Is The Most Efficient Hot Water System A Solar PV Powered Heat Pump?

pv powered heat pump hot water

Heat Pump Hot Water. Power it with Solar PV and its efficiency cannot be beaten by solar thermal. Bloody expensive though.

If you are looking for the most efficient solar hot water system then solar PV panels powering a heat pump are hard to beat.

While both the panels and heat pump will need to be proficient performers, it is possible for the combination to beat solar thermal hot water which is widely considered the most efficient method.

A typical heat pump hot water system is 3 times more efficient than a conventional electric hot water system that uses electrical resistance heating.  This is because rather than using electricity to create heat, they use electricity to move heat around.  Heat pump hot water works in exactly the same way as a refrigerator does, aside from the fact they work in completely opposite ways.  That is, a refrigerator pumps heat out to keep what’s inside cold, while heat pump hot water pumps heat in to keep what’s inside hot1.

Because heat pump hot water systems conserve energy they are eligible to receive Renewable Energy Certificates called STCs that lower their cost.  While these are the same certificates that are received for installing rooftop solar, the number received is determined differently.

While they are efficient, heat pump hot water has suffered from a number of drawbacks that have held them back from widespread acceptance.  These include:

  • High initial cost.
  • Poor reliability.
  • Expensive to repair.
  • High noise output.
  • Misleading claims by manufacturers.

Despite this apparent surfeit of dead albatrosses slung around heat pump hot water’s neck, I am confident the technology will improve and we are likely to see an increasing number of heat pump hot water systems in Australia.

What The Hell Is A Heat Pump?

In 1852 Lord Kelvin worked out the mathematics of heat pumps and, as a result, had the Kelvinator refrigerator named after him.  But — and this is an important point — not right away.  He had to wait until he was dead for 7 years before that happened.

Lord Kelvin

“Dear world — I, Lord William Thomson Kelvin, have won wide renown as the greatest scientific mind you have known since the days of Newton. In return for the scientific advances I have made, I would like a refrigerator named after me — The Williamator.
P.S.: Airplanes will never work.”

The first heat pump was built in Austria around 1856 by Peter Ritter von Rittinger2.  He developed a practical method of taking advantage of how gas heats up when compressed into a liquid and cools down when it evaporates to move heat around.  This meant heat could be moved into a confined area, such as with a heat pump hot water system, or out of a confined area such as with a refrigerator.  Household air conditioners are capable of either and can move heat out of a home in summer and move heat inside in winter.

Heat pumps can exchange heat between water, the ground, and air.  In Europe, where the temperature can drop to -30ºC just to punish people for not living in Australia, ground heat pumps are very common because the ground a few feet down is a hell of a lot warmer in winter than the air.  But in Australia where temperatures rarely reach freezing, almost all heat pumps use air and these are simply called air sourced heat pumps.

Occasionally heat pump hot water systems are referred to as solar heat pumps because 99.98% of the heat they pump originally came from the sun3.  But I think this misleading as they don’t require sunlight to operate.  After all, in winter almost all the heat an air conditioner pumps inside your home originally came from the sun and no one ever refers to it as solar air conditioning.

Lord Kelvin Quote

Heat Pumps Use CO2 As A Working Fluid

These days heat pumps often use carbon dioxide (CO2) as their working fluid4.  While CO2 is a greenhouse gas, the total amount is usually less than one kilogram, which is around how much is emitted from generating one kilowatt-hour of electricity from coal, so there is no need to worry about your heat pump roasting the planet if it leaks.  As long as it displaces more than one kilowatt-hour of coal generation over its lifespan you’re golden.

Also, one kilogram of CO25 is not enough to be dangerous to your health unless it is in a very confined space, such as your lungs.

Heat Pump Hot Water Is Around 3 Times More Efficient

The heating element in a conventional electric hot water system transforms 100% of the electrical energy it uses into heat and transfers about 98% of that heat to the water in the storage tank.  That’s close enough to 100% for me6.  But it can be far more energy efficient to move heat around instead.

An air conditioner is a heat pump and an energy star label for one generally looks like this:

Air Conditioner Energy Stars

Air conditioner energy star label. Image credit: Australian Government

And here’s a close up of the capacity output and power input figures:

Power Input and Capacity Output

On the right hand side within the far reddish rectangle is the power input for heating in kilowatts of electricity.  Next to that is the capacity output in kilowatts of heat.  So the air conditioner can take 0.86 kilowatts of electricity and use it to move 4.45 kilowatts of heat inside.  That’s a ratio of more than 5 to 1.

But below the 4.45 figure you’ll see it says the heating capacity at 2 degrees is only 3.6 kilowatts, reducing the ratio to 4.2 to 1.  This is because the colder it gets, the less effective the air conditioner’s heat pump is at moving heat inside because of the greater temperature differential.

Unfortunately, heat pump hot water systems are not likely to be as effective as this air conditioner.   For each kilowatt-hour of electricity they consume they will generally provide around 3 kilowatt-hours of heat, making them 3 times as efficient as a conventional electric hot water system.  The exact efficiency will vary through the year, because the greater the difference between the air temperature and the hot water temperature, the lower the efficiency.

