Are Off Peak Hot Water Tariffs A Waste Of Time For Solar Owners?

Off peak hot water tariffs and solar power

Last week Finn got me into hot water.  He did this by referring me to a study called::

“Analysis of electricity consumption and thermal storage of domestic electric water heating systems to utilize excess PV generation”

Let me translate that from Academeze into English for you:

Are electric hot water systems any good at soaking up excess solar electricity?

Here are some of the findings:

  • Electric hot water systems average 25% of a home’s electricity use. That surprised me. It’s more than previous estimates I’d seen.
  • If heat pump systems are ignored and only conventional electric hot water systems are considered, electric hot water averages 25% of a home’s electricity use.
  • Electric hot water can act as a battery, storing energy from rooftop solar panels for later use.
  • Even when no effort is made to increase solar electricity consumption, lots of the energy consumed by electric hot water systems can come from rooftop solar power.

The last point suggests that it may be cheaper for some households with solar panels to have their hot water systems on their regular tariff:

  • even if a controlled load is available,
  • even if the home’s solar system is relatively small,
  • and even if they make no effort to control the hot water to match solar electricity generation.  

After reading the study I was shocked at how little we know about Australian hot water energy consumption.  I’m not saying we should spend a lot of money investigating this, but if the boffins at ANTSO get over $300 million a year, surely we can spare a little to look into the appliances that make up a quarter of residential energy use1.

In this article, I’ll tell you all the other interesting things I now know about Australian electric hot water, including the study’s main discoveries.

The Study Is Paywalled

Update October 26th 2021:  Paul in the comments has kindly pointed out the study can be viewed online for free.

The study is by Yildiz, Bilbao, Roberts, Heslop, Dore, Bruce2, MacGill, Egan, and Sproul.  If you want to read it, I’m afraid it’s paywalled.  If you’re not a university student, you’ll likely have to shell out money to Elsivier — an organisation that pays people who write papers nothing but charges others to read them. 

The most positive thing I can say about that is, it’s good work if you can get it. 

But if you want to read a science paper without paying for it, you can just pick whichever of the authors appears lowest on the academic pecking list based on how many letters they have after their name and send them an email saying:

“Dear [NAME], your paper [NAME OF PAPER] was recommended to me by [RANDOM NAME].  I would be very grateful if you could send me a copy of either it or an earlier draft.  I promise not to use the information for evil.” 

You’d be surprised how often that works. 

Who Did The Study Study?

The electricity consumption and solar energy generation of 410 households with electric hot water systems and rooftop solar panels were recorded over a full year in three capital cities.  Locations and number of systems were:

  • Brisbane: 250 systems
  • Sydney: 100 systems
  • Adelaide: 60 systems

The other capitals were left out.  But given they can only study so much, these were the best cities to investigate.  Homes in Melbourne and Perth often use gas hot water while Hobart only has around 225,000 people.  Darwin’s population is even less and they don’t use much hot water in the Top End. 

Information From Solar Analytics

Information for the study was provided by Solar Analytics.  They supply monitoring hardware and software for solar homes.  They can also warn you if your solar system has a problem. 

I recommend good monitoring systems as they have positive externalities.  That is, they provide benefits for people other than their owners.  For example, they make my life easier because if someone asks me for help with their solar power system, working out what’s wrong is a lot easier if they have good monitoring.

Not A Random Sample

Because owning Solar Analytics was required, the households in the study weren’t a random sample.  This means the results don’t represent average households in Brisbane, Sydney, and Adelaide.  Instead, they represent homes with Solar Analytics.

Because Solar Analytics costs money, people who have it are more likely to be well off with larger roofs, larger solar systems, and larger overall electricity consumption. 

But because it doesn’t cost that much, I don’t expect the difference to be large. 

I have no comment on whether Solar Analytics users are smarter, better looking, or smell nicer than those that don’t have it.  These may be just rumours. 

Solar Analytics installation

Australian Hot Water Consumption Is High

The study gave existing estimates for the percentage of household electricity consumed by electric hot water for Australia and overseas:

  • Australia: 23%
  • Europe: 14%
  • USA: 18%

Australia has the highest percentage even though it’s the warmest of the three regions.  This odd result is probably mostly explained by this…

Percent of people who shower once a day

(Image: The Atlantic)

Mexican males are the only people on the graph more fond of a daily shower than Aussie blokes. A fact that can be celebrated for just A$77.45 on redbubble:

There Are 2 Types Of Electric Hot Water Systems

The study looked at homes with either:

  • Conventional hot water systems with a resistive element, or
  • Heat pump hot water systems

Heat pumps are far more energy-efficient than resistive water heaters.  The most efficient heat pump I know of gives 4.6 times more heat per kilowatt-hour than a conventional hot water system3.  A mediocre heat pump may only provide twice as much heat.  

While heat pump hot water systems are a great way to save energy, they’re only a small slice of hot water systems in use and 10% of systems in the study.  But that’s more than I expected and shows they are gradually replacing conventional systems. 

Power Draw

In operation, a heat pump hot water system generally draws 1 kilowatt or less.  For conventional systems, it depends on the size of the heating element which can be:

  • 1.8 kW
  • 2.4 kW
  • 3.6 kW, or
  • 4.8 kW

As you can see below, 3.6 kW is by far the most common element size:

Percentage of hot water systems with different size elements

Some hot water systems have more than one element, but these weren’t mentioned in the study.

The larger the element size, the faster the water is heated.  However, systems with larger elements often have larger tanks4 and so have more water to heat5.

Systems with smaller elements normally use a bigger percentage of solar energy.  This is because they’re less likely to draw more power than the rooftop solar system provides.  If you are getting a conventional hot water system, I recommend getting one with a smaller element where possible.

Hot Water Energy Consumption

From the information presented in the study, I could determine the households had…

  • An average total daily electricity consumption of 24 kilowatt-hours. 
  • Average daily solar energy generation of 19 kilowatt-hours.

Total electricity consumption was well above the 14 kilowatt-hours I would expect for randomly selected homes with electric hot water systems. 

Solar energy generation indicates the average rooftop solar power capacity studied is a little over 5 kilowatts.  While not large for a new system today, it is roughly what I’d expect for a random sample of solar homes. 

One thing the study clearly states is…

  • The electric hot water systems consumed an average of 6 kilowatt-hours per day.

This means electric hot water systems consumed an average of 25% of total electricity use.  This is more than any past estimate I’ve seen.  If only homes with conventional electric resistance heaters are considered it’s even higher.  If we assume heat pump hot water systems use one-third as much electrical energy, then homes with conventional systems use an average of 6.4 kilowatt-hours a day, which is 27% of total household electricity consumption. 

My guess is the percentage is high because the overall efficiency of other appliances in the home has improved more than the efficiency of electric hot water systems. 

Litres Of Hot Water Used

The amount of hot water used was not directly measured but was estimated to average 142 litres a day.  The averages for the three cities were:

  • Brisbane: 130 litres
  • Sydney: 147 litres
  • Adelaide: 153 litres

As expected, hot water use was higher in winter and lower in summer, with maximum consumption around 50% higher than the minimum.  No significant difference was found between days of the week.

