pay for.

            Your concept is not the basis of the Blog instigated by Mr Smith’s statements ,taken up by Kim Wainwright.

            Mr Smith’s proposal that it is more cost effective to use a resistive HWS with extra PV included as part of the system cost to provide that extra energy requirement compared to the Heat pump system with its respective sized PV requirement is the case being explored.

            The majority of responders to the blog (who have clearly stated that they have data to back their statements) have said that they can run their cheap and reliable and long lasting Resistive HWS for very little cost using power diverters or timers when on 6.6 kw (or smaller) Solar PV systems, without sticking their hands deep into their pockets for the heat pump HWS and inheriting reliability issues.

            The proposal put forward by Mr Smith was to include an enlarged PV array commensurate with the higher requirement of the resistive HWS, which not only gives greater surety of economical self powered hot water from FREE sunshine energy, but the additional energy captured can be used to offset that bought from the grid at high peak prices (think morning and afternoon) while using potential low priced FIT export during the middle of the day to heat the water.

            It is important to stay on topic or you will look like a salesman for an expensive brand of heat pump HWS.

          • Geoff Miell says

            Tim Chirgwin,
            I’d suggest you are ignoring/dismissing Kim Wainwright’s statement in the above post:

            A heat pump can get more PV self-consumption than a resistive element HWS because it draws less current, so there would be fewer occurrences where the grid would have to step in and supply the shortfall. Having a higher current draw, the resistive element would poke its head outside the solar yield curve more often, and the grid would have to take up that slack.

            A resistive element HWS draws at least 2¾ times (in freezing mid-winter), higher for milder winters, and near 6 times (in high summer) more energy compared with a quality HPHW system. That’s physics (& economics) you seem to be ignoring/dismissing. That’s more energy being consumed to heat water in a much less energy efficient resistive HWS that could be more useful providing energy to other high energy activities, like air-con, home battery charging, BEV charging, etc.

            The majority of responders to the blog (who have clearly stated that they have data to back their statements) have said that they can run their cheap and reliable and long lasting Resistive HWS for very little cost using power diverters or timers when on 6.6 kw (or smaller) Solar PV systems, without sticking their hands deep into their pockets for the heat pump HWS and inheriting reliability issues.

            I’d suggest they would do even better with a quality HPHW system in the longer-term. I think the problem with most people is they look at only the upfront costs and discount/disregard the overall life costs when comparing equipment.

            It is important to stay on topic or you will look like a salesman for an expensive brand of heat pump HWS.

            I’m providing an account here of my personal experiences with a HPHW system & related pertinent data. Do you have any direct experiences with HPHW to offer or are you just here as an anti-HPHW nark?

      • I don’t remember, sorry, was a few years ago and the person has now moved. But they had such a headache with the system, the installers constantly back out there to adjust it. They lived in Blackheath, so decently cold winters. It would have been high-end, they were a high-end sort of couple with technology.

        Again I’m an enormous fan of heat pumps, we’ve used them for our own heating for years, and I also have one in the clothes dryer. All without incident. But the hot water system heat pumps do seem to attract some negative reviews, particularly from people who live in cold areas, as we do. Would love to get one, but that makes me pause. The warranties were also not great, last time I looked.

      • Alan in Madrid says

        Are you sure about the COP? The water needs to be heated to 60°C to avoid Legionares, and when I looked at heat pumps they were good for my underfloor heating at 25-28°C, but for 60°C, the COP was much worse, even to the point where it was recommended to use supplemental heating, because the HP canta get to 60, and overstresses trying to.

        What about a comparison with solar thermal with resistive too-up? The panels get about 70% efficiency, compared to about 25% tops for PV. If its pumped, the pump can run off its own small solar panel next to the thermal panelto avoid running costs, or use thermo-syphon where the hot water tank is above the panel and heated by convection. I’ve known people with thermo syphon that then feeds an instantaneous water heater. In summer the water comes in at up to 90°C, then mixed down to 60, in winter it comes in warm and it’s heated to the required temperature with a gas or electric heater.

        • Geoff Miell says

          Alan in Madrid,
          Are you sure about the COP?

          See the Washington State University test report CoP figures as an indicator for the Sanden model # GAU-A45HPA heat pump water heater (HPWH) referred to in my comments at:

          The water needs to be heated to 60°C to avoid Legionares, and when I looked at heat pumps they were good for my underfloor heating at 25-28°C, but for 60°C, the COP was much worse, even to the point where it was recommended to use supplemental heating, because the HP canta get to 60, and overstresses trying to.

          My Sanden model # GAU-A45HPC HPWH produces hot water above 60 °C to fully recharge the tank daily, throughout all seasons. The Sanden HPWHs do not require supplemental heating in cold ambient conditions, provided the ambient air temperature is within the specified operating range (i.e. -10 ºC to +42 ºC).

          • geoff little says

            using a sanden heat pump will give very good thermal efficiency, COP roll-off at 60 C ??? expect a COP of at least 3.5 even in coldest Madrid winter. The system will provide a constant output of say 4.6 kW heating but will increase power input as required in the cold. To apply it underfloor heating, the hydraulic layout, you must have a cotrolled delivery buffer loop, that is the sanden heating loop HW storage vessel on a separate loop to the floor heat, that is a heat exchange loop with temperature modulating valve. If you would like, I can sketch one for you, and you can show it to your plumbing engineer designer in Madrid for approval. Madrid from recollection would only require floor heating say- 5 months of the year. I would look at a hybrid set up, That is, supplying domestic hot water all year round and tapping off floor loop in the colt season. To look at viability, balance of supply and demand, I would need to know floor area. DO NOT try and adapt sanden primary hydraulic circuit. just let it do its thing on thermostat as it is designed. Good luck with it.

    • Even in winter where the efficiency of a heat pump is reduced, it is still more efficient than resistive heating. It’s also very unlikely that solar will cover your resistive load during winter months.

      • Rod Webster says

        My solar covers 99.96% of annual resistive heating costs wit a diverter.

        • I have a diverter too (Powerdiverter), and I’m also an evangelist for them. But we certainly aren’t covering 99% of our heating costs with solar. Winter is the killer, but also wet weeks. I used to live on the edge by only “boosting” the hot water when I thought there was an imminent risk of the water going cold. But I miscalculated a few times and my wife ended up having a cold shower – not good for my health!! So now I play it pretty safe and boost more often (and also have a scheduled boost that runs late afternoon). One thing that I’ve done to help with this situation is to put a digital thermometer sensor at the hot water outlet. It’s not measuring the true temperature of the water, but proportionally it’s a good guide. The theory is that I’ll have a more accurate guide to when I need to boost.

          • Rod Webster says

            @Ben, I think there are diverters and there is Paladin. A temperature probe is inserted at the tank thermostat and a higher temperature thermostat lifts the off setpoint above the Paladin operating range. Paladin use the actual water temp to decide if and when a boost on grid power is required.It will not let the temperature fall below 40 deg C so no cold showers. I analyised hourly data collected by Wifi meters on the solar and HWS circuits. If I look back from today we have only used 1 kWh total during 4th and 5th of July 2023 in the last 12 months.

          • Thanks @Rod. That’s good info for anyone considering buying a diverter. A couple more considerations for people to think about:
            i. The general wisdom is that the diverter works better with a bigger hot water tank, for the simple reason that the bigger tank will store more heat and get you through a wet day or two without boosting. We actually have a smallish tank (160l), which I used to regret. But the more I think about it – the more I don’t think it’s not so bad. The good thing is that for a (non-Paladin) system, we don’t have to guess how much boosting is required – we generally just boost till it’s at max temperature. For a bigger tank with the same strategy, you might boost more than necessary and thus use more grid power than necessary. Plus, in winter we barely have enough solar power to fully heat our smallish tank, and outside of winter we don’t need to store so much because the diverter ensures it’s always being topped-up.
            ii. I spoke to a couple of home battery installers, and when I mentioned that I already had a diverter (and of course had to explain to them what it is because they hadn’t heard of it), they ran for cover. The diverter is already doing what the battery wants to do – ie. tap into excess power being generated. I have no doubt that a good electrician could do it, and choose the order in which devices receive that excess power, but the average battery installer is not a good electrician and doesn’t want to deal with “complicated” cases.

          • Rod Webster says


            The Palladin can run an optional second 40 amp SSR relay and has a mode to work with batteries but you will have to ask them about it. I think it charges the batteries first, then the hot water. The main thing is to keep all your generation on site…

            And yes, The Paladin can be much more frugal with Grid energy use becasue it knows the temperature and heats the water (if sufficient sunlight) through to 73 deg instead of the more normal 60 deg thermostat temperature so it can store more energy for the next cloudy day. If it can’t get to 73 deg, it does not really matter. Eventually it will catch up.

            We have a 250 L tank.

    • Darren Barnden says

      We built in 2007 and got a Quatum heat pump at around $3500. We were told it would last 15-20 years and save heaps on power. It failed at about 5 years and we nursed it to 10 years, having it repaired almost every year at considerable cost. When it died we needed a replacement asap, so we got a Bosch heat pump, hoping it would be more reliable. We were wrong. It’s around 5 years old and has failed three times. So far I’ve been able to get it working. We have very frosty winters and regularly run out of hot water. I don’t see the ” FREE” heat pumps being any better. I’ll never buy another heat pump HWS as long as l live.

      • I’ve had 3 plumbers tell me not to touch heat pumps as the maintenance costs and failure rates are horrendous. Contrary to what people will tell you, you can have a 315L resistive HWS in a new build in South Australia, providing you have at least 1Kw of PV panels on the roof. The Office of the Energy Regulator sees the storage HWS as just another form of battery.

        • Why is it always Gov regulation hampering efficiencies and looking to make a buck…

        • Hello, can you please elaborate further on this, I will be building a new house in the next year or 2 and want a resistive hot water storage unit with solar but thought the rules didn’t allow this anymore? There is no mains gas either.

        • Could you please elaborate? As far as I’m aware, you can’t install electric HWS in SA. Where is this information reference the 1kw of PV panels? Cheers

    • Super Puppet says

      I live in Myrtleford, NE Vic Alpine Shire… yep, pretty cold up this way.
      I have a 10kwh micro inverter solar panel setup and a Reclaim heat pump hot water system. Verdict? % would repeat the process if zipp was starting from scratch. Very satisfied with the outcome.

  2. LeroyBrown says

    I live in Qld so am thinking solar utilisation would be more like 80/20 for resistve HWS and 95/5 for the heat pump. I have sufficient solar so wouldn’t need additional panels for either option. A Catch solar relay on my resistive cost about $600. There is little separating the 2 options in terms of 6-year costs (based on 24c rate, 4c feed-in). However, when the time comes to replace the resistive HWS I think I’d prefer a heat pump so there is more solar available for other uses, such as EV charging. Where shall I send the spreadsheet?

    • Heat Pumps are definitely the go in warmer climates. Sounds like you’re all over it LB! 🙂

    • Kim Wainwright says

      LeroyBrown – yep I can see that. Different parameters change the equation, but then as you say, using less power allows you to use more solar for other purposes, which changes the equation again! It’s impossible to calculate the true figure, but at the end of the day, using less power is a good rule of thumb to go by for saving money, and saving that other thing… that little round thing we all live on.