If you look at information about heat pump hot water systems online you will often see they generally claim to be more than 3 times as efficient.  Unfortunately, this may not be the case.

Manufacturers May Mislead About Efficiency

A commonly given measure of efficiency of heat pump hot water systems is their Coefficient Of Performance or COP.  This is a straight forward measure.  If a heat pump uses 1 kilowatt-hour of electricity to provide 3 kilowatt-hours of heat it has a COP of 3.  If it uses one kilowatt-hour of electricity to provide 4 kilowatts of heat it has a COP of 4.

Most heat pump hot water systems state they have a COP of 3 to 4.5 or even higher.  But a COP of 4 doesn’t mean it will only use 25% as much electricity as a conventional hot water system.  This is because it will use energy for more than just operating the heat pump.  For example, when it is very cold ice will form on the outside exchanger of a heat pump and it may need to be defrosted every few minutes.  This either requires energy to power heating elements to melt the ice, or more often the unit will be designed to switch to using a standard heating element to warm water and so in cold temperatures it will function as a conventional electric hot water system.

As the highest hot water consumption occurs during cold weather, their reduced efficiency at low temperatures and their habit of either spending energy to defrost or simply giving up on operating as a heat pump, can considerably increase the amount of electricity they use.

Heat Pumps Can Be More Efficient Than Solar Thermal

Solar thermal hot water has generally been considered the most efficient way to heat water using sunlight.  This graph I stole from an Apricus solar thermal hot water brochure shows the efficiency of flat plate and evacuated tube solar thermal systems for both the total, or gross, area of the collectors and also for the smaller absorber area:

Solar Thermal Hot Water Efficiency

Solar thermal hot water efficiency. Image credit: Apricus

The “Delta-T” at the bottom of the graph is the difference between the air temperature and the collector temperature and is likely to average at least 40 degrees.  If we assume it is 40 degrees then the highest efficiency by gross collector area is around 54% for flat-plate solar thermal.

Heat pump hot water can be more efficient than this, although it often isn’t.  New solar panels these days are typically around 17% efficient and by the time the electrical energy they supply reaches a heat pump it is not uncommon for various losses to have reduced their output by 20%.  This will knock the effective efficiency of 17% panels down to 13.6%.  If a heat pump provides 3 times more heat than a conventional electric hot water system the total efficiency will come to 41%.

But if very efficient panels are used, such as SunPower’s X-22 panels that are 22.1% efficient, then even if losses drop that down to 17.7%, when used with a heat pump that is 3.5 times more efficient than a conventional hot water system the total efficiency will be 62%.

So for people who don’t mind paying for premium panels, using them with a high efficiency heat pump hot water system can be the most effective way to make use of limited roof space.

Heat Pump Hot Water Can Receive STCs

While heat pump hot water doesn’t generate electricity like rooftop solar, it does reduce its use compared to conventional hot water systems and a kilowatt-hour of electricity saved is as good or better than a kilowatt-hour of clean electricity generated.  For this reason, heat pumps qualify to receive Small-scale Technology Certificates or STCs as part of Australia’s Renewable Energy Target.

This page will let you download a list of all the approved heat pump hot water systems and the number of STCs created when they are installed.

If you want to know exactly how many STCs a system will receive, you can go to this page and enter its brand, model, and the postcode it will be installed in.

The exact number you will receive will depend on which zone you are in.  But these zones are different from the ones for rooftop solar PV.  Here is a map of the hot water zones for both solar thermal hot water and heat pump hot water:

Hot water STC climate zones

The number of STCs received depend on how much electricity is likely to be saved by a system in each zone and this is affected by amount of hot water people use and by its efficiency there.  So in zone 1 heat pump hot water operates efficiently all year round because it never gets really cold, but people there don’t use a great deal of hot water on account of how it never gets really cold, so zone 1 receives less STCs than colder zones.

What isn’t shown on this map is special zone 5 which is only in the Snowy Mountains and mountains of Tasmania.  These are the coldest places in Australia and you might think they would get the most STCs because hot water use there is likely to be the highest.  But because heat pump hot water will often operate with lower efficiency there, zone 5 can receive fewer STCs than zone 4.

The number of STCs received will be cut by one tenth at the start of 2022 and cut each year after that until they are phased out entirely by the end of 2030.

Solar Power, Timers, And Heat Pump Hot Water

I recently wrote about putting conventional hot water systems on timers so they will switch on during the day when they will hopefully be able to take advantage of surplus solar electricity that would otherwise go into the grid and receive a feed-in tariff.

It is easier to run heat pump hot water systems off surplus solar using a timer because they usually draw only one kilowatt or less, which is around half as much as even the smallest standard heating element used by conventional electric hot water systems.  This low power draw makes it much more likely rooftop solar will provide enough power to run them without needing to dip into grid electricity.

Because the average temperature is higher, heat pump hot water will operate more efficiently when run during the day and will be much less likely to need to waste energy defrosting or operating as a conventional hot water system.