Regional Variation In Energy Consumption

Hot water systems consume more energy in colder locations, for three reasons:

  • Water enters the home at a lower temperature in cooler regions.
  • Storage tanks lose heat faster when it’s cold.
  • But the main reason is people use more hot water when it’s cold.

Brisbane hot water systems used the least energy, while those in Sydney and Adelaide used similar amounts.  The daily averages were:

  • Brisbane: 5.6 kilowatt-hours
  • Sydney: 6.3 kilowatt-hours
  • Adelaide: 6.4 kilowatt-hours

Controlled & Uncontrolled Hot Water

There are two ways to run a hot water system:

  • Controlled — Something other than the tank’s thermostat determines when the hot water system turns on and off6.
  • Uncontrolled — Only the thermostat turns the element on and off.

Hot water systems can be controlled in a number of ways:

  • A system on a controlled load receives grid power at a lower cost at times set by grid operators.  Controlled loads may be called economy tariffs or off-peak hot water.
  • A timer7.
  • A relay that switches the hot water system on when the right conditions are met such as sufficient surplus solar generation.
  • A switch installed next to the hot water system that lets you turn it on and off manually — but only a nutter would do this8.  (Yes, I happen to have one of these switches…)
  • Use a Solar PV hot water diverter to send surplus solar power generation to a conventional electric hot water system. 

If a hot water system is on a controlled load, it won’t use any energy from your rooftop solar system — at least not as far as your electricity bill is concerned.  The other control methods — timers, relays, and PV hot water diverters — are effective ways to increase the amount of solar energy used. 

Western Australia is phasing out controlled loads and it is possible other states will do the same.  When low priced controlled load tariffs aren’t available it increases the benefit from having a different method of control that increase the amount of solar energy used by hot water systems. 

But even if a hot water system is uncontrolled, it’s still capable of getting most of its energy from solar, provided the solar power system is large enough and hot water consumption patterns are suitable. 

Hot Water Electricity Consumption Patterns

There’s considerable variation in when hot water systems consume energy.  It depends on hot water use and whether the systems are controlled or uncontrolled.  Six main patterns of hot water consumption were identified:

  • Morning and evening only
  • Morning and evening with daytime
  • Evenly distributed
  • Morning
  • Evening
  • Late night

Not all of a household’s hot water use occurs during the time period its consumption pattern is named after.  That just indicates when it mostly occurs.

In many homes, most hot water use occurs in the morning, less in the evening.  This means an uncontrolled hot water system will switch on in the morning at a time it can take advantage of solar generation.  It will also switch on in the evening when solar energy isn’t available, but its energy consumption won’t be as high.  This can result in lots of its energy coming from solar panels.  The amount will depend on:

  • The solar system size — the bigger the better as the more solar power is available the less likely grid power will need to be used.
  • The hot water system’s element size– here the smaller the better.  The lower the power draw the more likely there will be enough surplus solar power.  Because heat pump hot water systems have the lowest power draw, this makes them the best for maximizing the amount of solar energy used to heat water. 

If hot water use and/or hot water system energy consumption mostly or entirely occurs when the sun isn’t in the sky, then little to no solar energy will be used.  But on the bright side, if you have an uncontrolled hot water system, it’s not difficult to work out what sort of household you live in.  It mostly comes down to when the most showers are taken. 

Solar Energy Consumption Simulation

Information on the 410 households in the study was used to create simulations of how much solar energy hot water systems would consume.  Because these were simulations and not direct observations, you may wish to take any conclusions with a grain of virtual reality salt.

But because the simulations are based on real-life results they should still provide useful information.  I presume the authors of the study were very careful and diligent with their work.  Just don’t ask me to check the results9.

In a simulation, a home with the following characteristics was used:

  • Hot water consumption mostly occurs in the morning.
  • There is an uncontrolled conventional hot water system with a power draw of 3.6 kilowatts — the most common element size.
  • There is a 3.6 kilowatt solar power system.  This is tiny by today’s standards. 

This resulted in 48% of the hot water energy coming from solar.  An impressive amount for such a small amount of solar capacity.  The graph below shows the average kilowatt-hour of solar energy used per day by the hot water system in each season:

Average daily solar electricity used by hot water

While most solar energy is available in summer, hot water use is less and this resulted in summer having low average solar energy use.  It was higher in winter, despite lower solar output, because more hot water was used.  It was highest in spring thanks to good solar electricity output combined with high hot water use thanks to spring being colder than autumn.

The amount of solar energy used heavily depends on the household’s hot water consumption pattern.  If it’s not suitable it can be greatly reduced.  The graph below shows what happens if the household has an evening hot water consumption pattern:

Average daily solar electricity used by hot water - morning vs evening users

In this situation most of the hot water system’s energy use occurs at night, but some hot water consumption and energy use still occurs during the day allowing the rooftop solar system to contribute.  For this consumption pattern 28% of the energy used by the hot water system is solar.

A Queensland Example

Provided an electric hot water system uses a sufficient amount of rooftop solar energy, it can be cost-effective for it to be taken off a controlled load even if no attempt is made to control when it turns on.

My parents are in regional Queensland and currently they…

  • Pay 14.3 cents per kilowatt-hour for electricity consumed by their hot water system on a controlled load (tariff 31).
  • Pay 21.8 cents per kilowatt-hour for grid electricity (tariff 11).
  • Receive a 6.6 cent solar feed-in tariff for the energy they export to the grid.
  • Have a hot water system that consumes an average of 2.7 kilowatt-hours a day.

At the moment, all energy consumption by their hot water system comes from their controlled load and costs them an average of 38.6 cents per day10.  If their hot water system was taken off the controlled load and half the energy it consumed came from rooftop solar panels and half from the grid at a cost of 21.8 cents per kilowatt-hour, they would then effectively pay an average of 38.3 cents per day for hot water.  A saving of 0.3 cents.  This adds up to over $1 a year.

But that’s not all.  In Queensland, homes have to pay a metering charge.  Not having to pay this for a controlled load meter will save them 3.3 cents per day.  All up, this would save my parents around $13 a year.  Despite this windfall, my parents probably aren’t going to be in any hurry to call an electrician to remove their hot water system’s controlled load. 

But if someone in Queensland was wondering if they should put their hot water system on a controlled load I’d tell them not to bother if they have solar panels.

My Conclusions

While the study was interesting, it hasn’t changed my advice on how to get cheap hot water.  Just follow these steps:

  1. First, fill your roof with well-installed solar panels.  As much as will reasonably fit and your budget allows.
  2. If you can afford it, get a reliable and efficient heat pump hot water system.
  3. If you get a conventional electric hot water system, try to get one with a smaller element.
  4. Put your hot water system on a timer to maximize its solar electricity consumption.  Alternatively — especially if your hot water consumption is high — consider using a PV hot water diverter or a relay.
  5. Enjoy cheap hot water. 