      Where shall you send the spreadsheet? It would be good to post it up here but not sure how you could do that. I think SolarQuotes needs a way for users to upload and share files. I’ll have to talk to the IT guys.

      • Roger Davis says

        Great article, Kim, thank you. I am wrestling with a different evaluation scenario and am struggling to get the data I need. Our roof is simply full with 8Kw and no room for another panel. We currently heat our water with a gas ‘instant’ system so like Steve and John I’d like to see some simple comparisons to replace my gas hot water with a heat pump. We have just decided to no longer use our gas heater and make the hot water system our only gas appliance so I can finally get accurate costs for the current system.

      • Tim Chirgwin says

        Kim Wainwright,
        You have already charged the larger solar PV to the resistive HWS account in its capital cost,…it is not fitting that you should bemoan the fact that you cannot double dip.

    • This article is perfect timing. I was about to ask / check with your site the exact same comparison as I want to replace my gas hot water with a heat pump.

      The only other thing I need is a comparison of different heat pump brands like you do for the different solar inverters and panels.

      • I too would really appreciate an analysis of heat pump brands. I used SQ’s approach and tested SQ recommended installers’ recommendations for solar panels and battery nearly 12 months ago with excellent outcomes in Melbourne. Heat pump hot water is our next decision and while “good” units appear to perform much more reliably for longer, I really do not know which are “good” or perhaps more importantly which are “good enough” for our needs in cool but not cold Melbourne. The upfront costs range a lot and your guidance would be greatly appreciated.

      • Phill Motbey says

        I’m with John on this circumstances. Just starting to talk to plumber. In the Sydney basin, so those ‘artic’ conditions of homes up in the hills don’t apply.
        Phill M.

    • Marcus Gaiswinkler says

      I live in Hobart and have a cheaper Chromagen Midea 280L heat pump hot water system and 9.7Kw of solar but only at 8° tilt so really suffers low output in winter. My problem is my wife loves evening baths after work (she’s very hard working and had a stressful job) and goes close to emptying the tank on many evenings. I try to keep the Midea to run off its own inbuilt clock to come on after 10am after the solar kicks in and off by 3.30pm. but it only has 1 one-off time available. I like to shower before work in the morning but find I often have only warm water. If the hot water is 50-60% used up, and the heating cuts out at 3.30pm due to the clock, then I find that after nearly 20hrs without heat input the hot and cold layers mix and my morning shower is only warm.
      Can anyone recommend a great cost effective unit that has a better control system? At only $2k (not including install, the Midea is definitely hard to beat. Shame about the downright dumb controller…

      • Norman Price says

        I have a different make but talking to the importer’s tech support I found there was a ‘hidden’ menu where I could set multiple timer on/off’s but the standard interface was kept KISS. Maybe worth a call?

    • Wouldn’t a solar hot water system with heatpump support/backup be better? Or would it be too costly?

      • Holiday house rental. Occupied 50% of days, high winter usage. When occupied 6 to 10 persons. 630L storage. Just keeping hot for next guests expensive. Added solar tubes. During summer provides most hot water. Rest of year keeps water hot when vacant. Problem adding heat pump is if malfunction too hot solar water could damage heat pump if malfunction. We fixed by routing the active to the heat pump through the now redundant thermostat in the tank. If the tank too hot no power to heat pump.

    • Kent Taylor says

      Hi Leroy
      I utilise solar on timer for my resistive hot water system. The article does not analyse the benefits of using a 2.4 kw element over a longer time period. This minimises grid input especially on cloudy days. Nobody talks about the higher temperatures of domestic water in Qld which also reduces the kw required for heating hot water! Small battery installation not only reduces night time electricity usage but also minimises the impact of intermittent cloud cover while the hot water timer is operating. A few extra considerations to consider. I go for resistive with timer any day…especially in Qld.

      • Yes I agree. Australia is a vast country with many different climates so things can get easily misconstrued when assessing benefits. I trust Rewiring Australia and they note there’s very little difference between Heat Pump and HWS with solar. Hence why we’re having this engaging debate

  3. You can choose the 2.4kW element in the HWS and run it for longer. This would change the import percentage

    • Kim Wainwright says

      DW – yes, I totally agree. It would likely be less. I still think the percentage of grid power will always be higher for the resistive HWS though compared to the heat pump. And another thing that another commenter above (LeroyBrown) has pointed out – using less power allows solar to be used for other appliances which in turn changes the equation in the heat pump’s favour.

  4. Sadly, my favourite water heating solution is now out of fashion

    Gravity feed storage in ceiling space with thermosyphon solar thermal collectors should last 25 years or more but good luck finding a plumber who will install one.

    Our 3 panels and 320ltr tank used to give us 9 months of FREE heating with no boosting. I have since had added 1kW of off-grid boosting via a mini inverter which works well. Only 29kWh from the grid (manually boosted) so far this Winter.

    I recently replaced the tank (probably should have looked at a heat pump) The manufacturer is retiring and luckily knew a plumber with experience in them

    Tank $1200
    Install $1000
    Rinnai collectors $700 ea last time I bought one
    Off grid boosting and inverter $1500
    Electricity cost $12 per year

  5. Stefan Jarnason says

    Fair analysis. New heat pumps should easily last 10 years though (mine is over ten years old and the new one s are far better).

    There is one big unknown – electricity tariffs. Ausgrid is trialling even better tarrifs for controlled hot water in the daytime. So the winner is anyones guess.

    What I am sure of:
    * Both are miles better than gas ($$ and environment)
    * Getting them connected to the internet so they can be intelligently controlled will be a no-brainer – but sadly no-one is offering this yet……

    • Hey Stefan,

      The CATCH Solar Relay, connects to the internet and the hot water system to smartly control loads and inverters.

  6. Rod Kemp says

    This topic has been on my mind for some time as I am planning a new build. I’m leaning towards the heat pump solution myself. In the Adelaide Hills where I am, the output of my solar system during winter can be very low; at times less than 10% of the summer output for days at a time. At least we now have the time of use “solar sponge” tariff during the 10am to 3pm timeslot which is theoretically 25% of the standard tariff.

  7. I don’t understand why the resistive HWS scenario still includes 8kwh/day. This is very high usage and is highly unlikely for the average household.

    The extra 2kw worth of panels would actually cover that to a great extent.

    Either way, if the system does draw 8kwh from the grid a day, a catch power green would eliminate that cost for a total installed cost of around $750 I believe.

  8. Been through this so much myself it’s not funny. Love the idea of heat pumps and had one when we were living on Lake Macquarie NSW.

    Now that we have actual cold winters living in the Riverina region of NSW, I’m more inclined to go with a hot water diverted and resistive element setup.
    We have ample excess solar in the warmer months, say September – March or April that the hot water would be “free”….

    Problem with cold climates is that you really do need to go with a higher end heat pump, like the Reclaim mentioned in the article. I was quoted $5400 for that same model Reclaim in September 2022, so the price would be more like $5600+ this year.

    Would love to see the Govt. Rebates etc increased to make these Heat Pumps more affordable, but that’s a story for another article I reckon…

  9. I’m afraid trying to simplify this equation just doesn’t work. There are just too many variables.

    HP water heaters, good ones at least, are terrific. If water heating energy consumption is high then they will typically make the most sense when the time comes to replace a water heater but the equation is not so great when water heating energy consumption is low to modest.

    Before wasting big $ on anything, first you should work on reducing hot water consumption to begin with.

    We had a resistive element water heater already and the one it replaced lasted 30 years. The quotes for heat pump water heaters were eye watering, $3.5k more.

    We also already had a PV system and our water heater consumes an average of 5 kWh/day. There is simply no way the additional cost of the heat pump water heater made sense.

    You suggest the most expensive brands of heat pump are not a good comparison, yet they matter if you need the better refrigerant/performance for colder climates or if operating noise is a consideration.

    With heat pumps you also need somewhere suitable to locate the compressor. That’s not always possible. It wasn’t possible for us without considerable extra expense as there is nowhere simple to relocate the existing indoor tank, while a compressor would have to go on the roof. The quotes were eye watering.

    The cheaper brands have poorer performance, rely on a resistive element booster and come with crap warranty. Local plumbers advise against them because they are constantly replacing failed systems.

    Another factor is many PV systems are subject to export limitation, so production is “use it or lose it”. May as well use it.

    With solar PV we have eliminated grid imports for heating water. And it is powered by just 3.6 kW of PV. This is where strategic use of a larger tank matters as it can help ride through a bad solar day.

    I tried to post a chart to demonstrate our long term experience (I have 7 years of data) but the site refuses to accept image links.

  10. Peter Barreca says

    Excellent analysis Kim so thanks very much for all your hard work.
    I am looking at a similar scenario except that I will be replacing gas hot water with electic/solar hot water.
    The gas prices are skyrocketing and penalties are invlved if you with draw your gas service in soome states (VIC about $1000) so I have to act quickly before the cancer spreads to NSW.
    My research has revealed that in the cold climate I am in (Mittagong) the heat pump would not be that useful for most of winter becasue of the cold temperatures. And their reliability and service is questionable for some brands.

    I thought I need a special switch worth about $1000 to divert electricity from the inverter to electric hot water system so it doesn’t go to the grid. You don’t seem to have that in your calcs. Is their a cheaper way to do it?
    I have also gas cooking and space heating. The cooking can be replaced simply. The heating can be replaced with airconditioners.

    Once this is done I won’t have enough solar to meet demand without a battery but I will be buying electricity rather than gas and I expect my gas bill to be about $1200-$1500 a year. I have 6 KW system.
    Can you see another way to tackle this problem.

  11. A heat pump might be slightly more cost effective over time but that does not factor in the other reasons why I choose resistive+solar+timer.

    + Heat pumps are noisy – Resistive is silent
    + Heat pumps are very complicated pieces of machinery and require highly trained specialists to repair – Resistive are incredibly simple and every plumber in the world knows how to fix them.
    + Heat pumps might last 15 years – Resistive often last 30 years.

    I too know a number of people who paid a lot of money for a heat pump water heater and either couldn’t stand the noise, weren’t happy with the performance or got fed up when the thing broke (again). The wise ones chose to remove the heat pump and replace it with a resistive heater, more solar panels and either a diverter or timer.

  12. Dave Roberts says

    Why don’t people want a straight solar hot water system any more? What are the cost differences?

    • I have one but it basically is a resistive hot water system in winter.

    • Have used only solar with resistive backup for the last 35 Years Wagga to Port Moresby and several places in between. Have not had to use the resistive part since Wagga back in the nineties. Sized suitably they can maintain hot water for a couple of days.

      Off course I have never lived anywhere colder than NSW, so maybe less use further south.

    • Brian Rice says

      I wondered about a straight solar hot water system comparison too. We used to have a straight 2 panel solar hot water system with an electric element in the tank that was switched on manually if we felt the water temperature needed a boost. Located in the Macedon Ranges Victoria the system worked well without problems for 20 years and was still going when we sold the house.
      The water was usually hot enough for our needs and we only occasionally needed to switch on the electric boost.

    • I’ve got a 315L solar hot water system that was already on the house before my solar panels went on. I’m guessing it’s around 500kg all up with the steel frame to angle it north. A 1.8kW element that manages just fine with the 10kW inverter and battery.
      Trade pricing is anywhere from $3.5 -$6k depending on the type of unit. Then you have to get it up there.