Heat pump hot water also has the advantage of generally coming with their own built in timers.  Besides setting them to turn on when a surplus of solar electricity is likely, they also allow households that are on time-of-use tariffs take advantage of low cost off-peak power.

Solar Hot Water Diverters Cannot Be Used

Solar hot water diverters that send surplus solar electricity to the heating elements of conventional hot water systems cannot be used with heat pump hot water.  The type of current they supply is not compatible with pumps or electronics and may damage them.

Threshold Power Can Be Used

Devices that can send normal household AC power to devices when there is sufficient surplus solar energy to operate them can be used with heat pumps.  Examples are the Fronius relay and SunMate diverter.  These devices also permit users to select minimum activation times and activation delays after the surplus solar power threshold is reached to prevent clouds rapidly turning the heat pump on and off.

Controlled Load Or Economy Tariffs Can Be Used

Heat pump hot water systems can be used with controlled load tariffs, which are also known as economy tariffs.  Note there are some systems where the manufacturers recommended they have access to electricity for more than 8 hours a day, which can make them unsuitable for most controlled load tariffs.

Lukewarm Water Is Dangerous

No matter what method is used to power a heat pump hot water system, it is a matter of life and death that the storage tank reach a minimum temperature of 60 degrees every three days to control dangerous microorganisms such as Legionella.

They Aren’t Cheap

An installed heat pump hot water system is likely to cost $2,500 to $4,500 including the reduction in price from STCs.  Households in Victoria can also receive Victorian Energy Efficiency Certificates or VEECs which are worth approximately $500 when replacing an existing conventional hot water system.

The total cost is often at least 2 to 3 times as much as an equivalent conventional electric hot water system.

Warranties Are Often Short

Warranties for heat pump hot water systems are often horrifically short and may be as little as one year.  The longest I have seen is 6 years.  While they may have much longer warranties on their storage tanks, that is not the part that is likely to break down.

Their short warranties suggest their operating life cannot be expected to be long and combined with high initial cost and high cost of repairs, many people don’t consider them an attractive investment.

They Can Be Noisy

Some heat pump hot water systems generate a considerable amount of noise.  The quietest I am aware of is the Sanden Eco Plus which operates at 37 decibels.  This is as loud as a quite fan operating a low speed, which shouldn’t be surprising because that is exactly what makes the noise it produces.

Because they are similar in operation and power use, most heat pump hot water systems make noise comparable to room air conditioners, which can range from a mild hum to a mild cacophony7.  Some manage to be so loud they have been banned from operating at night so they won’t disturb the neighbors.

Potential Savings

The most cost effective location in Australia to install a heat pump hot water system is likely to be Western Australia.  This is because controlled load tariffs that lower the cost of using conventional hot water systems are no longer available and the cost of electricity is high.

In Perth the large majority of households use natural gas hot water systems.  But if there was a family that, despite the warm climate, was a very large user of hot water and had a conventional electric hot water system, they could potentially reduce the number of kilowatt-hours they use to heat water annually by 1,500 with a heat pump hot water system.  If all their electricity came from the grid it could save them $400 a year.

Elsewhere in Australia the savings generally will be far less.

If my parents in rural Queensland were huge users of hot water and were able to cut annual electricity use by 1,500 kilowatt-hours by installing a heat pump hot water system, as their hot water system is on tariff 31, they would save around $260 a year.

Elsewhere in Australia where controlled loads are usually lower and feed-in tariffs sometimes higher, the benefit is likely to be even lower.

Environmental Benefit

While the economics of heat pump hot water are not yet favorable, it can provide considerable environmental benefit by either greatly reducing the amount of fossil fuel a home uses to heat water, or alternatively by freeing up rooftop solar generated electricity to be sent into the grid where it will reduce fossil fuel generation.

But many households will find they will be better off economically and provide greater environmental benefit by using the money to instead install a larger rooftop solar system than they would have otherwise.

Those who have already installed as large a solar system as is practical may find heat pump hot water an effective way to further reduce emissions.  There are many people who will find that worthwhile even if the return on a heat pump hot water system is extremely low or even negative.

The Future For Heat Pump Hot Water May Be Rosy

While heat pump hot water hasn’t been a roaring success in Australia so far, in the future its prospects may improve.  The amount of STCs it receives won’t start to be reduced for over 4 years and in this time costs are likely to fall and reliability improve.  A number of nations overseas are working on improving heat pumps and we will benefit from their efforts.

But one drawback I can see for heat pump hot water is that the cost of operating conventional hot water systems may fall as as the cost of rooftop solar continues to decline and increasing renewable generation puts downward pressure on controlled load tariffs and the off-peak rates of time-of-use tariffs.