  1. While I’m sure studying ants is important, I’m certain they’ll still be around after we’re gone, which may not be long if we don’t clean up our energy act.
  2. Clearly an Australian academic
  3. Note sometimes the efficiency of heat pump hot water systems are exaggerated as only the efficiency of the heat pump is given without taking into account other energy consumption by the unit.
  4. The most common tank size is 315 litres.
  5.  But because hot water rises to the top of the storage tank where the outlet is, a larger element generally means you don’t have to wait as long until you can have a shower if you run out of hot water, regardless of how large the tank is.
  6. Although the thermostat will always cut the power if the water gets to temperature
  7. It’s well past time all hot water systems came with one of these built-in.
  8. Note water in a storage tank has to regularly reach at least 60 degrees to stop legionnaires disease, so a nutter who isn’t careful could wind up nibbled on by the parrot of death.
  9. I studied statistics so long ago if we wanted to do a T-test, we first had to invade Ceylon to get some.
  10. My mum says it’s worth it.
About Ronald Brakels

Joining SolarQuotes in 2015, Ronald has a knack for reading those tediously long documents put out by solar manufacturers and translating their contents into something consumers might find interesting. Master of heavily researched deep-dive blog posts, his relentless consumer advocacy has ruffled more than a few manufacturer's feathers over the years. Read Ronald's full bio.


  1. Lionel van den Berg says

    What about using solar hot water and only using the booster during solar PV producing time periods?

  2. Any idea on how this works for a tankless (instant) water heater vs a gas heater? Is it worth having a gas connection to a house only for a water heater?

  3. Doug Foskey says

    I have mentioned using solar pv to heat water in previous discussions.

    I built 2 units in Lismore, where the landlord pays for power & covers the cost in the rent. (It is private metered to ensure ´reasonable´ consumption that is detailed in the lease.)
    I have found that the most economical way to run hot water is to use a time switch to turn the load on about 10am, then off at 2pm. It is important to use a contactor to switch the load because the timer contacts are not man enough to handle the slight reactive load. I fitted 2400w elements to the hws.
    For a new house design, one could consider using smaller tanks really close to the water usage point to minimise water heat losses. You can even have a tiny tank under the sink, fed by a larger hw tank near the bathroom. this reduces heat loss in the lines. (The little tank is also heated by PV).
    I am a firm believer in resistance heated hot water, using a slightly larger PV system. Usually about 1kw equivalent of PV is ´used´ by the hot water heating, & an extra 1Kw on a solar PV is less costly that a ´wet´ solar powered hot water system compared to cheap storage hw tanks. Also the installation costs in a new build are much less.

    The occasional time that solar pv does not supply enough energy to heat the water are few. the occasional day one may need to pay for say 5kw of peak power are few.

    Personally, I do not like Heat pump HWS. THe running cost savings do not in my estimation offset the extra purchase costs, plus the additional complexity requiring maintenance. Storage HW is cheap & reliable. I do not consider the ´smart´ switches are value either. (abt $700 retail + installation)

    Timeswitches: Ensure that the time switch is battery backed, & definitely use a contactor to switch the load. (As an energy coach with Enova I have seen timeswitches with welded contacts causing unbelievably high bills, & time switches that have not been reset to standard time after a power outage, hence the recommend for a battery backed clock.)
    Remember too, if the HW needs a boost, one only needs to press the override button on the timeswitch.

    • Michael Paine says

      Re “You can even have a tiny tank under the sink, fed by a larger hw tank near the bathroom. this reduces heat loss in the lines. (The little tank is also heated by PV).”
      That gives me another idea for boosting a conventional (controlled) HWS with surplus PV – add a modest size conventional HWS on the supply side of the controlled HWS and have that extra HWS on a timer so that it is operating when excess solar is likely to be available. That way the controlled HWS receives pre-heated water and so uses less power when it comes on at night.
      This would be similar to the dual element HWS that I mentioned below (above?).
      This is all about storing PV energy at home and avoiding feeding into the grid at low rates or, worse still, being charged for the privilege (since our governments have failed to plan for storing “excess” solar themselves).

    • Got a catch diverter on my system in vic , haven’t had any issues 2 years since stopped using off peak, 3-6 kw element 350 litre hws no cold showers no going back.

    • Alan Dobell says

      A HWS element has negligible reactance and is purely resistive . The only reason time lock contacts fail is that they are not rated correctly for current. Nothing to do with reactive load.

  4. Great one Ronald 🙂 I’ve Got an iSTORE 180L for the Last 3 Months have not paid one cent for Hot Water, They are Classed as Solar so you do get a Rebate on the STC’s and that also helps with the Price of the Unit, My 180L only uses about 450Watts per Day, there is only two of us in the House!
    a 270L uses 1KW to recover 100L so great maths on that size as well 🙂
    I Agree Solar Hot Water Systems on the Rooves of Houses, if not prepared for it, Are over Rated. And Costs are Overboard.

    Thanks for a Great Article, Sharing this all over my FB Page right away 🙂

  5. We have a good solar hot water system in Brisbane- only two of us in the home now, and haven’t needed to boost in at least eighteen months or more.
    Even when we had a tribe (4 kids) living here, the solar hot water didn’t need much boosting.
    And yes- we were very strict on shower times AND had a very water efficient shower head, which is a no brainer, saving both total water and hot water.

  6. Thanks Ronald, nice article.

    I am curious about the desire by governments(?) to remove controlled loads. It seems to me controlled load (at least that managed via ripple controller or smart switch) represents a great opportunity for periods when the grid needs to add load. A grid stability option of sorts, particularly for those mild sunny days.

    Hot water is an interesting optimisation challenge and an example where the optimal solution varies for each individual household situation. Being wired the way I am I looked into it with forensic precision given I have years of 5-min energy interval data to play with (private smart meters for the win).

    We have 11kW solar PV and also 3.6kW resistive element 315 litre hot water system on a (ripple current) controlled load (5kWh/day). Regional NSW. All electric home.

    To make worthwhile the expense of removing the controlled load circuit and having a timer relay switch installed instead, I need FIT to be 7c/kWh lower than controlled load tariff to break even within a reasonable time frame. That includes accounting for the reduction in daily fee for the CL.

    The model tells me with a simple timer relay switch and optimising the start time I can achieve the following proportion of our HW energy from solar PV, based on heating element size:

    3.6kW: 70.4%
    2.4kW: 76.0%
    1.8kW: 78.1%

    I can squeeze another % or two with a programmable start timer or via the solar excess relay controller built into my inverter.

    With a smart diverter (like Ohm Pilot or Catch) I can get to 85.3% powered by solar PV, so these devices, while clever, represent pretty poor value for us compared with basic relays/timers.

    Keep up the good work.

  7. Thanks for a clear and informative explanation of the topic, Ronald.

    To my mind the most important reason for getting out of controlled load / off peak water heating is not cost savings, but because it always uses fossil fuel generated power. The rationale for offering off-peak rates is so that coal-fired power stations, which are slow to shut down and start up, can be kept running all the time.