      Mine works well most of the year. Although, there’s no way I’d go down that path if it wasn’t already there.

  13. Malcolm Dent says

    What about 3 phase electric Instantaneous, in the bathroom within 500mm of the shower-head, versus 2 phase heat-pump outside the house, both with 6kW solar? (Our lifestyle will allow us to shower during the day if desired).
    I did some rough ‘back of the envelope’ calculations and the two seemed to be roughly the same–what the instantaneous lost in resistive heating, it gained in not having to keep a lot of hot water sitting around.

    What about 12kW solar?

    • 3 Phase electric would be up to 27kw power draw for 12 lpm, you would need a monster solar array to produce that Let alone the inverter requirements across each phase. Plus with no storage you don’t have hot water once the sun goes down. Typical heat loss from a 250L modern tank is around 1.3-1.7kwh per day.
      Heat Pumps are single phase and use typically less than 1kw flat out.

  14. We took the plunge last month. No more gas HWS/stovetop means $1500 AND 4 less bills over the year.
    That is before the price rises at the end of this month that were just announced…
    I had a good chat with one of the HVAC supervisors on site a few months ago and the Reclaim gets his professional seal of approval.
    When mixed with a new PV+Storage array and the removal of gas bills entirely ($1 per day supply charges alone over 6 years without any inflation are over $2000).

  15. Philip Ripper says

    I have just finished grappling with this issue and decided to go down the quality heat pump route. The biggest deciding factor for me was fitting the 850 watt draw of the heat pump under the available solar graph vs the 4800 watt draw of our existing 25 year old twin element unit the main element not working.
    As others have mentioned the extra solar available to charge an EV or home battery will be cherished in the not too distant future I hope.

  16. We’ve been using a Catch Green solar diverter to heat our 250l HW tank for 4 months now (2 person household) and have operated solely using solar (the water has always been hotter than it was when heated off-peak!).

    The conclusion for us, based on no new panels, a $1000 solar diverter (for resistive element HW), 100% solar for resistive element, 70% solar for heat pump, and 6c/kWh FIT is $3550 for resistive element and $3350 for heat pump. So, resistive element wins, even with the additional cost of the $1000 solar diverter!

    This outcome even more compelling given we’ve already got a resistive element HWS, and that some solar will be @ 0c/kWh, as we’ve got 12kW of panels export limited to 5kW on single phase.

  17. Wolfgang says

    And for anyone in NSW it is currently possible to get a government subsidised heat pump replacing an existing resistive HWS for $33. No, that’s not a scam. Now do the numbers ?

    • Tim Chirgwin says

      The practical use of resistive HWS using free sunshine is to all our benefit, while a subsidised heat pump benefits you at everyone else’s cost, and will cost you dearly when you have to replace the lower quality systems that have been offered. The low cost units typically have low COP and depend on resistive elements to back them up, with increased maintenance issues,…and when running the resistive element cost the same energy as standard resistive tanks.

      Have you costed out the long term benefit?

  18. Colin Martin says

    There’s always one area that is left out of this exercise.. With Powerwall 2 batteries.. (PW2). We invested in 3 x PW2s and 13kw Solar, 10kW feeding in Melbourne.

    We are on ToU and the only Peak being used is 0.3 kWh/day average over the last year. Mainly because the PW2’s are always cycling, import and export

    We will be swapping our Gas Stainless Steel Aquamax mains pressure storage HWS with a Rheem Stainless steel, exactly the same functioning. Just a swap out, and a new electrical connection. Why ?… We are a 24hr house with 40kWh in the PW2s and the Gas HWS is 22 years old and only had a thermostat and pressure relief valve replaced in that time. No noise or moving parts.

    I have scheduled our pw2 charging that it benefits from the Solar. Our new August 1st Off-peak rate is 17.82c/kWh

  19. Peter Johnston says

    With a 10 kw system and a catch diverter both systems would cover my hot water AND household use 95% of the time so the only difference is the missing 5.2 cents fit we get by heating the water so I’d basically get 30 cents a day more fit back with the heat pump equaling about 110 dollars a year
    Having to replace the heat pump twice as often and twice the cost makes it out of the question!
    Also they’re noisy!
    Also being export limited to 5 kw the resistive uses heaps of power otherwise just wasted!

  20. Very interesting, thanks for writing.

    For my household, I already have an 8kw of system, capped at 5kw limit. I use very little as my house is well insulated / designed.

    I don’t disagree with your analysis and I’ll look at your spreadsheet. But, I was thinking in my situation the sunk cost of the system and limit would sway the equation to the kettle. Any thoughts?


    • Matt …. given anything above your 5kW export limit is essentially “free power” (no FIT opportunity cost lost if you use it), I would anticipate smartly diverting this power to a resistive element HWS would be the to go (we’re in a similar situation with 11kW of panels export limited to 5kW, and have heated our water with 100% solar for the last 4 months …. )

  21. I have just transitioned to heat pump from gas boosted solar. I’m definitely enjoying my constant temperature showers, but already can see my energy use rising more than I thought. I didn’t go the cheap route.. I have wifi access to the controller of the system, thank goodness.. the first day it was running but something switched it off.. day 8 happened again. I did have to do the wifi set-up myself but only did so when we didn’t have hot water 2 hours after install.
    On the issue of solar running the heat pump. Well also have to consider if a boost is needed after the sun drops. I have 5.6kw panels with 5kw inverter and 10kw battery. My battery has been charging during the heavy overcast days and sometimes isn’t enough to run the usual load. So I can say my usage during winter is going to be at least double than pre heat pump. IF you can get a solar system with electric back up.. that would be a better option.
    At the moment I’m happy with the heatpump. I like my water constant temperature and we’re using less water as it seems to run warmer quicker. Instead of 10 litres its now just 2 or 3 before hot water reaches the furtherest point.
    Also no more click click boom noise waking me during the night.gas back up kicking in.

  22. I have 2 units where I pay the power bill, in Lismore, NSW. These have a 180L resistive HWS, with 2.4Kw elements installed. TRhese units were built 6 years ago. They run for 4Hrs/day, 10>14:00. Yes, I pay for power occasionally, but the total electricity usage for both units is not high (less than 10Kw/day avg off the grid, inc HW. The Solar PV is 15Kw, 4Kw facing E, & rest West.

    It is important to fit a contactor to take the current, even with 2.4Kw elements. (16A timeswitch with welded contacts convinced me of that!)

    The advantage of using resistive HWS is they can be mounted close to the demand. In my case, close to the bathroom/laundry, & abt 5M from the Kitchen. It is possible to have 2 or more HWS, & really close connection meaning a big saving on water use. (eg an 80L for the Kitchen, & say 200 for the bathroom/laundry). It is also possible to have small HWS fed by a Solar roof mounted Heat tube Heat exchanger HWS. This means pre-heated water into the HWS, & heat top up from solar. The heat exchanger means there is no risk of Legionaires contamination in the hot water (because the coil is flushed every time the hot tap is turned on.)

    btw, I agree with the 8Kw/day for an average small family HW use, & the 2Kw additional solar.

    I am not in favour of Heat pumps due to complexity. Also the HW lines can be long, wasting water (due to the centralised HWS location). I agree, however that the TCO costs are probably about the same. For my rental situation, simplicity & serviceability are a bonus. (nothing worse than a tenant ringing to say they have no Hot water!)

    I am currently planning a PassivHaus retirement house. Water usage is one design consideration for PassivHaus. I will probably use a Heat Exchanger Heat tube HWS, feeding the Laundry, & small PV topped up HWS for Kitchen & bathroom, fed off the Heat tube HWS. All running on Rain water.

  23. Brad Griffin says

    Have just switched to a PV diverter system (Solahart) and whilst the upfront cost is certainly higher it is proving quiet effective in that since installation 3 months ago have used no grid energy to heat the water and has gone up to 3 days without sun. We are a household of 2 and during winter experience sub zero each night. other with heart pumps in the area have indicated are not overly effective during the colder months. Lost feedback payment is averaging 0.90c per day against offpeak cost of $1.80 so im about .90c per day ahead.

  24. What I would really love to see is great pump+ pv compared to solar hot water plus booster.

    • Kim Wainwright says

      David – the spreadsheet will easily do that without modification. In fact you can do a lifetime cost analysis of any electrical appliance with any other. In the case of a thermal solar HWS + backup element all you need to know is the kW rating of the element and the estimated percentage it will be running.

      For example if the backup is on for 2 hours a day over 4 months that would be:
      120 days / 365 days = 33%
      at 2 hrs per day
      So, for the spreadsheet operating costs of the solar thermal HWS:

      Load = backup element size
      Daily run-time = backup element run-time
      Off-peak tariff = electricity tariff
      Off-peak % = 33% (from above calculation)
      “Free” solar tariff = 0 (Zero because the booster is on when there’s not enough sun)
      % solar = Also 0 (zero, as above)

      I’m in the same boat. I’m waiting for my solar thermal HWS to die so I can put in a heat pump. This will happen in the next 5 years because I get about 12 years out of them where I live. Let me know how you go.

  25. Darrell Martens says

    Good analysis … if you do a “whole of system” analysis when you know you’re going full electrification (including EV), you find HPHW is even better and beats out even solar thermal hot water. There are system synergies not apparent in simplistic analyses.

    Here’s a couple of teasers to get people thinking …

    Solar panel efficiency of around 20% times HP COP of 4 means the water gets about 80% of the energy incident on the Solar panel (20% from the panel and 60% from the air), whereas you’d be doing OK to get 80% of the energy incident on STHW panel into the HW. So they’re kinda level pegging when the sun’s shining. But when it’s not, HPHW wins. Capital cost of HP is lower, so HP comes out on top.

    HPHW can be cheaply set to run when the Solar panel installation you have is at maximum power, which as it happens is when the HP is running at best COP. Also even reduced sunlight is likely to be generating enough power to run the HP without the grid. Not a theoretical result – I have the data from my own installation.

  26. Craig Iedema says

    For another $50 (i.e. $300), you can get a smarter solution than a dumb timer (Shelly 3EM + contactor is one) that would change the mix to 90/10 for a restive HWS. On a sunny June day in SEQ, I can heat a 1.8kW and 3.6kW Restive HWS entirely from PV production with lots left over. The day needs to be very overcast before we use lots of grid energy to heat our HWS.

    That said, even putting these much more optimistic values and my current rates, a Heat Pump comes out in front by about $500.

    When the current units die, they’ll be replaced with a Heat Pump.

    • Craig Iedema says

      As I think about this. I’m not sure that including the entire cap cost of the Panels in the calculation is reasonable. They last 25 years, the 6 years of this calculation is only 1/4 of their life. I think that should be factored in the panel price of this calculation.

  27. You are probably right either way but you’ve made a potentially unfavourable assumption for the resistive heater, dumb resistive elements have one major advantage over a heat pump, you can use a diverter instead of a timer. This means you can 100% match the heating to excess load, scaling up and down variably vs running at 100% and *never* import from the grid. Whereas the heat pump on a timer *will* import because heat pumps generally can’t cycle that fast when a cloud moves over etc.

    The diverter costs a bit more than the timer though, as it needs to know exactly how much power you have excess to send to the element and actually control the element rather than a relay.