  1. I am working on creating the next leap forward in household energy efficiency by combining refrigerators and heat pump hot water systems.  All I need to succeed is work out how to get them to mate.
  2. Ritter means “rider” and is the Austrian word for knight, so after Peter Rittinger was knighted he became Peter Ritter von Rittenger.  As Rittinger is Austrian for “close rider” and the name Peter comes from the Latin word petra meaning rock, a literal translation of his name is Sir Rock Rider Close Rider.
  3. Less in geothermally active regions or Maralinga.
  4. Fun Fact: While we usually think of a fluid as a liquid it can also be used to describe a gas!  Okay, I admit this fact isn’t as fun as it would be if nitrous oxide was used instead.
  5. One kilogram of CO2 gas is 509 liters, which sounds like a lot, but it is about half a cubic meter in volume and will rapidly disperse under normal conditions.
  6. I have some 100% insulin on sale if anyone is interested.  It’s almost certainly less than 2% anthrax.
  7. Cacophony — The sound made by a fake caco.
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. those heat pump hot water systems seem dead in the water.i was going to install one, and then after reading the amount of negative comment i decided it was a waste of money and did not justify the extra cost.
    They seem just a niche as most reviews claim inefficiency, cold water, noisy and so on ….just not worth the trouble at the moment.

    • Des Scahill says

      I fully agree with you Eric. I’ve been through 3. The first one packed it in within a week, the replacement second lasted about 3 weeks, and the third chugged away until the warranty period expired and then expired about a year after that. In my darker moments I wonder if there’s some ‘planned obsolescence’ by the manufacturer going on. They’ve used heat pumps in NZ for decades and they seem to function fine over there. They’re not worth the trouble. Get more solar panels instead.

      I now use a stock standard 250 litre hot water heater on a relay timer, which means that a fair proportion of the power needed for hot water ends up coming from my panels. Had to fiddle a bit with the turn-on time for the relay timer and change our consumption patterns for cooking and dishwasher etc a little, in order to maximise self-consumption. Basically, if you set the relay timer so that the hot water system draws power during peak production times and work your other consumption outside that period as best you can, you do get a noticeable benefit. The hot water cylinder and heater will probably last about 10 years or more (that seems to be the ‘norm’). Our hot water system is 3.6 kw that was already in place before I got solar panels, Initially, it was using 5 kw a day, that’s now down to around 2 to 3 of billable usage with the rest coming from our solar system. A heat pump would probably drop the total usage to 1 kw, but the economics simply don’t stack up, and evacuated tube approach is at present not worthwhile unless maybe you have to replace the hot water system anyway.

      My general advice would be to get more solar panels if you don’t have a 5 kw system already, and change consumption patterns. Also – I wouldn’t ‘fixate’ on just hot water costs. What matters most is to maximise your overall kwh self-consumption while at the same time minimising your billable kwh consumption. That usually means you have to make ‘changes’ from time to time in response to seasonal influences and your own varying patterns of usage with items such as pool-pumps, air-conditioners, electric heaters, dishwashers, cooking etc. .

    • Stephen Grenville says

      Sanden het-pump hot water
      I bought a 300 litre Sanden heat pump in November and while it functions well technically, it seems to me to have a serious design fault.
      I set it to operate in the period when the PV panels are supplying power – 8am-6pm and of course it doesn’t heat water in the block-out period beyond these hours. The sensor which activates the pump is located about 2/3 of the way up the tank. This means that by end of the ‘on’ period in the late afternoon, we might have drawn down, say, nearly 2/3 of the hot water in the tank without triggering the pump to heat the whole of the tank. Thus we can go into evening ‘block-out period’ with little more than 100 litres of hot water, and run out (just two of us!) before the operating period begins next morning.
      On later models, it seems possible to fit a WIFI link which might allow this problem to be ameliorated with ad hoc adjustment. But what is needed is a setting which ensures that the system ends the ‘on’ period, in the late afternoon, with a full tank of hot water. Of course, we can work around this deficiency, but this drawback should be clearly known to buyers BEFORE they install.
      Stephen Grenville

      • Geoff Miell says

        Stephen Grenville,
        I bought a 300 litre Sanden heat pump in November…

        What’s your outdoor unit model?
        GAU-A45HPC or
        See the rating plate on the rear face, near the side where the electrical supply & water recirculation pipes connect with the unit.

        If it’s the GAU-A45HPD, then the Sanden Wifi Controller is compatible for it.

        Thus we can go into evening ‘block-out period’ with little more than 100 litres of hot water, and run out (just two of us!) before the operating period begins next morning.

        Have you actually run out of hot water?

        I’d suggest you perhaps may need to adjust your block-out timer setting or adjust your daily usage pattern.

        This thread at Whirlpool may be of interest:

  2. Thanks Ronald

    One thing you didn’t mention is the relative area on the roof taken up by either solar thermal collectors or PV panels. I haven’t done any calculations but based on my own installation (of both of these) I would guess that the PV needed for water heating would require more space than the thermal collectors. This becomes an important issue it roof space is limited.

    • Ronald Brakels says

      Hi Rod. Normally flat plate solar thermal will beat using PV panels and a heat pump, as far as roof space is concerned. But high efficiency panels used with a high efficiency heat pump hot water system can beat flat plate collectors.