    My question: I am trying to choose between (1) getting a solar diverter from my 6.5 kW PV system to my 37 year old solar thermal HWS with electric (off peak backup) or (2) ditching the solar thermal and just getting a new heat pump system. I’m in Melbourne.

    Any suggestions?

    • christopher says

      I installed Catch power diverter and stopped using controlled load, 9.8 kw panels 315 litre hws 3.6 element, works a treat been operating since may bearing in mind export 5kw limited feed in. Cheap solution, only paid $350 for tank 18 months old (house demolition) have had few usually get 10 years out of them. How do you get such long life from yours, must be very high quality?Chris L. Lara victoria.

  8. Ian Hammond says

    When I installed solar panels my, hot water system was a conventional hot water system with an element on controlled load.

    My solar feed-in tariff is actually slightly higher than my off peak hot water cost so I didn’t see any need to make a change to my hot water system.

    If solar feed in tariffs drop or off peak hot water prices increase then maybe that would prompt me to look at it again but the savings would have to be significant enough to go to the effort of a change.

    In my local area I am expecting Endeavour energy to start switching some of the off peak electricity to times when there is excess solar energy being produced. Endeavour energy are working with Intellihub to get smart meters to do this. When that goes ahead it may mean that much of the electricity for my off peak hot water system will be coming from my own solar panels in the middle of the day and I may neve see the need to change from off peak hot water to some other system.

    • Michael Paine says

      Electricity providers switching on my hot water heater when there is excess solar on the grid would be useful, but only if they didn’t charge me more than the lousy feed-in tariff.

      When I replaced my hot water tank (direct heating) a few years ago it was wired to the supply-controlled circuit like the previous one. I have no control over when that is heating the water (tariff is 13c). I have even found on some very cold days in Sydney that the controlled circuit was not on long enough to produce a reasonable amount of hot water for the day.

      However I should have paid a little extra for a dual element hot water system. That would have allowed me to control when it is heating – like when I am generating excess solar.

      • “I have even found on some very cold days in Sydney that the controlled circuit was not on long enough to produce a reasonable amount of hot water for the day.”

        Controlled loads are required to be on for a minimum number of hours each day. In Ausgrid area CL1 is required to be on for a minimum of 5 hours/day, and available for up to 9 hours/day, and longer on weekends.

        If your water heater is unable to heat enough water in that time then either there was a grid power outage on that day or there is something wrong with your set up.

        5 hours x 3.6kW element = 18kWh, which should be ample energy to reheat even a 315 litre tank.

  9. To let someone make a helpful decision, all info must be considered.

    With regards to hot water, I think we have some info missing.

    We need TCO which means total cost of ownership. Therefore we must add how much the hot water service costs and long it lasts. This is significant and cannot be omitted because it could totally change the choice. A good start would be the warranty length in the absence of end of life info.

    • Peter Fuller says

      Hi Peter you make a good point and one that was high on my list when we had to replace our hot water heater about 3 years ago. We have 13kw of solar PV on 3 phase and we have CL1 in NSW (off-peak). At the time I found that heat pump hot water systems where 3 times the price of a standard resistive heating element water tank of similar size, heat pumps are noisy but the main issue for was heat pumps are much more complex and almost impossible to get fixed if they fail and they do. Also there is the sunk energy and materials in the manufacture of the system compared to a simple tank with element. At the time we had a feed in tariff of 9.5c/kwh and the cost of CL1 was exactly the same so i figured we just use the grid as a time shift ‘battery’. The bigger the difference in FiT and CL1 rate the bigger the incentive not to use CL1 and explore other options

      • Viv Donovan says

        I am a aged pensioner who turns off power when not in use. I have solar panels on roof at retirement village and no idea what my water heater is. Am I doing any harm by turning my hot water switch off during the day and turning it on at night late when I go to bed. I shower in the morning and wash the dishes once a day at night. My bill seems a bit cheaper than my neighbours.

        • Ronald Brakels says

          Hi Viv, Ronald here.

          You could be putting yourself at risk of serious harm if you expose yourself to hot water that hasn’t been heated all the way to 60 degrees. This is because legionella bacteria can grow in lukewarm water and cause serious health problems. The risk of infection increases as you get older. The bacteria die quickly at 60 degrees, which is the temperature hot water heaters are set to, but if you turn off your hot water system late at night they could potentially multiply through the day and if you start using hot water in the evening before it reaches 60 degrees you could be at risk. If it has time to reach 60 degrees then I presume the water will be safe, but note I am not a medical professional. I think the safest course is to leave the hot water system on all the time. This will result in a little more electricity consumption, but I’m sure it’s worth it.

          UPDATE: If your hot water system is left turned on overnight, then it will hit 60 degrees for an extended time so you are likely to be safe. But note I am neither a doctor or a plumber, so this is not professional advice.

          • Ronald, I see no difference between Viv’s mode of operation and how single-element controlled load hot water storage systems operate, with a once per night heat cycle. We have millions of those in operation with little risk of legionella.

            An overnight power supply window will easily get the water hot enough, and provided the tank is large enough (for single pensioner even a modest 125 litre tank is large) then it will provide ample and safe hot water for the next morning, day and evening right through until the next heating cycle.

            Legionella is not a risk when used in this manner.

            For an aged pensioner I’d say they will barely need more than 3 kWh/day for a resistive element water heater (with half of that being daily heat losses), and even a low power element is going to do that in less than two hours. Maybe 4 kWh in colder climates.

            The natural heat stratification of water in a tank is a wonderful thing and is one of the basic physics reasons why we can run hot water systems in this manner. The water at the top of the tank where it is drawn from will remain very hot and safe.

            That said, Viv may be better to heat the water during the day from their solar PV but they will need to ensure the water heater cannot operate during peak tariff hours unless manually over ridden (e.g. via a boost option).

            Much depends on how the metering works in their retirement village and what sort of electricity tariff regime they operate with. Pray they are not stuck inside an embedded network.

          • Ronald Brakels says

            Hi Alex

            From Viv’s description it sounds like the hot water system is left on overnight, which guarantees it will hit 60 degrees, and so – as you say – it should be safe. I was concerned that, like some people, she may only be turning it on around the time hot water is required to save money or to be kind to the environment, which can be a risk.

            I’ll update my previous comment.

  10. An interesting discussion and it’s horses for courses. In your Queensland example you make an assumption that your solar panels have the excess capacity to provide half the consumption of hot water usage. In my case the excess solar (solar buy back) would only provide less than one third of my hot water usage (Hunter Valley NSW). It would cost meover $100 a quarter to change from my night dedicated circuit charge to day grid costs plus solar. Unfortunately a) I don’t live in QLD and b) I have an old solar system that produces less per Kw than new systems.

  11. Isn’t there a situation where a household has say a 9kW solar system but because that can only inject 5kW into a single phase for export they might as well burn the excess heating water without the expense of a reverse cycle water heater

    • Dominic Wild says

      I am wondering if your idea can be put into place via a three-phase inverter and dedicating one phase to the H/W heater? If you place more than 6.6kW or max. of 5kW export on your roof, you lose your feed-in tariff, you may as well use it all.