    Of course if you have several days without much excess and your usage is more than that, the heat pump will be more efficient regardless of where the power comes from.

  28. Scott Phillips says

    I think your missing the point!! Every house already has a hot water! So adding solar is something their going to do, to cover some of their piwer bills.
    So no up front costs for the electric hot water. Now, as recorded nearly every house has excessive export, given away for next to nothing.
    So do the calcs on
    1. Adding a timer, cost $230
    2. Adding a power diverter, cost $1,000
    3. Replacing existing hot water, include removal, waste disposal, relocation of plumbing infrastructure and a new heat pump! $6,000 easy.

  29. Great info, thanks!

    Improved energy efficiency is the only way we can make our planet cope with all the people we’ve put on it, but a heat pump presumably requires more energy to build than an old-style water heater, not to mention the solar panels etc.. Without a large amount of research, the best proxy we have for this is overall cost.

    On balance, I agree with your conclusions, but I want to get there with a fair fight. To really look at how much money ends up in people’s pockets, I suggest you add in a row with a negative cost for each installation, for the money the homeowner would make from their solar panels if they only fed into the grid, i.e. about -$1000 for the 2kW system (averaging 8kWh/day over 6 years) and -$250 for the 500W system (2kWh/day). Then there is a real cost to the homeowner when some is diverted to water heating as you have shown. The heat pump still comes out well ahead. That would also allow people to estimate the impact of changing the size of the solar array.

    Do the solar yield vs. load graphs use a solar panel installation 4 times bigger for the resistive water heater? If not, on days with reasonably good sunshine like the one shown, the resistive load won’t be running off the grid much at all. However, over the seasons, on days where panel output is low, the resistive water heater gets really expensive, so I’m guessing your numbers might be about right for that reason. People should dial down the proportion of energy taken from the grid a little as the number of panels increases, while noting that on a dull day, 10 times not much is still not much!

    Not an expert, just an interested amateur.

  30. Gary Phillips says

    Ok but what if you add a $700 Catch Smart meter/switch to a resistive and solar system. That would help ensure that unused PV would heat the water rather than being fed into the grid.

    This is also looking at it from putting either in new but the solar and Catch system can be added to and existing resistive system without replacing it

    • Gary …. yes, exactly what we did, and it works a treat (we’ve now heated in “solar only” mode for 4 months, with 2 in a household and 250l resistive element HWS),

    • Look at my previous two answers. This is what I am currently using:

      6-solar panels of 305-watts
      1-ELWA (resistive element) works directly off the solar panels and or alternating current depending on time of day ir weather conditions
      1-hot water cylinder (from 150-L to 300-L). I have a 150-L one
      1-solar air conditioner works directly off the solar panels and or alternating current depending on time of day ir weather conditions

      NO INVERTER is necessary

      NO permits are necessary since there is no inverter

  31. I’m on my second heat pump (first one did a great job and lasted over 10 years). I also installed a 10kw solar system in December. My logic was a lot simpler than all the number crunching above. For the first hot water heat pump, I knew the roof was old and needed restoration. By using a hp I could use renewable energy without having to do my roof straightaway. When the first one died, I needed a replacement quickly as I needed the hot water. I went with a hp again to get renewable energy without waiting for installation of solar panels. My roof was already fixed and I thought solar panels would be easy to add later. It was. Now I understand the benefit my hp provides me is it still operates even on cloudy days and my electricity consumption is reduced. This year, I’m monitoring the surplus energy production with the view of adding on to my system with an appropriately sized battery.
    The only downside of a hp I’ve experienced is the noise it creates. When I i stalled my second heat pump it was much noiser than my first and what the supplier had indicated it would be. I had them remove it and replace it with a different model.

  32. Rod Webster says

    The analysis is heavilly skewed by some unrealistic assumptions about resistive. My 5kW system handles my hot water needs. Given the normal minimum install today is 6.6 kW, there is no reason to increase the panel count. My Palladin diverter has achieved 99.6% solar and 0.04% grid over an analysis of 365 days of hourly metered data. This results in a total annual operating cost of $152.41 ($1.88 Grid, $150.33 FIT forgone). So removing panel costs and adding in $1k for the diverter, we end up with resistive marginally in front @ $3193 resistive vs $3549 heat pump over the project life. Yet again, this proves the total lack of understanding of how diverters work, particularly the Paladin, which has very smart algorithms to both store energy from one day to the next and minimize grid consumption.

  33. It would have been good to compare a resistive with heat exchange panels vs the heat pump instead. We have a 315 litre electric storage hws with two heat exchange panels on the roof. For four months of the year we turn the electric boost off, as the panels are enough to heat the water by themselves. That’s pretty close to free hot water. In winter, we need to use CL to boost the heat, but I have been considering swapping it over to a timer and using only one of the two 1800 watt elements and hope that our 5 kW PV will cover most of the load. Our hws is a 16 year old stainless steel tank unit that cost $2500 installed, and in that time has needed one circulating pump replacement, which was around $300.

  34. I have a stored hot water system in my roof. Capacity 300 litres. Installed June 1995 with 2 passive solar panels ( water flows through copper pipes when heated by the sun) My average daily hot water cost last year (2022/23)was 51cents a day. ( or just over 2 kwh a day) We live in Adelaide. The panels face west. Family of 3. In summer our hot water costs are virtually zero, in winter we average 8 kWh a night ( J tariff – controlled load) We have a pump on the outlet of the tank to get decent water pressure. I bought a spare tank while I could in 2018 ($850) and have it stashed under my deck. Replacement solar panels are still available. ($1080 for 2) I check the tank twice a year. I realise this type of HWS is not for every one, but I don’t think there would be a cheaper option for our use.

  35. Duncan McKillop says

    Thank you for the article and I’ll ponder the spreadsheet in due course.

  36. A third option which does not seem to have been included for consideration is that espoused by an article in SolarQuotes in
    February 2020 – “The Best Way To Heat Your Water – Solar PV Or Solar Thermal?”

    Using a variation of Michael Bloch’s “Solar PV Diverter” proposal, basically I have not been out of pocket on the power bill front for the last 10+ years.

    (North, East and West facing ~6.6Kw PV on the roof; 5.4kWh battery; Time of Use electricity tariff; dedicated solar hot water system on the roof; a manual booster on the HWS with no power draw the rest of the time; located Far North NSW; 50,000L swimming pool running a pump and salt water chlorinator every day; and two adults in a very large, all-electric house with a wooden fireplace.)

    The system is well over 10 years old and the cost of a thermal solar hot water system has probably well blown out since installed. Also when the Solar Hot Water System was installed the old (standard electric) HWS was (fortunately) nearing end-of-life and was replaced with a Solar model at the height of the Federal Government’s financial stimulus exercise in 2009.

    However, for a massive part of the year the HWS is turned off from the booster which is only turned on when it is noticed the hot water temperature is starting to taper off, at which time the booster is turned on for a brief time (almost certainly in double-digit hours over an entire year), usually during the Off Peak billing period, or when the PV solar system is running gang busters (which is unlikely, because if the hot water temperature is not ideal, it is usually a weather-based issue). And obviously the whole system was installed piecemeal over a number of years.

    Anyway, definitely works for me, especially given the swimming pool in the equation and bedtime virtually never before midnight/1AM with quarterly electricity bills averaging $0 over any 12 month period.

  37. Tim Brooking says

    We have 6.4 on the roof Nth facing on 30° pitch and 5kw Fronius. Regional Victoria With a green catch that only diverts excess solar and a 320 litre tank 3.6 kw element our hot water bill is zero except for the half dozen times we set the catch to 2.5 hour off peak grid power in the middle of winter and we have gusts. So over the 3.5 years since installation our cost is the new water tank and the green catch. No way a heat pump can compete with that. As a matter of interest our panels produce max output in the middle of winter on clear cold days. A win win for hot water

  38. There is perhaps another solar option.

    My-PV has a DC resistive heating element that uses 750 watts from solar panels for around AUD$1,200.


    When there is no sun it uses the same heating element at 2,000 watts off alternating current.

    The installation does not require pipes; therefore, installation costs are simple. There are two PV solar cables from the resistive heating unit to the solar panels which are connected in series, and there is a standard alternating current cable to an electric outlet for days without sun.

    In addition, this installation does not require an inverter since the DC resistive element works directly off the solar panels.

    Maintenance costs might require at some point replacing the heating unit. However, this could be a DIY repair.

    Either alternating or direct current will increase the temperature in a water cylinder above 60°C.

    The solar panels can be installed on a dual axis solar tracker from Eco Worthy for around AUD$660. This will extend the morning and evening solar hours for the panels.


    The controller might need to be replaced at some point; however, this is, also, a DIY replacement.

    Add the cost of solar panels to the above, and you have a simple, efficient solution for heating water.

    My-PV has a calculator to estimate the percentage of alternating current required at different locations.


    Additionally, excess energy in the summer could be utilized for a dual direct / alternating current air conditioner, also, without an inverter.


    • By the way the above setup with 6-solar panels produces at my location in excess of 2,500 kWh.

      To extend the energy a bit more into winter, I have been speculating of adding one additional solar panel. This would increase solar production to about 3,000 kWh.

      The maximum amount that I can add is dependent on a few constraints:

      1- the heating element allows a maximum open circuit voltage of 360 VDC at 10-amps.

      The amps are somewhat less of a problem since the heating assembly can limit higher amps to 10-amps.

      2- There is a second constraint due to a ground fault protection device installed which limits open circuit voltage to 300 VDC from MidNite solar.

      If I add more panels, I might have to replace it with a GFPD from Morningstar which has a tolerance of 600 VDC.

      3- Another limiting factor would be the breakers themselves which have a tolerance of 300 VDC, and I would, also, need to replace these.

      Keep these constraints in mind when you plan an installation.

      We don’t use hot water everyday; therefore, there is surplus energy most of the week. This surplus energy is used by the air conditioner unit.

      During the summer we have seen a decent reduction of electricity from our electric bill.

      In addition to having less parts that can go wrong such as with a more efficient heat pump hot water unit this setup does not have the problems of leaking water pipes or additional water pipes associated with hot solar water collectors, and there is no need to replace the collector itself every 15-years.

      This setup works in the winter unlike solar water collectors, and if something should go wrong, I believe that I could handle it on my own. This should limit future maintenance expenses.

  39. Hugh Spencer says

    Has it ever occurred to folks to use less hot water?? (shock horror).
    We have an evacuated hot water solar system – and though we have a tad too much shading – BUT even in poor weather, it manages to supply adequate hot water for our needs (mostly showers). We also have a gas booster (one of those little camping doovers) which we use in poor weather.
    Evacuated tube systems beat flat plate hands down. Plus they are entirely passive.
    But heat pump systems are quite complex bits of refrigeration technology – and are subject to all the ills thereof (think refrigerant leakage – GHG’s, controller failure and so on).
    But at the base of all this discussion – is we have become socially programmed to expect 80+ C hot water whenever we want it. You can wash dishes quite well in cold water (that’s what kids are for) ditto washing clothes.

    • The water still cools if the heat pump isn’t working.. I already have appliances that heat cold water, and I do wash clothes in cold water, apart from wool..
      from what I can tell at the moment is that my dish washer uses less power than the heat pump..
      juries out here, BUT I still enjoy constant temperature hot water over fluctuations I had with the Chromogen gas boosted solar.