      • Steve Hiscock says

        Hello Sir
        While I appreciate the time and effort put into the article on Solar hot water, your suggestion is completely out to lunch.
        Using FPC to generate hot water straight up is by far more efficient than your suggestion of producing electricity with PV then using that electricity to do the same thing!!
        It’s a no brainer.. FPC’s are 70-75 % efficient. PV is at best 18% efficient. After that, your doing exactly the same thing.. Heating water.. Everything else is the same..
        What you suggest is the most INEFFICIENT way to produce hot water. Doesn’t matter how efficient your heat pump is. Your losing energy both ways, Firstly by PV… then by heating water with a heat pump , which is already hot from FPC systems..
        I don’t know that you are smoking, but I sure would like to try some!!
        I’ve been doing solar for 20 years.. The concept you suggest reminds me of the people who said “why don’t we put generators on all the wheels of the car, then it won’t use any energy to travel cause we are producing as much as we are using!!!”
        Standard physics my friend.. The more direct the conversion, the more efficient!!
        You really need to do your research… The world really isn’t flat, even though some still suggest it is… It doesnt mean they’re right.
        Take care and thanks for thinking about it.. Steve Hiscock
        [email protected]
        Living in Bhutan, worked in Australia, come from Canada.. Lots of solar experience!!

        • Steve Hiscock,
          You state:
          “FPC’s are 70-75 % efficient. PV is at best 18% efficient. After that, your doing exactly the same thing.. Heating water.. Everything else is the same..
          What you suggest is the most INEFFICIENT way to produce hot water. Doesn’t matter how efficient your heat pump is. Your losing energy both ways, Firstly by PV… then by heating water with a heat pump , which is already hot from FPC systems..”

          I don’t think you comprehend what Ronald wrote in his post.

          Firstly, Ronald nominated an Apricus solar thermal hot water system in his post above, and kindly ‘walked us through’ why the Apricus FPC is of the order of 54% with a Delta-T of 40 °C. He also clarified why he chose the gross curve, see his comment at:

          I assume the Delta-T is the difference between the FPC water inlet and outlet temperatures. In winter, the Delta-T would be higher and therefore the efficiency would lower.

          Steve, what brand(s)/model(s) of FPC can you please nominate, that demonstrate(s) your claim that “FPC’s are 70-75 % efficient”?

          Secondly, Ronald ‘walked us through’ the energy efficiency of the solar-PV/inverter system. I take that to mean that the conversion of available sunlight energy to 240 V AC electrical energy is typically 13.6%. But some panels, like SunPower’s X-22 panels that are 22.1% efficient, coupled with a high efficiency inverter could achieve of the order of 17.7% overall.

          Thirdly, your comments suggest to me you misunderstand the physics of air-sourced heat pumps and the demonstrable efficiency gains for refrigeration, reverse-cycle air conditioners and hot water heat pumps.

          For a Sanden Eco® Plus Hot Water Heat Pump System, with the ambient air temperature at 32.45 ºC (90.41 °F), cold-water inlet temperature at 18.74 ºC (65.73 °F), with water outlet temperature set at 63 °C (145.4 °F), the CoP is rated at 5.96 – or 0.84 kW of electrical power consumed provides 4.99 kW of heat to the water. At lower ambient air temperatures, operating down as low as -10 ºC (14 ºF), the CoP would be significantly lower – I suspect about a little less than half for my unit. Sanden units don’t have secondary heating elements, but do have a frost protection mode.

          See also my comments at:

          So, an air-sourced hot water heat pump (at ambient air circa 20 °C, with CoP of nominally 4.0–4.5) coupled with a solar-PV/inverter system (with overall solar to electrical energy efficiency of about 17.7%) yields an overall energy efficiency of about 70–80%, which it seems to me is better than a FPC system with an efficiency of about 54%. In winter, overall efficiencies would be less for both the heat pump (lower CoP) and FPC (higher delta-T) systems.

          There may be other considerations, like roof-space limitations (for me), and budgets.

        • David Morgan says

          I disagree, for the following reasons. In we only consider a simplistic instantaneous definition of efficiency, ie output over input, in this case, heat into the water for a given amount of radiation on the roof, then the systems are about equal. One kW of thermal radiation on a solar panel produces about 200W of electricity, but the heat pump uses 200W to pump at least 4 times that amount of heat into the water, so that the water gains close to one kW.
          If we consider a broader definition of efficiency, including averaging over a year, then the photo voltaic heat pump solution is much more efficient for the following reasons. If we install sufficient passive solar panels to heat our hot water in winter, then we will be massively over-supplied with hot water in summer. About 4 times in Melbourne. And of course the ratio for the sunniest day to the cloudiest day is much greater than that Not only is most of that summer heat wasted, but it is expensive to dissipate, either by running hot water down the drain, boiling water, or isolating and draining the panels. On summer days, the fraction of radiation that makes it into you water tank is limited by the heat required to bring the water to temperature. So efficiency for a summer day is very small, depending on how many days you are prepared to use gas or electricity to boost the panels. The “annual efficiency”, how much of the radiant heat on you panels over the course of a year made it into the water tank, is likely to be 10-20%. With the photo voltaic heat pump solution, at least 20% of that extra energy is converted to electricity and sold. Defining efficiency as how much income generated per square metre of roof over a year, the photo voltaic heat pump solution is much more efficient.
          Of course this will be less so in the tropics, where the solar radiation varies less from day to day.