      • No. 3-phase inverters require a correctly synchronised grid frequency signal to all three phases to be able to operate. IOW you can only install them where there is a 3-phase supply.

      • > If you place more than 6.6kW or max. of 5kW export on your roof, you lose your feed-in tariff, you may as well use it all.

        In Queensland you can have a system more than 5kW in size as long as the inverter is export-limited to 5kW (single phase).

        If you have 10kW of panels with a 8kW inverter you have ~3kW of unused capacity in the middle of the day which is wasted unless you self-consume it.

        A HWS on a timer which starts at 10am would only cost a few hundred watts of lost exports, the rest would come from the increased production.

    • Martin Holden says

      I have a larger domestic solar power system on my house that has been there for a few years now, it took a lot of research to find a power diverter that would sell my maximum of 5Kw to the grid and then put what if anything was left over into my HWS, it has to be a factory special order to be able to do this but it works great now, if you dont special order it will work like any other diverter and put all the power into the HWS until it is hot before selling anything to the grid, now I sell my full 5Kw to grid and it still gives me free hot water from power that I would otherwise thrown away, so it is a win win for me but you do need to have enough panels to be able to sell 5Kw as well as put some power into the HWS for it to work for you.
      They are made by a company called Power Diverter Australia and are called a Powerdiverter, they are easy to Google if you want to contact them.

    • Martin Holden says

      Hi Mark,

      I have the system that you are asking about which also works well for me so far about 4 years now, it is a Power Diverter Australia diverter, it has to be specially ordered to do what you want it to do when you order it though, it will sell 5 kw to grid and put whatever is left into your Hot Water System, it also has a grid connection to heat up the water on rainy days so it works well for me, maybe try to Google Power Diverter Australia and see if it will do what you want it to do??????

  12. Mark Symonds says

    Im in Brisbane with a 6KW SMA inverter, with SMA energy meter.
    In your Queensland example, surely if your going to the expense to have an electrician to remove the hotwater from the controlled system you would also pay to have an electronic timer and contactor installed?
    Most days in qld I pay zero for hotwater because I have it set to come on at 12pm, for up to 4hrs. On average its on for 45mins. It draws approx 4kw, and being thats its a 12pm, which is the peak for solar around here, my system has already flatlined at 6kw output. So after the 1kw the pool is drawing, I still have 1kw left.
    For free monitoring I definitely recommend “Sunny Portal” thats included free when you install a SMA energy meter. So easy to see what you are using from solar, using from grid, overall production and feedin.
    Not sure why anyone would actually pay for service that does this.
    I get notified by SMA if I have a fault and I get a daily production email.

  13. Soccerstar says

    I’m on AGL Solar Savers. I pay 10.78 c/kWh controlled load 2 and get 12c/kWh FIT, so my solar is essentially heating my hot water (indirectly) and cost is about the same. Don’t think I’d change anything to save a few dollars.

    • Well in this instance you would lose money if you switched your hot water heating to solar, as you would be forgoing the feed in tariff to heat your unit.

      • The respective tariffs for CL and FIT are not the only consideration though.

        For example, in our area controlled load also incurs an extra daily service fee of between 12-15c/day (depends on retailer but the underlying distributor fee is ~11c/kWh). For us this effectively adds another 2-3c/kWh to the CL tariff.

        On the flip side there is the capex required to make the change (electrician to remove the CL equipment and install an alternative HW circuit controller), and that can be quite a chunk of change and very hard to recover if the amount saved each year is small.

        I calculated I need my CL tariff to be at least 7c/kWh more than FIT before the cost of removing the CL and replacing with a simple programmable timer switch was worthwhile. In our case expensive solar PV diverters won’t ever make financial sense for us, the capex can never be returned within their expected lifespan.

  14. I have a regular “resistive element” hot water system. Not sure of size – I think it may be 3.6kW. I have a timer in the fuse box which comes on at 11am and goes off at 2pm every day to make sure it runs off the solar. The water is still hot for morning showers and whatever else we need, even in winter. It seems pointless letting the supplier control the system when I can do it myself cheaper – free.

  15. Dominic Wild says

    Plumbers in WA install only cold water for dishwashers (Bosch) and the Bosch washing machine only allows cold water, but plumber installed a hot tap there too. Question: Have the bean counters at Bosch decided to save a few cents by using cheaper plastics in valve bodies, pipes, etc? Whilst the first lot of water comes in cold and could be used for a quick rinse, a lot of money is wasted heating the later water once you have solar H/W.

    The Fronius Symo tells me the production on my mobile and I decide to switch on the H/W system via WiFi relay and eWelink app on mobile when I have 2,400W spare in the home, but clouds are a problem.

    With the Fronius and its standard Data Manager board and software it is possible to drive a 12V relay coil for x minutes when y Watts are produced or more for a total of z hours, etc. All it needs is an installation of a relay and feeding the switch board from I/O1 and GND with the 12V DC from the push-down block of the Data Manager. That gets rid of the cloud problem and the varying output. I decide what is running in the house and the Fronius tells me the production, but not the surplus.

    • Modern washing machines and dishwashers have been heating their own water for a long time, this is normal. It’s also efficient as the machine only heats the water it actually uses and to the temperature it requires, and is doing so in situ so there are no transport losses of hot water in the plumbing. My Dishwasher uses ~7 litres per cycle. I’d lose at least that much in pipe transport losses alone if the unit had to wait for HW to arrive from the HW tank.

      Temperature control also matters. This is especially important in dishwashers where the water needs to be very hot for hygiene reasons, while a plumbed HW supply is typically not hot enough.

      Given the above, the same rule applies to these appliances as with many others – run them when there is ample solar PV available.

  16. Just a few points:

    As far as I know, power companies don’t adjust their time-of-use times for daylight saving time. So be careful to allow for that when you setup your timers.
    Also, when there is not enough solar energy through the day to get the water up to 60 degrees, watchout for the Legionella bacteria.
    That also applies if you are using a timer to switch the HWS on at off-peak times.

  17. Well, that does not add up. Half a tank, 150l water daily used. Alright, heating 150l of water from 15C to 90C (75 up) takes 13.1kWh. On top of that 300l tank up to standard would disperse another 3.5kWh daily. Say, 16kWh daily, not 6 or 7.

    • Ronald Brakels says

      I get a different result, but the the kilowatt-hour consumption does seem off the for litres of hot water used. For Adelaide the mean temperature is about 16 degrees so the water will need be be raised by about 44 degrees. To raise one litre of water 1 degree requires 4,184 joules so getting 153 litres to 60 degrees would require 28 megajoules which is 7.8 kilowatt-hours. The heat pump hot water systems that are part of the sample will reduce that, but it will still be over the 6.4 kilowatt-hour daily average they give for Adelaide. Especially since there will be some heat losses while the water is sitting in the tank. So well spotted that the study’s figure doesn’t seem quite right. Since they could only infer the amount of hot water use I guess that’s where the problem is — unless there’s something I’m overlooking.

    • Is it 153 litres of water from the HW tank, or 153 litres of tempered hot water supply?