  40. The issues I see are climate related, heat pumps struggle in frost climates. Also circuit cards are a common fault with heat pumps/ split systems.

    The lack of refrigeration mechanics for heat pump repair also a worry

    Heat pumps are very efficient so have a place but I think we need to have multiple solutions and options.

    I work in facilities maintenance so having lots of discussion on this topic

    • They use heat pumps in the UK.. colder than here, even in Vic.. although the cold didn’t seem as chilly even at 3deg.. than here at 5. I hung washing and it dried on such a cold day.. so does the humidity help with the ambient heat in the air

  41. You could also assume if you replace the anode in 5 years by replace the system in 6 that you just decide to not replace the anode. Saving another $200.

    Also you would assume technology would be better and you would get a more efficient heat pump after another 5 years of R&D.

    Saving even more.

    To the point of its it worth replacing the system even if it’s not broken?

  42. James Morrish says

    In central Queensland, heat pumps have gone out of favour for life and reliability concerns. We have a resistive HWS with a 1.8kW element retrofitted and simple dumb timer. We run at around 60% self supply from a 6.6kW roof top. Just need to run the numbers plus trust your gut.

  43. I’m not sure it’s fair to account for the capital cost of the solar panels as well as the lost feed in tariff in these scenarios. It should be one or the other.

    However, I still think the heat pump would come out well ahead. 2KW of panels would be undersized for the resistive hot water system. In the winter months, hot water usage would be significantly increased when solar output is low. For us, it’s the difference of 2kwh/day used for hot water in summer vs 10kwh/day in winter.

  44. Lyle Essery says

    Ive got a 2.4 kw off grid solar array with 10 kw of AGM ES. Ive derated the element of 315l resistive element HWS to 1800 watts and fitted a Tuva internet switch which switches the HWS on and off depending on hours after sunrise and hours before sunset. The tuva switch is also conditional on weather and only triggers when sunny. I also have a lux meter relay that further toggles off the HWS should bands of cloud appear and toggles on again when clear. This helps prevent the offgrid ES from using stored energy to supplement when solar energy is too low to fully power the HWS. A heat pump could not handle all the switching and a heat pump is not able to derate its load. Restive HWS arnt as energy efficient as heap pump but more than make up for it in there simplicity and ability to tune to loads to the system capacity by selecting appropriate elements size. And rapidly switch when required. And furthermore their big capacity compared to most heatpump makes them a battery of sorts , storing many kwh of energy for up to a week. Thus you can have many days of no full sun and there is still lots of hotwater available.
    Cost is very reasonable. $900 for 315l HWS, $25 for tuva internet switch and $5 for photo relay . Add labor of around $300 and there is change from $1300.
    Big capacity Resistive element HWS with derated element and cheap smart switches smash heat pumps out of park . Especially if you got a offgrid ES and are derating your solar energy production as batteries are fully charged by 11am or you have a grid tied PV and are exporting to grid for next to nothing. No point in saving enery with heat pump if you get nothing for the saved energy.

  45. Patrick Weir says

    April 2006: 400L / 4.8kW “Resistive” HWS: (cost = $1000): 5 sac. anodes (done by a naughty person): tank leak.
    Dedicated cct. cost during 2022 = $321.
    May 2023: 400L / 1.0kW Acclaim HP HWS (cost = $5200), not dedicated cct.
    Energy retailer tariffs were 0.16 and 0.37, daily charge was 1.028.
    New tariff was 0.27 and daily charge was 0.83.
    Dedicated circuit “concession” was an illusory benefit?
    Now: a warning letter of steep tariff increases (keep share dividends coming?)
    My Powerwall 2 might have a better show of running the new HWS?
    I studied Kim’s Excel sheet, but I do not yet seem to fit into it.
    I have 10kW 3ph solar (shaded).
    As Kim says, there are many variables to be considered, and what I have written here is (mostly) what has already happened.
    One thing I know for sure is that my next bill will arrive soon, but I am not confident that I will be able to decide how well I am going.
    Maybe in a few years from now I will be able to apply the RetrospectoScope, and work out more exactly what has happened.

  46. What about those evacuated tubes with a tank? Are they not good or just out of fashion?

    • I have had an evacuated tube SHW system for around 10years, and find it awesome!.. For over half the year, hot water cost is zero, as the system has more than enough hot water.
      There is also an instant gas hot water system that is turned on during runs of cloudy days over winter,
      And i also have the option of using an electric element that was installed in the solar hot water storage tank. I plan to install some solar PV panels soon, that could also be used to power the electric element in the tank.

  47. Dennis Nord says

    I am wondering why Mr. Smith didn’t consider the older, passive hot water collector? I am using a second system as my first one started leaking after 20 years and the 2nd generation system might be a little better than the first.

    No electricity is needed as a passive system. It preheats water before any water tank if/when that’s needed. For our house we turn 2 valves in summer and bypass the electric resistant heat water heater. The actual water temp at the tap goes up (there’s a scald proof temp valve in the system) and we have adequate hot water throughout the summer and fall, no electricity involved. During the remainder of the year it supplies pre-heated water to the hot water tank.

    For financials I have nothing to contribute. I recently looked for my invoice on the newer system for a friend and I don’t have it. I also don’t keep any records that I can identify as the cost for my electric resistant heat water heater,. Maybe it is similar to Mr. Smith’s?

    Anyway, I know these simple passive systems are used world wide, in some cases as the only heat source in the water system so it seems they are worth considering. In fact it could be used in both the scenarios you have so far to see what the relative costs are if this is used as a pre-heater in cold months and sole heater in the warm months!

  48. A heat pump is 3-4 times more efficient than resistive heating. It is true that it takes time for hot water to get to the dishwasher (same.as when you open the kitchen tap) so it would still need to use some energy to get the water to the desired temperature, However it is misleading to say that the dishwasher uses less power than the heat pump. The dishwasher uses both more power and more energy to heat water up.

    • Rod Webster says

      Efficiency does not translate to least cost. I checked my data today. In the last 365 days, we have used just 1 kW of grid power on hot water heating with resistive and a Paladin diverter. That was on 4th and 5th July just gone. I’ve yet to see data from heat pumps or timer relays that gets anywhere close. I also doubt the catch power can achieve the same result.

  49. I’d like to see a solar HW system thrown into the equation.

    I have used an ESP32 to monitor my tank’s water temperature so I can decide whether to switch on the booster. (I can also automate this process via a home automation system)

    During sunny summer days we don’t use any boost. On sunny winter days we can get by without a lot, but with two rainy or overcast days I’ll need 7-10kwh to get the water to 60+. Anything over 55C will get us a hot shower if we are careful.

    So far I have about 6 month’s data and I have probably been a little generous with the booster to make sure we don’t have cold showers. It’ll make for interesting reading after 12 months.

    The thing holding me back from putting the heat pump at the top of the replacement list is the number of reviews (and comments here) that talk about control boards burning out – and the $1k+ repair cost.

    • We have a similar approach but more manual. I have a bluetooth sensor on the tank outlet pipe and will manually boost at the meter (2 hours) to top up

      I am not paranoid about Legionella and anything over 40C (at the sensor) is ample for a bath. Last night’s bath cost me $1.92 for 5.6kWh. Bringing the 320ltr tank up from 36C to 42C

      This Winter we have used 39kWh to boost from the grid. I have 1kW off grid PV boosting too but next to useless this time of year.

      • Our Chromagen HW tank has a thermowell on the top, so it lets me drop a thermistor down about 150-200mm, so I think I am getting reasonably accurate readings. If I get it up to the mid-60s once a week, I figure that help sanitise it.

        I had it on the outlet pipe like you, but found I had to run the hot water to heat that pipe up for a reading. This also made me aware of how much heat is lost through the fittings acting like radiators – and out the top of my tank.

  50. Tim Chirgwin says

    I downloaded your spreadsheet and being the very savvy buyer I bought a 1.8kw element or (2.4kW as standard), I bought PV at 45c/Watt (can be bought for less than this), changed the power purchased to 25% (as I added in a Catch Solar Relay and the power draw is close matched to the PV so assigned, while the HP draws twice that of the panel so assigned (1kW draw HP to 0.5kW of PV)), and changed the solar to (-25%) to correctly give credit for power exported (there is 4x more power to export from the 2kw array than the HP’s assigned 0.5kW array.)

    As the capital cost of each system included the solar to run it (averaged over the year) there was a need to buy power sometimes (but with the catch solar relay this would cleverly limit the pull from the grid and 25% is a very generous allowance in favour of the HP system that does not have such smarts). At other times the same amount of power drawn was actually a credit sold (remember averages) which earned 6c per kWh.

    I noticed that your formula was double dipping by “charging out the self generated power used at 6c” when the cost of that power generation capacity was very clearly included in the purchase price!

    We now have the Resistive HWS (via your spreadsheet) over 6 years costing $3210.00, and the HP costing more at $3529.00! I added on an extra $319 to cover for the extra cost of the smart catch relay over the allowance within your spreadsheet.
    So now the two options are even priced at the 6 year period (BUT,… your Heat Pump is already completely out of its warranty by one year, and the Resistive is still going with plenty of tank warranty on its side.

    When the first replacement of the warrantied Resistive is due, the PV included in the capital cost will still be under long warranty, and by the time the 2nd Resistive HWS completes it warranty period you will have bought

    5 expensive Heat Pumps to compare.

    We don’t need a new spreadsheet to see the huge cost of HP systems.

    • Craig Iedema says

      The 0.06c is the cost of foregone FIT.

    • Rod Webster says

      Installing solar panels is a seperate investment decision to water heating. The spreadsheet attempts to add it in to disdvantage Resistive as it increases capital cost. Any basic rooftop solar plant has more than enough capacity to run a resistive HWS (with Diverter) or a Heat Pump. The days of FIT are certainly numbered. The grid doesn’t want our generation even now. It won’t be long before .AU goes the way of other countries where you are charged for the power you export. So keeping your generation on site is the objective long term. If you don’t have an alternative use for the energy savings of a HP on site, then there is no point increasing your capital cost to reduce power consumption. I’m sorry but when you can get your running costs down to $0.30 per annum (like I have with a diverter), The Heatpump with its higher capital and running costs will never be a rational investment. Plus you can seperate the investment in a diverter from a system replacement so it can be added any time and be paid off by the savings including FIT forgone in 2-3 years.

  51. Tim Chirgwin says

    There will be occasions when very high users of water will find a heat pump works out cheaper in theory (and perhaps in practice if they have no solar at all) , but when the water doesn’t heat up well due to the cold weather (without buying the much higher priced Heat Pump), or you have worn it out faster through overuse, or the fins are corroded from salty air, or you have “p….d” off the neighbours with your heat pump running near their bedroom, or you are fed up with the wasted time waiting for hot water to arrive due to limited HP placement options, (along with wasted water and lost heat), then the misgivings of so called energy efficiency will come to haunt you.
    Indeed Mr Smith is 100% right when he says that free energy is free, whether it is free sunshine energy turned into electricity and hot water directly, or whether its into electricity to drive a machine to capture free sunshine heat energy from the air!
    If the PV is included to capture free sunshine then it all comes back to cost of the gear and its longevity.