        • David Morgan says

          Another way to look at it is this. Whenever the hot water tank is up to temperature in an FPC system, the efficiency drops to zero, as there is no “output” (heat going into the tank). For a system designeded to minimise boost, this will be most of the time on most days of the year. However the photo-voltaic-heat-pump system simply switches its “output” to electricity to be sold, making it much more efficient most of the time. Again, away from the tropics.

        • Ian Garradd says

          you are right, FPCs are the most efficient use of space….on the face of it

          The problem with solar thermal is that it is a waste of space after the tank is full of hot water, which can be as early as 10 am on a summer day.

          Solar electricity is useful 365 days / year, all daylight hours.

          That Is the benefit of solar PV, & where there is limited roofspace, in conjunction with a heat pump

  3. Stephen Fuller says

    What about solar thermal collectors with heat pump back up? Heat Trap Solar has this system available and it includes a synthetic low pressure storage tank reducing the problem of tank corrosion and other maintenance costs.
    As I understand it, Heat Trap Solar systems attract the highest level of rebates under the RET.

    • Ronald Brakels says

      This is an incredibly energy efficient system, and because the heat pump should only need to operate occasionally, hopefully that will result in it lasting a long time. Unfortunately, these kind of systems are pricey, even with their high number of STCs. But they could definitely appeal to people who are looking for the best efficiency they can get.

  4. Erik Christiansen says

    Apropos combining heating and refrigeration, I’ve for some time been planning to put something like the Bolt-On heat pump up in the roof space so it cools up there using the same input energy which is heating the water. Insulation blankets at the ceiling joists and between roof and sarking should allow the oven-like summer simmer to be dropped significantly, reducing the power requirement for an air conditioner below.

    What of winter? The wood heater and over 100 Ha of forest take care of that.
    (All of the reduced winter output of the array can be better utilised than heating water. Leave that to the inevitable summer surplus, I figure, when the heat pump will also be more efficient, as it’s not pumping so many degrees uphill.)

    Some heat pump manufacturers require acidity reduction if used with tank water of pH < 6.0, if the warranty is not to be voided. That appears to be little more than a tube filled with limestone chips, or similar, so it doesn't seem to be a big deal. A little lime is good for the garden, if the greywater is used there.

    That Sanden's noise level is the best I've seen. (But compare it with a spiffy glass kitchen rangehood: 54 dB on speed 1 (of 3).
    The Bolt-On and Chromagen were both at 48 dB when I last looked

    I'll be looking for the best quality compressor, in pursuit of maximum durability.
    The whole thing is only an air conditioner run in reverse, after all. (Admittedly at higher temperatures – the enemy of longevity.)

  5. What you seem to ignore , from the Apcicus chart you show is that the evacuated tube at the temperature you use is about 72% efficient. Far better then what a premium solar panel will give you plus a heat pump according to your calculations.

    So if you want to argue that solar electricity using a heat pump is more efficient you may want to revise your figurers.

    The real advantage of a heat pump combination for making hot water is that if you have an regular hot water heater using a resistance heating element using a 4.5 kW heating element (typical hot water heating element) it will be much more efficient to preheat your hot water with a heat pump using say 1200 watts of energy to get your hot water up to 120F or so and not use the 4.5 kW heating element at all. I myself have a 50 gallon hot water tank with a 4.5 kW heating element


    • Ronald Brakels says

      Hi John.

      The line on the graph you are looking at is for the evacuated tube absorber only. That makes up less than 60% of the area of an evacuated tube system. To get the figure for the entire area covered by the collectors you need to look at the lines for the gross amounts.

  6. Ross Hastings says

    Dear Ronald
    The best engineering solution is always to keep it as simple as possible.
    This is achieved for solar hot water by using a remote tank and pump and a suitably sized and designed panel.
    By adding a heat pump to a solar PV system is the exact opposite to simple. It is the most complicated option and the performance benefits may not necessarily be better than solar thermal.
    I understand that mine is just one view and that there are many ways to skin a proverbial cat.

    Ross Hastings

    • Erik Christiansen says

      The problems with heating water on the roof are twofold:
      A single unit is inadequate in Victoria in winter, needing supplementation from the grid or a wood heater; and the roof area cannot power other loads when it’s not used for heating water. (Winter: all the time, given a wood heater. Summer: after lunch.) The space is then wasted.

      I’ll be off-grid, and it’s well worth $2.5k to be able to heat enough water for one person by lunch, with power for some other purposes all the while, then have full array power for e.g. welding + aircon + breadmaker in the afternoon. (Even with evacuated tubes, the water won’t be hot enough for welding. 😉

      With array output only 1/3 in winter, having the extra utilisable array area all day in winter is better use of limited roof area, especially when off-grid.