      The water exiting the HW tank is blended with cold water by the tempering valve to keep the tap supply hot water outlet to no more than the regulatory 50C limit.

      If so then the 153 litres will be made up of hot water from the HW tank and cold water from the cold water supply and the relevant effective energy consumption calculation would be for raising 153 litres from 16C to 50C (or whatever the temperature the valve is regulation to).

      The actual temp at the tempering valve might be a little higher than 50C to allow the water at the tap to be at the 50C limit.

      But for reference, heating 153 litres of water from 16C to 50C = 6.05kWh. If the tempering valve is set to 52C that’s 6.41kWh.

      • Well, the text says all about tank so I believe it is half a tank, not 150l of tap water which can be also mixed further down on tap itself. 60C is minimum allowed but while heat pumps keep just above that, I saw electric heaters set 80-90C as it makes them storing more heat in the same tank size at heat dispersion cost. BTW – for 315l tank maximum dispersion loss allowed would be just below 3kWh/d, however, for modern and as new condition. Otherwise 3.5-4kWh. Whatever makes sense in this calculation, it never gets below 10kWh/d

        As you mentioned that is not even clear what sort of water discharge they mean and that puts it even further from scientific paper. It sounds like one of our bold TV experts in every field just making hist world changing discovery that if he walks longer along supermarket aisle, he may find same good cheaper.

        • It would be very unusual for a large HW storage tank in Australia to be set to heat water higher than 70C. Most heat to between 60-65C.

          As for standing losses and daily consumption not sure where you are getting your data from. We have a 315 litre electric resistance hot water tank and our average HW energy consumption this last 12 months has been 4.9kWh/day. Standing losses represent about half that.

          If you do set the thermostat very high then you can expect standing losses to increase as the rate of heat loss is driven by the temperature differential.

    • Geoff Miell says

      You state: “Alright, heating 150l of water from 15C to 90C (75 up) takes 13.1kWh.

      Per UK Health and Safety Executive:

      The primary method used to control the risk from Legionella is water temperature control. Water services should be operated at temperatures that prevent Legionella growth:

      * Hot water storage cylinders (calorifiers) should store water at 60°C or higher
      * Hot water should be distributed at 50°C or higher (thermostatic mixer valves need to be fitted as close as possible to outlets, where a scald risk is identified).
      * Cold water should be stored and distributed below 20°C.

      It is necessary to keep warm water at a temperature of 55–60 °C to inhibit the growth of legionella bacteria. Water at 60 °C (140 °F) can induce scalding injuries in less than 3 seconds, while it takes 10 seconds to get an injury at 57 °C (135 °F) and 1.5 to 2 minutes in 52 °C (126 °F) hot water.

      I’d suggest heating water above 65 °C for domestic purposes is entirely unnecessary and potentially dangerous.

      Heating 150 litres of cold water from 15 °C to 65 °C requires 8.72 kWh. A 3.6 kW output water heater would take 2 hours 26 minutes (146 minutes).

      Heating 150 litres of cold water from 5 °C to 65 °C requires 10.47 kWh. A 3.6 kW output water heater would take 2 hours 55 minutes (175 minutes).

      • Michael Paine says

        Re “I’d suggest heating water above 65 °C for domestic purposes is entirely unnecessary and potentially dangerous…”
        New hot water installations must now have a tempering valve that adds cold water to the hot outlet if the temperature exceeds roughly 55C. This means that, in theory, excess solar can be used to heat the water to well over 65C and so store more energy for later use. This assumes that the tank and equipment can take the higher temperatures (effect on warranties and longevity?) and that the tempering valve operates as intended. Not something for DIY but there could be a commercial market for this approach.

        • Geoff Miell says

          Michael Paine,
          You state: “This means that, in theory, excess solar can be used to heat the water to well over 65C and so store more energy for later use.

          Alex above stated earlier (Oct 26): “If you do set the thermostat very high then you can expect standing losses to increase as the rate of heat loss is driven by the temperature differential.” I’d suggest that’s wasting energy unnecessarily.

          You state: “This assumes that the tank and equipment can take the higher temperatures (effect on warranties and longevity?)…

          I’d suggest cycling over a higher thermal range increases stresses on the tank vessel seams, ports and fittings that could reduce longevity.

          You state: “… and that the tempering valve operates as intended.

          Per Clause 1.9.2, of AS/NZS 3500.4, the outlet temperature of tempering (or thermostatic mixing) valves must not exceed:
          * 43.5 °C for childhood centres, primary and secondary schools and nursing homes or similar facilities for the young, aged, sick or people with disabilities; and
          * 50 °C in all other buildings.

          Tempering valves are mechanical devices that can fail. Some suggest tempering valves should be replaced every 5-8 years.

          If the tempering valve fails to a state that allows insufficient tempering (or fluctuating tempering), permitting hot water at well above 60 °C to reach the point-of-use hot water taps, then there’s a high risk of near instant full thickness scalding. Why take the risk?

  18. Randy WESTER says

    “Australian Hot Water Consumption Is High” Only compared to the amount of electricity used to heat buildings in those cold climates.

    (We use about 100 to 110 GJ of gas per year in Alberta, but seldom use as much as 1.5 in June, so probably 90% of our heating energy is not for hot water. We’d use electricity, but 85% of it comes from gas and coal anyway. We’d use air ti air heat pumps, but the C.O.P. in winter is low, too)

  19. Geoff Miell says

    You state: “Heat pumps are far more energy-efficient than resistive water heaters. The most efficient heat pump I know of gives 4.6 times more heat per kilowatt-hour than a conventional hot water system[3]

    The Coefficient of Performance (CoP) for heat pumps is always dependent on the ambient operating conditions.

    Sanden states in its promotional information:

    In fact, the Sanden Eco® Plus Hot Water Heat Pump System consumes 0.84kW^^ of electricity to generate 4.99kW^^ of heat (COP = 5.96^), which equates to 20% of the energy used by a conventional electric system. Consequently, with the Sanden Eco® Plus Hot Water Heat Pump System, you can save up to 80%* of your conventional electric hot water energy costs!

    But this claim is dependent on the specified operating conditions:

    ^^ at 32.45 ºC ambient / 18.74 ºC cold water inlet

    And from the Sanden Eco® Plus brochure (Nov 2018), the:
    * Ambient Air Operating Temperature range is -10 ºC to +42 ºC;
    * Refrigerant is CO2 (R744);
    * Hot Water Outlet Temperature Setting (Nominal) is 63 ºC.

    The rating plate on my Sanden Eco® Plus includes:
    * Model: GAU-A45HPC
    * MAX Capacity: 6.0 kW
    * MAX Rated Power Input: 2.3 kW

    As the ambient air temperature (and water inlet temperature) reduces the CoP will reduce. I’ve been advised by a technical representative during a phone conversation that for Sanden heat pump units, at 0 ºC ambient air temperature, the CoP reduces to around 2.76.

    The Sanden heat pump unit’s compressor, fan and water pump are all variable speed inverter driven. I presume the rated 2.3 kW electrical power draw occurs only at the lower end of the ambient air temperature range and draws substantially less at the higher end.