    • Geoff Miell says

      Tim Chirgwin,
      …but when the water doesn’t heat up well due to the cold weather (without buying the much higher priced Heat Pump)…

      It pays to check the system’s environmental operating range specifications will suit the intended location. Both the Sanden Eco® Plus & Reclaim Energy HPHW systems say they will operate effectively between -10 ºC and +43 ºC.

      I know from years of experience my Sanden Eco® Plus certainly works effectively below freezing. For example, on 21 Jun 2023 the minimum reached down to -7.2 ºC.

      …or you have worn it out faster through overuse…

      That suggests to me the storage tank(s) are under-sized. Go with a bigger tank size, or use less hot water.

      …or the fins are corroded from salty air…

      I’d suggest salt air corrosion would also be attacking many other appliances around your home, your vehicle(s), etc.

      …you have “p….d” off the neighbours with your heat pump running near their bedroom…

      For Sanden Eco® Plus & Reclaim Energy HPHW systems, coupled with solar-PV, either system should normally be set to run during the day to take advantage of self-consumption, except if the ambient air temperature drops below 3 ºC whereupon the unit runs briefly at short intervals to avoid the system freezing.

      37 dB, measured at 1m distance from both the Sanden Eco® Plus & Reclaim Energy systems, is IMO very quiet. My air-con outdoor unit is rated at a noisier 60 dB.

      “…you are fed up with the wasted time waiting for hot water to arrive due to limited HP placement options…”

      I’d suggest the same lag problems would exist for a similar sized tank resistive element electric system.

      Mr Smith is 100% right when he says that free energy is free…

      The equipment providing/converting the energy to heat the hot water is not free!

      • Tim Chirgwin says

        Geoff Miell,
        Clearly Mr Smith understood that the very expensive heat pump was a close match to the resistive tank on the basis of ability to heat when the weather is cold and a closer match on warranty period ( although he should have chucked a couple of hundred extra on for the same tank size for the resistive ), but as the figuring clearly showed a bias against the HP, Mr Wainright chose a less efficient and lower warranty unit to justify his arguments, using “savvy” to ease in the paper tiger.

        To suggest that larger tank size or to use less hot water is a way to justify the unreliability of high water use through a HP is not even amusing.

        To suggest that the HP fin corrosion common from salt air causing early problems is somehow to be ignored by talking about rusty cars is off topic,… resistive HWS, especially those placed inside near the demand for hot water do not suffer this at all (and save time and hot water).
        Expensive heat pumps might be quieter as you say, and affordable ones might be more noisey,..doesn’t change economics though.

        Mr Smith is 100% right that free sunshine is free, and the machines to capture that free energy need to be costed,…you should quote the complete sentence to justify your credibility???

        A large number of experienced people are describing how they have set up their resistive HWS to run much cheaper from PV, not to mention that the extra Pv generation during mid day is all surplus and receives low FIT. And further that large PV paid for by capital savings instead of buying high cost heat pumps, also give extra generation morning and afternoon that directly offsets the purchase of peak power that is up around that 55c/ kwh.

  52. David Gammon says

    Hmmm. Just putting in some rough estimates for comparing a heat exchange (as in panels on the roof to directly heat the water) with electric boost and using controlled load rates vs the heat pump. assuming no solar PV used at all, no timer needed, and no electric boost needed for 50% of the year (in reality it would be no boost at all needed for 3 or four months, and a fair amount in winter months). Using SA rates, as I live in SA, and I have a solar HWS with elec boost.

    Even without the benefit of using any existing PV instead of using controlled load, and without trying to factor in the inconvenience cost of a failed heat pump and having to organize to replace it much sooner than the resistive unit, it doesn’t look good for the heat pump……

  53. Just to add my 2c worth … I think some of the article really misses the point. The main issues for me are the cost of investment and the return on same. We replaced a “typical” resistive HWS back in 2020 … and we did go reasonably high end … Sanden 315 L unit, and yes, we did get an STC rebate. Total cost was $4,990.91, less $909.09 STC … so all up $4,081.82.

    The Sanden uses a German heat pump, and is installed with a “premium” tank made locally by Rheem. By comparison, a Rheem replacement at the time would have cost $1,800 for a comparable size, resistive unit configured for Dedicated Circuit (Off-Peak). So the “investment” is approximately $2,300

    Looking at the operation of the Sanden, it consumes 1 kW, and runs for somewhere between 45 min and 75 min. …. depending on how much usage and time of year … so say around 365 kWHr for a full year. The last full year we had the previous unit, it consumed 1948.5 kWHr … same usage pattern as now with the Sanden. So, as advertised in the Sanden brochure, it uses less than 20% of the energy a normal off peak resistive unit would.

    Looking at cost … the real cost of running the Sanden is the power from the Solar system, that would otherwise be fed into the grid … at the current infeed rate or 7.5 c/kWHr .. that equates to $27.375 per year – a pittance really.

    For the resistive unit from the past, the Dedicated Circuit usage cost is (as of 1st August 2023) $0.282656 per KWHr … so total running cost would be $550.77.

    At that rate … the Sanden will pay for itself in 4.4 years …. a bit over a year from now!

    Given the Sanden on average uses 1 kWHr per day … so 1 panel is more than adequate … but why would you go to the expense of adding any panels – to get the 7.5 c / kWHr infeed? Don’t see the justification.

    We have an 8.8 kW system, with 14 kWHr battery … and I’ve got to say, the result is great

    • Tim Chirgwin says

      Mistakenly you are justifying the huge cost of the heat pump you purchased by billing the cost of running the heat pump on the loss of FIT of 7.5c per kWH by using your existing solar panels, while you are calculated the very cheap and reliable resistive HWS usage at the rate of 28c being power bought from the grid!!

      Please compare usage correctly Pv or your comparison is meaningless.

      Your solar Pv should return its investment cost (even if selling ALL of the power produced with no self consumption in 10 years at 7.5c FIT), and this power can easily be used to run your higher draw resistive on smart timers or diverter, and give you extra power morning and afternoon to reduce your peak rate priced purchase of power.

      Recalculate your figures based on PV usage alone and compare your heat pump with a $27 bill p.a. against $146 (using your figures) for a cheap reliable quiet resistive HWS. At a saving of $119 per year it will take 19 years before you get a ROI, and if you’ve had to borrow the extra cash then you will never get a ROI for the extra money spent. The warranty does not last 19 years nor can be expected to faultlessly run that time either.

      It would appear that those who support the heat pump want to justify their own decision, while those who argue for the resistive with extra PV are penny wise.

  54. Rod Webster says

    You should get way less than 25% with a catch power, but consider a Paladin instead. Its a bit smarter. The last 12 months, I have spent 30 cents on heating hot water with my diverter (eg. 1 kWh) so the gap is substantially in favour of Resistive (with diverter)

    • Tim Chirgwin says

      Rod Webster,
      I agree with your point of very low grid purchase % when installing a power diverter (or solar relay) to regulate the cheap and reliable Resistive HWS,.
      Could you tell us your PV system size and kWh of electricity used for resistive hot water?

      Of course with the relatively low price of installing large PV systems, low FIT and maximising self consumption (especially if your system is export limited) the much smarter move, and with better ROI is to use Resistive HWS than paying a fortune for Quality heatpumps (or multiple cheapies) to obtain comparable warranty and reliability (even if with some additional running costs).

      The added advantage of a larger PV system (that is not running the HWS during the morning and afternoon), is the increased generation available to self consume when you are home, rather than purchase peak power from the grid at very high rates per kWh.
      At 55c/kWh this offset alone could well be saving 55c/day extra in winter and $1.65/day in summer for the extra 2kw array installed as part of the proposed resistive with large solar.

      Some would say that their HP are effective at low temps, but fail to account for the much higher capital cost (and replacement cost yet to come) for these units, or only partially quote descriptions of free energy in sunshine to misconstrue that it means “energy captured without cost”, obviously in an effort to try to discredit the economical system cost of “resistive with lots of PV”

      • Rod Webster says

        Tim, I have a 5 kW Solar system. Daily average HWS consumption over 12 months is 5.17 kWh which comes from an average of 26.6 kW of daily solar generation. There is definitely no need to add extra panels for HWS when most starter systems installed today are 6.6 kW. What people do not understand is that Diverters only need to keep the water to 40 deg C and go over 60 deg C once every few days (solar has always covered thet before its necessary). My Paladin Diverter also stores surplus energy by heating the water to 73 deg C when the Solar is available to carry over for cloudy days.. Even in the middle of winter, today our water was to temp (73 deg) by 11:20 am. Plus the diverter heats at the full 3.6kw element rating if solar is available. If even 100 W is available above needs, it goes to water heating. If you turn the kettle on, the diverter instantly reduces the amount sent to the HWS so Grid power is not required

  55. You can get a Chromagen installed for $33 in NSW (price as long as it is a resistive replacement)

    I only replaced my 300l resistive with a new one 12 months ago.

    Thoughts on Chromagen – they seem to have a bad reputation?

    • The plumber I’ve used for a while described Chromagen as cheap builder-installed heaters.

      We’ve had a solar HW Chromagen for 8 years and the only issue was the wiring from the thermostat to the element deteriorated and was causing high resistance. An electrician replaced that with silicon insulated wiring for about $120. (The actual wire cost me $2)

  56. We bit the bullet and upgraded our HWS to heat pump. Over the past few years we installed initially a 6.2kw array, and subsequently a second 6.2kw array (we have one of those oddity houses with 2 phases of power).
    The house had 2 storage HWS, and even with the 2 solar systems our power bills were still not as low as we’d hoped.
    We switched both the units over to heat pumps in January this year (iStore 270l), and for the first time we had bill credits right through until end of may (it’s very cloudy here through winter). We have the timers on the HW units set to run during the solar sponge window during the day, given that it’s the time of day to be most likely sunny, and if not it’s the cheapest power rate we figured this was the simplest way and it’s been working great.
    We did peel the stickers off one of the units which sits right near our driveway, purely for aesthetic purposes as those logos are pretty in-your-face and didn’t add much to the look of things, but otherwise the units have run so far exactly as hoped.

    Wish we had done it sooner!

    • Tim Chirgwin says

      Have you costed out how much you would have saved if you ran your existing resistive HWS on solar power (increased the PV still) with the added cost of a power diverter or catch solar relay, ( instead of having it on the controlled load tariff) to see whether you would have been financially better off? Did you consider Future maintenance of shorter life heat pumps?

  57. Chris Thaler says

    Now for my tuppence worth; Our monthly controlled load usage (two oldies) was 172 KWh @ $.020 per KWh. This is with a relatively new 250 litre (Australian made) resistive hot water heater. The original 350 litre unit failed at 31 YEARS of age !!
    Replaced for $764.00 by myself. C.L. power costs $0.20 per KWh which equates to approx. $1.14 per day. I now use a time switch Mon to Fri accessing our solar production as available during the day and am yet to receive a bill for the changed circumstance. How does this stack up against all the wayout & wacky assertions made previously.
    I estimate that over a ten year period our power charges will average $744.00 per annum, at current rates, for all use. Currently have a solar total of 6 KW across our 3 phase supply and use approx. 242 KWh per month.