    • I would have to agree with you Ross, all we are trying to do is to help the planet a bit and to save a buck in the process. Now we have pumps and plumbing and fans and noise et all, and you could also preheat the incoming water by running the piping along the underside of the roof first too. And then,

  7. I have a Chromagen 170L hotwater heat pump in suburban Melbourne.Initial cost was under $1000 installed,it’s set to heat water from 10am till 5pm.It usually acomplishes that to 60.C by 1.30pm.The operation is quite and never intrudes on the neighbours or inside our home (the continuous barking of neighbourhood dogs is far more troubling.)
    During the last week or so of very cloudy weather our 4.59kW solar PV has easily handled the load of charging the hotwater and running a 3.5kW reverse cycle aircon at 23.C all day long. Total daily cost of running has been around $2 a day plus daily supply charge.
    We had gas hot water prior to the heat pump and I know which method I now prefer.

  8. Great article.

    We just posted it to My Efficient Electric Home (link below) where there is heaps of heat pump discussion going on as I write…


  9. I have a Bosch Compress 3000 heat pump that cost $1830 installed after STCs. This is only marginally more expensive than an equivalent resistive hot water system. So far it has worked flawlessly.

    It can run down to -7 degrees on heat pump mode only, but also has a backup 1600W element. But the best part is that the element can still heat the water if the heat pump unit is no longer serviceable.

    So, even if the heat pump dies just after its warranty expires, I can still use it as a conventional hot water system. For the marginal cost difference, it was a no brainer to choose the heat pump over a conventional heater. (My roof is full of panels so not able to add solar hot water, but even if I could, the competitive price of the heat pump wins out anyway).

    • Ronald Brakels says

      That’s a good point that is can be possible to run a heat pump hot water system as a conventional hot water system after its heat pump dies. Its 1.6 kilowatt element is small so it may not suit some households. It will take 8 hours for a 1.6 kilowatt element to heat its 270 liter tank from 20 degrees to 60 degrees. Fortunately the normal controlled load period is 8 hours, so, with heat losses, that would almost be enough to fully heat a tank in winter.

  10. I’ve recently replaced a 1.8kW, 120L electric storage tank with a 275L Hydrotherm heat pump system. We’ve more than doubled the amount of hot water we can make and store, using 10% less power and selling 10% extra back to the grid (those are Winter figures too, Summer will be much higher). Whilst that doesn’t sound a lot, imagine the same scenario if we had a 4kW electric storage tank instead! We run the Fronius smart relay so this is all under solar power except for a handful of times each year when the required 800W of excess solar doesn’t get generated throughout the day.

    I will say that I was concerned about all the bad reviews that heat pumps seem to get but then I found the Hydrotherm. They have some of the longest warranties (if not the longest), the customer service was amazing and I spoke to several plumbers about them and they all said the same thing – they all ran one, they install them for their family and friends and they don’t recommend anything else.

    Time will tell on the longevity of the system but for now, I’m extremely happy with the decision.

    • Hi Gavin
      Would you be able to update how your Hydrotherm hot water system is going after 2 years. I’m thinking of getting one, also which state are you in as we are in Vic.

  11. Thanks Ronald
    So in terms of using excess solar PV energy – am I correct that a Fronius relay (with smart meter) is able to help prevent the heat pump regularly cycling on and off during days where the solar PV panels are frequently tipping above and below the threshold power level? (eg. 2kW). Before I read your article I started wondering whether using the simple timer option would be a better option (despite being less efficient) if using a smart meter risked wearing out the heat pump due to regular cycling on winter days.

    • Ronald Brakels says

      That’s correct. And my understanding is you can set the relay to threshold level where it will switch on the heat pump can be lower than the level at which it turns off, so there can be a safety margin to prevent rapid cycling there.

      • FYI exact Fronius settings are:
        1. chose by *power production or * power surplus (in case of feed-in limits)
        2. thresholds on and off (in Watts)
        3. duration (minimum duration per on signal and maximum duration per day)
        4. desired duration per day (minutes) and is to finish by ##:## time

  12. I have a sanden eco plus (new model, COP of 5 (under test conditions)).

    I cannot hear it when it’s on. Fan points at solar panels to cool them down.

    They now have a 6 year warranty (15 years on the tank).

    I have set it via inbuilt timer to run in middle of the day when my PV is running at its peak. This also eliminates freezing likelihood.

    My experience to date is that it uses 1kW/hr for about 2 hours per day (slightly more in middle of winter, less in summer). In inner Melbourne, 315L tank. If I didn’t have PV, it would cost about 40c per day for the hot water. With PV, it’s free. And given the low usage (1kW vs 9.86kW DC PV system), it’s highly unlikely to ever draw power from grid.

    Only risk I can see is that it fails (and they do, I realise that) and will need repair. I take that risk for the ability to live with a very low power usage house (electric only, gas abolished).

    • Derek Harbison says

      The Sanden Eco® Plus Hot Water Heat Pump is different to most heat pumps as the refrigerant (CO2) is held under much higher pressure than other heat pumps. This produces some interesting results as the COP of the unit at 20 deg is 5. The unit becomes less efficient as the temperature rises above 25 deg which is not how other heat pumps respond to Australian conditions.