    If the heat pump is programed to operate around the middle of the day to take advantage of energy from an available solar-PV system, then the ambient air temperatures in nearly all of Australia would be above 0 ºC, which I’d suggest per the data I see, is still much more energy efficient compared with electric resistive hot water heaters (and gas).

  20. In QLD, if you want to save the metering charge which is 3 cent a day, Energex will charge you roughly $130 to remove the controlled load meter 🙂

    • That seemed to be the ONLY thing Ergon didn’t charge us for. They have a monopoly up here in FNQ and charged us over $1,100 for an ‘overhead service upgrade’ for solar, which took 2 guys about an hour. This was on top of the electrician’s $1,500 to actually upgrade the meter box. A $5,500 6kw system ended up costing $9,000. It is pumping out over 30kw per day at the moment though 🙂

  21. Very useful, thanks! We are in SA and have just been told that SAPN have removed Controlled Load as an option for us and households are being switched to TOU for all meters. Timely info for us as I’ve been scratching my head as to what to do about our hot water now!

  22. Frank Perry says

    I live in Rockhampton, Central Qld and have a 6.6kw Solar edge system with 3.6 of that facing n.e, and 3.0 facing s.e.. My best generating day has been 42kwh and the average daily generation over the last year has been 26kwh.
    Prior to installing solar I had off peak electric hws, the solar installer convinced me to do away with the off peak and he put a timer on my hws so it heats between 11.00 am and 1.30 pm only.
    We are a household of 2 adults and have never run out of hot water despite showering more than once a day and using hot water in the washing machine

  23. Hi Ronald,

    I thought I’d let you know that many Australian universities have a policy of Open Access to academic research and papers. UNSW is no exception. While a paper may be published by a “paywalled” venue, that often is not the only place the paper is published and a simple search of the author’s affiliated institution will provide leads. UNSW’s policy is at and the particular paper you refer to appears to be openly available at
    The days of locked down “paywalled” publications may be numbered.

    • Ronald Brakels says

      Thank you very much for that, Paul. I’ve updated the article with the link. (Now I just have to hope Finn doesn’t want his $50 back…)

  24. Mark Gibson says


    I noticed the para about “of the other Capitals …”

    The other capitals were left out. But given they can only study so much, these were the best cities to investigate. Homes in Melbourne and Perth often use gas hot water while Hobart only has around 225,000 people. Darwin’s population is even less and they don’t use much hot water in the Top End.

    I’m a little surprised that you left of THE Capital – Canberra?!

    Happy Solar panel / Battery / Heat pump user in the nation’s capital.

    • Sandy Brucesmith PhD Elect Eng. says

      About 3 years ago I noticed the cost the electricity for heating HW even with the controlled load.
      I changed the element to 1800 W and connected into the regular house power and added a timer to heat between 10AM and 1 PM each day.
      Our solar use is the lowest during this period.
      My system is 6KW.
      From the next bill the savings were significiant.
      If for any reason such as laundry the power switch is just turned off for the period.
      We have never had no HW with a family of 2.
      Usual bill is $60 pm and considering the ducted air runs all daytime on many occasions I am quite pleased.
      The biggest bug are the fixed line charges that are more than energy used plus the solar power is automatically disconnected during the day if there is a line fault. I understand the reason however a sensing system should be used to allow your own solar system to be isolated from the grid until grid power is resumed, without the need for a battery.
      Sandy Brucesmith PhD Elect Eng.
      Port Macquarie

      • Sandy, there are solar PV inverters which can, without a battery, isolate from the grid to continue to provide the household energy supply during grid outages. You just didn’t buy/install that functionality.

        They are somewhat limited in what they are capable of supplying because balancing power production and consumption in a microgrid (an absolute must in any electrical energy system) is much trickier without a battery, but they do exist.

        For a solar PV system to operate with a level of usability more suitable for grid outage backup it’s generally far better to have a battery to provide system ballast.

        Like all things, solar PV systems have many options and some specialist features like this are not high on the priority list for the vast majority.

        • I have given my old (12 years), decommissioned 1kW solar roof top system to a friend, who intends to use it as a stand-alone system for out buildings on his farm. My rational is, that if he feeds the old Ecokinetic inverter with 240VAC from a 12V car battery via one of these inexpensive DC to AC inverters, which campers use in their caravans, this would simulate grid connection to the Ecokinetic inverter. I would greatly appreciate any comments on this idea.

          Taking this idea further, I operate at present a larger rooftop system via a 6kW inverter, which shuts down if it looses signal from the grid. If I were to install a break-before-make switch and then feed the 6kW inverter from the same 12VDC to 240VAC inverter during grid outages, would I be able to restore power?

  25. One day I bought a house with LPG instant HWS, LPG cooktops and mains (methane) gas pipe within range to connect. Then I installed panels..

    I found many of questions above which I had to ask myself:
    – Is it worth to have mains gas while have panels?
    – Instant gas or tank/electric HWS?
    – How to balance load to stay within PV?
    – How to see $$ return from panels?

    … and many more.

    I believe I solved most. I’ve got 16c FiT which will expire in April. That makes my PV electricity expensive and in fact more expensive than my off-peak worth 14c only. This shifted my focus a bit and some loads into night but I remember it won’t be forever. So how did I solve these problems?

    – I’ve added heatpump HWS. It was expensive, $5k installed. I compared it with standard electric HWS: $2k installed + $600 wiring +1500 PWM to adjust power. Nearly the same, in fact heatpump was $4.5k if I didn’t want to keep instant LPG as a backup. With 6 years warranty for pump TCO seems lower and it fits within PV even in cloudy day.

    – The above made mains gas idea gone. My 16c export wasted into hot water makes my annual hws run $180. Yet it is about get cheaper when FiT drops. Even free gas from pipe won’t do because connection fee is well over my heatpump run.

    – I kept LPG HWS as it does not cost for standby and brand new HWS will last for a while. I let it run one day per month when I switch to gas. It will do backup when pump fails.

    – Balancing load turned a bit tricky. I looked at commercial solutions and honestly found nothing. My inverter was installed with metering and after a year I see it wasted $500 as it is difficult to use it for anything. Because I could not find useful solution I thought about implementing open automation. While idea was born as related to PV solution, whole automation evolved a lot further when I found it useful and amusing. I skip unrelated details, however.
    To balance load I implemented various sensors such as metered plugs, power points, central metering and also some native IOT implementations embedded in appliances. I also use BOM forecast and ambient sensors exposed by people around. As I merged my aircon, its sensors are also used. Then number of rules – some examples:
    – Heatpump works in “controlled load” mode. It has no power unless it is time to heat-up or outdoor temperature drops below 6C to let it antifreeze.
    – Time to heat-up is based on scoring. It takes: export over 2kW, ambient temperature delta to forecast (various threshold), ambient temperature itself and time left till sunset. Aims to start when is warm, lots of electricity and not too late. It may skip one day in bad conditions or run shorter cycle.
    – Aircon runs when I have surplus daytime. In winter I used night pre-heating with runtime calculated from forecast. I am getting aircon people to add DRED card which I could use to throttle it or stop when Sun is not up to the job. Not yet installed due to last lockdown.
    – My dishwasher is paused when competes on water heating with washing machine until laundry heats up to resume dish heating but only when PV does not cover everything.
    – To be aware of energy flow I’ve added a few “export lights” and my automation plays announcements whenever substantial event happened so that my wife always knows if there is enough to run oven 🙂

    – I see my returns in pvoutput paying little donation and exporting data from open metering. Calibration made discrepancy compared to official metering below 0.5%

    Yes, I found some people in NZ trying to offer DRED signaling for aircons and people on OZ offering PWM regulator which takes weather forecast. I also found Solar Analytics. In fact these systems inspired me. However, I concluded these don’t operate together and are horribly expensive.