  58. Andrew Leask says

    In your battle of the HWS’s is it seriously realistic to suggest the life of a Heat Pump HWS (HPHWS) is only 6 years and, at that point, the whole of the system needs replacing? Further, a replacement HPHWS is unlikely to attract much inheritance way of rebates, if any. My HPHWS is only 18 months old, but my previous experience with gas/electric HWSs gives a life achievement of 18+ years. I would hope to achieve >10 years life from my HPHWS, which would swing the argument further in favour of this type of system.

    • The use of the shorter time was to take into account reports from reviews and some of the comments here, of the electronics getting fried. I have read several reviews of one of the more expensive HPHW systems where the main control board needed to be replaced because the manufacturer blamed a power surge and would not cover it under warranty.

      It was taking a “worst-case-scenario”.

      My air-conditioner (same technology) had a main board component fail and that cost $1k.

      The resistive heater is a much more simple beast.

  59. The article, analysis, and discussion here is an exemplary illustration of the absolute mess we have got ourselves into with electricity in Australia.

    The evidence here about just one power matter among many – our hot water – is that unless you are a competent meteorologist, electrical engineer, economist, accountant, and lawyer (to work out warranties and liabilities), and have ample time on your hands, you can never be sure of what combinations of technology and circumstance will work best for you.

    Oh for the days of the simple switch and cheap, reliable electricity!

    • Robert, many of the people commenting on this site are enthusiasts or those with the time & inclination to optimise their systems to the nth degree. The rest of us can get big savings with little effort by just installing something better than we have now & then ignoring it. Any reliable heat pump, or solar (electric boosted) or if you have PV, a simple electric storage tank with a timer, will save you plenty over any gas system that you’re replacing. You may not save as much as is possible, but you’ll be way ahead of where you were & you can get on with the other things in your life that don’t involve obsessing over every kWh.

      • Thanks Peter, I do agree with your thoughtful reply and you identify a very good principle.

        There are exceptions, as always, and once having made a basic move – say only installing solar – the next might be counterproductive in terms of costs and benefits. Installing an expensive battery is one example, and another might be replacing good quality resistive HWS with a heat pump.

        Making that next move is where either being a multi-disciplinary expert or enthusiast comes into play.

  60. From a carbon footprint point-of-view during the actual use of the HWS, the graphs indicate that the heat-pump is much smaller than the resistive element type. Is this a fair reading of the data? (My main interest is in decreasing our domestic carbon footprint.)

  61. geoff little says

    I was selling, specifying., crunching numbers on HWHP from 1990 till I retired in 2016. Now I just do occasional consulting to Govt on commercial HPHW systems, mostly peer reviewing other party’s specifications. Last count there are 42 HWHP brands/ models on the aust market, 75% are rebranded Carnot cycle systems of cut and paste design dubious performance, greenwashed advertising and proven chaotic reliability. About 5 Carnot cycle systems are acceptable for performance and reliable lifecycle. The best technology by performance for domestic HWHP in all climates and applications are the 3 brands of transcritical CO2 systems.
    The customer who buys in the goldrush of advertising will get what he pays for. The cycle of goldrush and bust has been going on since the big boys in water heaters panicked after Quantum market share bolted, The bust came when their reliability was so bad that the technology as a whole was burnt badly for reputation. I see it happening all over again. The esoterica of comparative numbers are all very well, but meaningless to the dismayed consumer who finds his beat-system-purchase has died after 5 years, & replacement is at ‘full price’ as STCs and state rebates are for 9 years. Numbers aint worth the paper they are written on if the choice of brand and model is not fit for a reasonable lifecycle of service.

    • Well Geoff, you sound like your opinion is worth having. What brand/s would you recommend for a typical family of four in Melbourne?

      • geoff little says

        For Melbourne climate, for medium to large household HW requirements, the high performer is CO2 refrigerant transcritical technology. Would give equal weight for different reasons to the longest market incumbent, Sanden, to the Mitsubishi Reclaim because it has its electronics control on a wander lead, and third to Revere from Automatic Heating Global [AHG], headquartered in Melbourne. Revere is the brand with least exposure; however, these are rebadged as Revere from Itomic in Japan, a world leader in HW systems. AHG are not just salesmen, their engineering and commitment to CO2 refrigerant technology is as good as it gets. OK– moving next to Carnot refrigeration cycle HWHP systems. This technology was developed and patented in Australia by Melbourne University and the Siddons family in the 1980’s. Their ran it alone as Quantum until very badly copied by the big players, The brand name Quantum has been rebuilt in recent years as I think the best option. The 850 watt compressor, ideal size and good design means no axillary RESISTIVE element is needed. Note they employ natural refrigerant with a global warming potential of 3, not 1300 as for synthetic R134A. [ that is a significant environmental tick in my book!] Quantum have a range for any household size. For medium to smaller households, Stiebel Eltron are an alternative choice. Reliable, good service delivery, however with what I consider a drawback however of needing resistive boost, mainly because they have a smaller watt compressor. 550 watt if I remember correctly. After that, the other 37 are mostly rebadged generic systems from China, originally badly copied from Quantum when their patent expired. Suggest check reviews, online– some appear OK, quite a few have a high failure rate. Low price and give-aways are not an indicator of good performance or a reasonable service life, which in my view can be 15 to 20 years. Note, welcome any comments from any of the 42.

  62. Steven Thompson says

    Recently had solar fitted to our home we moved into two years ago. It was fitted with an existing standard hot water system that was only a few years old. As part of our 6.6KW system we had a Green Catch Power relay installed . This relay heats our hot water from our solar panels rather than from off peak this is serving us well and has reduced the amount of electricity we are buying to heat water to less than $5.00 a month. The solar panels are heating our hot water but if the conditions are bad and no solar it will still heat from off peak.

  63. Gregory B Bell says

    The suppliers have gamed the system on COP which is dependent on the temperature difference. The one I looked up only heated the water to 32 degC (so it looked really good on a COP basis). Typically the temperature requirement is 60 degC and ambient temperature is less than what they claimed for typical use. There is a more balanced view from the ACT government.- https://www.climatechoices.act.gov.au/policy-programs/sustainable-household-scheme/buyers-guides/singing-in-the-shower-a-guide-to-hot-water-heat-pumps

  64. I have an absolute ton of SA interval data that I could dig into to explore any of these hypothesis.

    But it boils down to:

    1: Resistive coupled with a small element on a timer if you already had it existing is an extremely cost effective option if you already have solar, especially since short of the tank splitting – component repair is trivial.

    2: Plenty of levels of government are throwing bucketloads of money at heat pumps. If you spend say $1500 on a Hydrotherm, it’ll pay itself off in threeish years. Which is what I did as the aforementioned resistive was 13 years old and I figured ‘why not’

  65. I am 100% go for heat pumps but it’s important to do your research and weed out the sub standard units.

    My first Heat pump was a Dux airoheat which turned out to be the biggest pile of garbage I have ever bought. It was continually leaking and would often leave us with no hot water. after 5 years of struggle and many call outs, I binned it and went with a kelvinator brand which had the heat pump seperate to the tank. That unit was brilliant and lasted for the 6 years I continued to own the property, it was still going strong when we moved. the only issue was a failure of the controller board which had been infested by ants. this was replaced at no cost to me. I dont think these units are available anymore.

    Im about to have a reclaim unit installed to replace the resistive HWS that came with my current home. I think with the continually rising costs of electricity this is a smart move for my family. Currently my HWS system uses 14-20kw per 24 hour cycle on off-peak rates. With the new reclaim unit I will run it from 11:00-2:00 in winter to use my solar power and it will only use around 4kw at mostly 0 cents. The reclaim units are expensive but my opinion is that in the long term it will save me alot of money. I looked at getting a diverter but still felt the Heat pump we the way to go.

    • Tim Chirgwin says

      Can you practically keep your existing resistive HWS and spend the same money you plan on spending for the heat pump on extra PV and diverter/timer and allocate the 4 kwh you say you have surplus now, and come out financially and green credentials in front?
      Have you actually costed out the both the options and could your share your figures?

  66. SO we use pv to run a heat pump at what cumulative in efficiency.How about we use thermosyphon panels PROPERLY installed with which ever backup is cheapest ie [dare i say it] instant, My experience is that enough panels is enough hot water. If you rely on a big cylinder and resistance element you often get a cold shower at BED TIME . or you wait till the element heats the top of the tank PRESUMING you have a TWO element HWS .
    i suspect keep it simple may be way cheaper and reliable.
    address 3305

  67. Elaine White says

    We are in Canberra and installed a Quantum heat pump HWS in 2005. With semi-regular servicing it is still going strong (replacement compressor in the mid-teens). So it may or may not be just lucky, but we have had a good long life out of it. It was on off-peak before we got solar panels, now we just run it off the solar and grid to ensure water is always hot. It is still very cheap to run.

    • geoff little says

      good post on this brand–quantum is one of the few Carnot cycle brands i would recommend. checkered history since 1988, now back action with good product. [ see my general comment on this blog]

  68. Kajal Mukherjee says

    Hello there! I live in New Zealand and only recently have installed a 5kW PV system. My next project is to go Heat Pump HWS. At present my water heating is using piped gas. So it is not costly and being instant inline heating, I’ll have continuous HW till the gas is available.
    I am keen to still move over as I feel I should reduce my fossil fuel burning.
    What dillema I have is, is my thoughts cost effective? Or am I being stupid by trying to save the planet? Gas price is on the rise and will keep rising, wherever we’re in the world. But still. What do you think?

    • Kim Wainwright says

      Hi Kajal. You’re certainly not being stupid trying to ‘save the planet’. I’d suggest that those who don’t consider ‘saving the planet’ when making decisions around energy use would be the stupid ones if they’re living in a country with a high living standard and think they can carry on like they used to. We have to stop burning stuff. It’s up to you whether you want to be part of wearing that cost right now or pay later.

  69. Dan Phillips says

    I would say that from the point of view of energy efficiency – A heat pump with a COP of 5 is consuming much less energy than a resistive HWS. Less energy consumed at your house is less fossil fuel burned at the power station.

    Depending on which state you live in, the fossil mix of your energy may differ.

    My house uses a lot of hot water – due to teenagers..

    I’m a heat pump convert, my old resistive used $900 more of electricity per annum than my Reclaim model does.
    It is also almost silent.
    It runs down to -10deg.
    it has timers to run at different times – which you can change easily. The new model is wifi connected apparently.
    In Vic I got a $1500 rebate on my old HWS when I changed – so the overall cost for me was <$4000.
    and realistically I expect it to last 10 years.

  70. Stuart Gluth says

    We are 2 people living in an apartment so we need the compressor on the roof and the storage tank inside, precluding installing a connected unit. We are continually ‘warned’ about cheap heat pumps from a particular country, while at the same time you are advocating not purchasing an (over) expensive heatpump. We are aiming to replace our instantaneous gas hot water to get rid of the service supply charge and save ourselves and the planet. What level split heat pump would you reckon would suit us?

  71. Stuart Gluth says

    We are 2 people living in an apartment so we need the compressor on the roof and the storage tank inside, precluding installing a connected unit. We are continually ‘warned’ about cheap heat pumps from a particular country, while at the same time you are advocating not purchasing an (over) expensive heat pump. We are aiming to replace our instantaneous gas hot water to get rid of the service supply charge and save ourselves and the planet. We have (nominal) 6.3Kw solar but live in cloudy low lattitude Melbourne. What level/brand split heat pump would you reckon would suit us?