  26. In South Australia ( and some other places) we have an off-peak tariff called a “solar sponge” between 10am and 3pm, designed to access the abundance of solar power available in the middle of the day at very cheap prices.

    It seems to me that a house with solar PV could utilise this as an adjunct to cover the days when local solar production was low, so water could be heated with solar PV on a sunny day and solar sponge tariff on a cloudy day.

    Also in SA, most new homes have to instal an energy efficient HWS which means heat pump, gas or solar. Resistive element HWS can only be used in limited circumstances.

  27. Here is an interesting thought:

    Most PV systems have inverters sized so that they clip total available solar (to enable a decent output on cloudier days and because inverters are expensive.)

    A simple contactor with a Solar Insolation Meter (does one exist?) or a new generation of inverters to switch in/out excess panels based on whether the insolation is above the inverter capacity. By switching the DC (excess) feed it could be diverted to a DC element in your tank (still keeping the basic AC element as a back-up system).

    That way you get your PV and hot water from existing system (with a little wiring on the DC side) for very little cost.

    We need to start thinking about DC infrastructure within homes. We convert PV DC to AC with losses and then back to DC for eg LED lighting, inverter based appliances etc. This will be decades away and for now simple solutions easily achievable are required.

    So anyone know what kit there is out there to do this?

    [I haven’t researched it further because I am totally off-grid with plenty of power for instant water heating]

    Ronald – maybe talk to the inverter manufacturers?

  28. One thing Ron. You are not counting the missed export potential in your parents calculation. I would argue they’d be losing money by switching their water heating to utilise the solar. I know you’re aware of this concept as the same idea applies to batteries.

  29. It is not easy to produce a 50Hz 230v AC supply to simulate the grid and fool a grid connect inverter into operating. Whilst voltage variations can be tolerated to a certain degree, the acceptable tolerance of frequency is quite limited and must be maintained within very close limits. I have my house wired with a changeover switch to allow an alternator to be connected during power outages, but I have never been able to get my inverter to sync. (In this case the frequency varies as the load varies and the motor speed changes). A solid state device may work better but only if the frequency is accurately maintained across a range of loads.

    • I have petrol generator with inverter and it has nice wave keeping 50Hz regardless of load. Thing is my string inverter will burn it trying to export. It does have limit though, however, manual says setting 0 limit means export 0-100W. I thought about putting 300W load on generator side before export clamp. This way PV could feed back up to 300W without reversing generator inverter. Didn’t test it as I didn’t want to burn either.

  30. Kerry Ward says

    With the help of an electrician friend we have finally changed over from our expensive LPG gas instantaneous hot water to a electric boosted solar hot water system. We probably wouldn’t have gone down this path ,but for bad advice. When we ordered our solar PV system 3 years ago we ordered a solar diverter. However due to the provider not doing their research nor an inspection prior to installation , we couldn’t fit the diverter ,due to having a meter box remote on our shed ,no designated hot water line from the shed to the house and 3 phase. We also bought a new 400 litre storage tank at the same time. I am not a fan of heat pumps so that wasn’t an option.
    So we decided to get 2 roof mounted hot water panels and the circulation pump/controller. We managed to get a 1200W element for the tank. The electrician then wired up a separate circuit for the HWS with a din rail mounted timer in the switchboard set to run from 10.00am to 3.00pm. AGL now offers a solar soaker rate of 21c/kwh during this time period ,so not too expensive if we do draw power from the grid.
    We also set up a further system inline to utilise the excess from the solar PV panels as much as possible.This system is a poor mans diverter and cost less than $100. It consists of an adjustable light control sensor feeding a normally closed contactor with a switch to over ride the light sensor if needed. The light sensor only has 0-500 lux range ,so was set up to only get indirect daylight. We were then able to adjust it so that it opened the contactor when the PV panels were putting out roughly 1800W.
    It has been cold and damp the last few weeks ,but it is supplying all the hot water with no problems for 2 people plus visitors.

  31. Kerry Ward says

    Meant to say “it opened the contactor when the PV panels roughly less than 1800W”

  32. Peter Olsen says

    Regarding the possibility of legionella, I found this website which provides an interesting insight into the issue:

  33. I have a 10Kw PV system with a Fronius Primo inverter, Fronius smart meter and contactor to use surplus solar for hot water. I am exporting to the grid average about 12kWh a day in winter. Tariff is TOU peak 3pm-9pm. I have a 250L HWS with a single 2.4Kw element. I have found that in Melbourne over winter sometimes in the morning for showers the water could be hotter. Some showers late night and mornings. Is it possible to have the hot water heated early in the morning from off-peak? How would this be done?

    • Peter Olsen says

      It depends on your energy supplier. Some providers allow HWS to be switchable between solar and off peak, other’s don’t. Fortunately mine does (Endeavour Energy around Wollongong way in NSW). (An issue where the government should step in and require that it be allowed.)

      I have just had a 10.1kW solar system installed that uses both. The HWS solar diverter is from If you look on their website you will see a diagram for switching between the solar diverter and off-peak.

      It just requires a SPDT contactor. The common terminal connects to the HWS. The N/C terminal connects to the solar diverter. The N/O terminal connects to off-peak. When the off-peak power comes on overnight it pulls in the contactor which switches the supply from the solar diverter to the off-peak supply. If the HWS thermostat is closed then it will heat the water.

      When the off-peak supply stops at 4am or whenever the contactor will open and connect the HWS to the solar diverter again.

    • With your export amount you have plenty of excess for HWS, my system 9.8Kw catch power diverter 3.6 Kw element 315 litre HWS grid tied system ditched off peak heats on thermostat if needed provides most of hot water needs in winter. I’ve found in Victoria with this system hot water needs can be met with 2 to 3 hours solar generation, often clouds appear afternoon by which time needs are met.

  34. Now that I can see my power load on my inverter app I have suspected that off peak solar hot water back up seems to run most of the day and night and is now only 10-20% cheaper. My solar heater does very little as the off peak appears to run whenever we shower or use HW for any time of day. It seems pointless to have solar. So I have been looking to for something to meet my confirmation bias! Very grateful for the academic rigor in this. It appears I have two options, a simple timer and manual switch or a more comprehenvie diverter? Can any electrician do this? will it impact the solar inverter?

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