  72. Not much of Australia where the Minimum temp goes down to -10 let alone the Maximum that low (during the day when panels are working)

    And where is thermal hot water in this (with a resistance booster in some cases) cheaper than both and my father ran his for 25 years with the booster turned off before the tank gave out

  73. zimeadime says

    I would love to know what a recommended heat pump for hot water is.

    The only ones that I am aware of are the CO2 ones, which are the most expensive, partly because the compressors are inherently more costly to manufacture but also maybe they have better construction and insulation.

    I would love to if there are any good brand units that use standard refrigerants.

    There is an AFR comment piece that says that one of the big issues with them is that there are no standards thanks to the ‘war on red tape’ of a previous government so unlike resistive hot water cylinders there are no minimum insulation levels and a star rating for them. Basically anything goes and buyer beware.


  74. It seems if you have a resistive HWW (or a solar HW with a booster like me), a Catch Power or Paladin device is the go.

    1. Are there any views on which is better?

    I get the impression that the Paladin is more expensive, but perhaps has more features or can be “programmed” a little more than the Catch???

    2. Is the extra cost worth it?
    3. If I eventually get a heat pump, is there a market for 2nd hand devices?

  75. Thanks SQ for yet another great article (even if most of the analysis well beyond my comprehension) and to your generous commentariat.

    (Another suburban Canberra denizen – living in 55 yo small (3br) freestanding double brick retrofitted w double glazing, good insulation (ceiling – R7, under floor sprayed, pellets in wall cavities), 6.6 kW solar, hydronic heating (7 wall panels) via heat pump that rarely catches sun in winter and frequently has ice on exposed coil/tubes – which we have coming on at 0600 rather than the recommended 0500 but works well for retired couple and dog, all of whom rarely emerge before 0900-1000. Getting a rough idea from our solar inverter’s app, our small old style electric HW tank (which we decided to stick with to replace a similar 15 yo tank which had been starting to leak) appears to use a lot less electricity than I had been led to believe (could it be that newer models of that very basic appliance are more efficient??) but maybe a timer as suggested above could still be a good idea. With winter temps often below freezing, can’t say we are enamoured of having water tanks, tubes or pipes on or in our roof.)

  76. One important aspect in the whole heat pump vs resistive argument is that quite a lot of people (me included) already have an old school resistive element hot water system.

    Provided the system is big enough and still has a decent service life left then throwing the old system away to fit a heat pump system doesn’t make much sense, especially from an environmental perspective.

    I already had a 300 litre resistive element system that was installed only three odd years ago so I put extra money into having the solar panels to run it. I have 8kw of solar and the hot water system only runs for two hours from 10am to midday, which is plenty for most occasions as it turns off after one hour normally (two person household).

    The 8kw of solar generates so much power at the middle of the day that the hot water and the pool pump can operate at the same time and normally won’t draw any grid power or if it does (overcast) it’s very minimal. Even in the depths of winter (as it is now) the system generates about 4.5kw from 10am so 3.6kw for the hot water still leaves overhead for the pool pump to operate.

    Of course anything else above that demand is most likely going to use grid power but that’s only a problem on non working days of the week when someone is actually home between 10am and midday. Timing the hot water to turn off at midday also leaves you some spare solar capacity for cooking lunch assuming you start a bit later than usual.

    When the existing hot water system craps out I might look into replacing it with a heat pump but then again maybe not as a heat pump with moving parts can’t outlast a resistive heat element with no moving parts, end of story. As soon as you have to replace the heat pump or replace some major component in it you’ve probably done any dough you saved by having the system in the first place.

  77. Looks like the COP of 4 is only above 20deg C air temprature what if COP is only 3 in winter when we do most of the water heating ?

  78. Michael Dunn says

    I’ve just installed a dual 215L heat pump system to replace a 415L electric storage. There are govt incentives for this program and the cost was about $700.00 . What prompted me was the huge hike in prices on July 1. I already have 15kw solar plus battery, but in winter the solar wasn’t enough to heat the water, which I was doing on night off peak which just bacame ridiculously expensive. I’ve set up the heat pumps to run 9am to 3pm and now my Winter solar is covering the whole peak period from 7am to 10pm. Saving is $8.00 a day compared to winter on the new expensive tariffs. Have cut daily winter grid import by 20 to 25kwhrs.

    Anyone who doesn’t believe heat pumps are the future, think again!

    I believe the reason the govt is specifically subsidising a changeover from off peak electric to heat pumps which can run off solarcyear round is to help eliminate the overnight demand before the coal fleet is retired. The offer is only available if you have an electric hot water system .

  79. ARENA published a report (prepared by UTS) on 16 May titled “Domestic Hot Water and Flexibility“. On page 5 there is a simple decision tree that provides “electric heat pump + DRED” as an option. This looks to me the best of both worlds: use a diverter (like Paladin) to toggle the heat pump according to the prevailing solar production.

    Another option that I would like to see explored more is instantaneous electric. I can see benefits from the lack of need to pre-heat and store water. There’s even the option of placing the heater closer to where the water will be used (even indoors), reducing waste.

    • Rod Webster says

      DRED (demand response enabling device) is totally different to the way a Paladin HW difverter works. A DRED enabled heat pump can have its output reduced by the grid supplier to cope with peak demand. It must be fitted (or able to be fitted) to Air conditioners sold in Australia. If a HW heat pump is DRED enabled, you could reduce its consumption based on available power. (Paladin have another device that can do this). Becasue a DRED changes consumption in discrete steps and heatpumps re not designed to be constantly switched on and off, its not trivial to use DRED to map to your solar availability but I have done it with Home Assistant on a MItsibushi Air Con. The Paladin diverter on the other hand is infinitely variable in real time (50 x a second).

  80. I have a 7Kw solar system installed and have had a Bosch heat pump for the past 6 years.
    The noise does not bother me as I have it set up to only run at daylight hours.
    It only uses 500 watts when running so it’s almost completely covered by solar even on cloudy days.
    I live in Brisbane so it is warm temps and optimal for it to run.
    However, when it dies I am going back to a normal hot water system.
    It is just too complex and prone to breakdown, and plumbers cost a fortune. I think I was pretty lucky; only the clock broke on mine, I could work around this by resetting the clock twice a day on the hot water system, and I still had hot water.
    But this took months to fix and ended up costing me over $800 even when Bosch covered the costs on the parts (which they did not have to do, it was out of warrenty)
    I am no longer eligible for any rebate as I have already used it, and even if I was, I think i would still go back to a normal hot water system with a small element and a diverter. I got 5 good years out of it, I may even get 5 more, but I am currently not planning on doing it a second time.

  81. Fascinating debate. It seems to me there is no clear winner in the choice between the two electric water heating technologies since there are so many variables including: different state power costs and subsidies; different climates; specific site issues (location and wiring); how much solar PV you have; if you are export limited or not; availability of capital; NPV discount rates; price inflation assumptions; interest rates, whether you have a battery etc etc.

    I was going to suggest that the initial spreadsheet was a frivilous comparison because, historically, the best idea was to just put on the biggest solar system you can fit/afford and then make a separate decision about water heating. However as FIT disappears to zero this may not be the case going forward, and matching the solar system size to the (planned) total home load starts to make some sense.

    Getting rid of gas water heaters will be good for the planet, so if you could all please do that first and also get rid of your gas space heaters and use your reverse cycle aircons. I’m happy for you keep your gas cooktop since the gas burned per year is a tiny amount. I cut off our natural gas supply and converted the cooktop to LPG so as to save the daily supply fee and avoid replacing a large freestanding oven with gas cooktop. Uses three or four BBQ bottles a year now so has paid for itself inside a year and only minor inconvenience to swap over.

    If we must burn gas it should be in gas turbines to efficiently make electricity to supply our efficient home heatpumps (for water and air). Or perhaps it could be used to co-fuel the remaining coal fired power stations whilst we run them out?

  82. Ken Graham says

    Solar direct hot water heating or are gravity feed roof mounted tanks not a thing in Australia?

    20+ years ago I made a copper tube solar heater for a house in Mexico and 15 years ago I used Pool solar panels for solar hihwbottom of solar panel. The only pump is to get the water up to the tank in the first place. . The house has a 750 litre hot and a 750 litre cold tank side by side with connection at the bottom so if our well pump fails water levels fall in the 2 tanks until hot water stops at about 1/2 tank level, warning us it is time to check for a problem. This still leaves us with 750 litres of water !/2 of each tank which means things like the toilet still work

    The tanks must be protected from rain to prevent immediate cooling and from sun to prevent the insulation disintegrating.

    The Pool panels were guaranteed for 10 years so are on borrowed time. Over the years we found to keep them in place from high winds it was necessary to use strapping across them. Aluminum channel from a Glass wholesaler has proven to be the best and quite affordable, I think the channel type was used to make screen doors.

    If making your own collector, I found Aluminum for a base was available inexpensive from a place that dismantles and rebuilds damaged Semi Trailers. I suggest making the collector as long as possible to reduce quantity of T fittings. Here Copper tube is in 6 meter lengths so would suggest 6, 4, or 3 meter. Aluminum bottom sheet, either no top sheet or if Glass, try to find affordable Safety Glass, Maybe Patio Doors.

    Do not worry about the water cooling in the solar panels during rain as they are below the storage tank and the water just will not circulate.

    For size calculations 15 years ago I used https://natural-resources.canada.ca/maps-tools-and-publications/tools/modelling-tools/retscreen/7465 though at that time it was a Excel spreadsheet with Macros.

    • Craig Iedema says

      Hi Ken,

      Yes, they are , or should I say were. A company called SolarHart sold many off-the-shelf units that work in the manner described above (possibly 1 million+).

      They have somewhat fallen out of favour in recent times, favouring PV powering a restive or heat pump HWS. The logic is that once the HWS has finished heating, the power can be used elsewhere.

      Is this logic sound? I’m not sure, it would be worth a blog article.

  83. Very useful analysis and discussion. Thank you one and all.
    It convinced me to rip out the still functioning 14-year-old 315l RHWS and plug in180l iStore to my panels and battery set-up. I had already switched off the Controlled Load on the RHWS. Its a scam, at 25c/kWh; so the switch to the heat pump cut around 2kWh/day on my Peak use.

    The thing that surprised me has been how quickly the heat pump re-heats after 2 showers and a bath. Its ready to go again within a couple of hours – all at about 500 watts.

    Amusingly, one of the salespeople I spoke to, asked me if I had run the volume drop (315 to 180) past my wife. Implying that the entire 2-3 people for a 180 iStore pitch was not credible and I was chancing marital friction by being cheap.

    Based on my experience, provided the 2-3 people can wait 2 hours between baths; they could have 7 baths or showers each per day with a 180-litre heat pump. The old 315 litre Controlled Load RHWS set-up would never handle that sort of use.

    Ah, tip. Adjust the tempering valve for best results.

    • Great insights – thanks for sharing. Being bold and experimenting with smart, clean tech seems the best way to find out what’s best because everyone’s requirements are different and the is so much misinformation and generalisation out there. Like people thinking they’re EV batteries will go flat. Why? Same
      concept as an ICE car but you have the convenience of home charging. Bizarre

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