      This has consequences: if you are running a Sanden unit off your Solar PV system, you may want to look at WHEN the unit is running. As the optimal time of day to run the unit will be earlier in the day in Summer and may be in the afternoon in Winter, depending on where you live. To work this out, one should note the temperature (where the unit is located) through the day and monitor this over a few days. Then set the timer on the unit to correspond to match the optimal temperature conditions. The aim is to have the unit in the ‘sweet spot’ as often as possible to use the least power. You would still be mindful not to set the unit to run too early, as you must provide 1kw of power continuously for the unit to run as designed. It is a mix between when your solar system is ‘online’ and when the temperature is optimal. Then fit a two hour window around that.

  13. Richard Winter says

    Thanks for the great article Ronald.

    I’m considering installing a Chromagen 280 litre hot water heat pump in a new house build in Denmark, Western Australia and like the idea of connecting to a Fronius inverter and smart relay with premium PVs (funds permitting!). Is it possible for a heat pump to be connected directly to a DC PV system, or does it have to be AC managed through an inverter/relay such as the clever little Fronius? I want my heat pump to get my PV generated electricity rather than being ‘fed’ by the expensive grid. I’m mindful of your comment that diverters won’t work with hot heat pumps given their fancy electronics.

    • Ronald Brakels says

      Hello Richard

      Heat pump hot water systems need to run off AC. It is possible DC units exist, but I am not aware of any. The 280 liter Chromagen apparently uses less than 1 kilowatt when using its heat pump, so provided you have a large solar system one option would be to just use a timer and hope you’ll have enough surplus solar power to run it. If you don’t and it needs to use grid electricity, then it won’t use much. It does have a 3 kilowatt heating element that it will use when the air temperature drops below 5 degrees, but that rarely happens in WA, especially if you are running it during the day.

      I’m afraid its warranty isn’t fantastic, but I assume you are okay with that.

      • Bernard P says

        Can you point me to a reference for the 280L Chromagem using <1kW? All I could find was a reference to 2kW input power on their web site (actually 2.15kW in the specs document).

        • Ronald Brakels says

          Hello Bernard

          I probably got it from this brochure here:

          Which shows 1 kilowatt of electrical power draw by the heat pump produces 4 kilowatts or more of heat. But looking further I see that is misleading as further down it says the heat pump can only provide 3,000 watts of heating so the heat pump should only draw around 750 watts. But note the system can draw power for things other than the heat pump, such as deicing its coils and it also has a heating element for use in very cold temperatures when the heat pump may no longer be effective.

        • Richard Winter says

          Hi Bernard

          I haven’t seen any information on Chromagen running at 1KW, maybe Ronald meant the Sanden Eco pump? We ditched the Chromagen after hearing about their tendency to fall over close to the 5 year warranty; also chews up the power when the element comes on. We went with the Sanden Heat Pump EcoPLUS and included the water filter & calcium cartridge – its more expensive, quiter but should last us longer here in the south coast where water has a lot of calcium carbonate.

          • I just got the information that Sanden does not have any dealers in WA due to water quality. That’s out of the question than.

          • Hi Richard,
            I have a Chromagen now. It uses around 900W or so – indeed under the 1kW mark as Ronald was thinking. No problems, thus far, after about 9 or 10 months. I basically never see it drawing more than my 5kW system can produce anytime of the year (but see below), unless I’m running the oven, washing machine, etc simultaneously.

            Interestingly, its own controls let you set a timer operation, but the tank overrides this occasionally (I think to heat above 65 deg C to sterilise), sometimes in the middle of the night! But mostly it runs between 9.30-15.30 according to my program. When it does its overheat/sterlise thing, it appears to sometimes use the immersion heater, so then it can draw around 3.6kW, if I recall correctly, but only for short periods.

            In short, my tank is drawing something like 95% (my guesstimate) of its power off my panels and not the grid, which is more or less what was after.

            Here’s hoping the thing lasts a good many years…

          • Ooops! Massive mistake in my post. We got an Evo-Heat, not a Chromagen. My brain is obviously fried from all that good hot water we have now :/

  14. You get what you pay for. Sanden is more expensive than other brands but better quality. According to Sanden, their heat pump has an expected life of 15 years (warranty 6 years on heat pump 15 years on tank).

    You may not have heard of the company before but Sanden provide aircon to car manufacturers as well as refrigerated vending machines. Since 1943.

    Most complaints I saw when I was researching heat pumps 2 years ago came from cheaper brands.

    However, the technology has improved a lot too so heat pumps are more reliable than 5 years ago.

    I used Pathfinder plumbing in Brisbane. Neatest plumbing job I have ever seen. The guy knows his stuff. Highly recommend if you live in the Brisbane area.

  15. Richard Winter says

    Hi Dennis and Ronald

    I agree with you that the Sanden Eco (now PLUS!) heat pump offers longer warranties on its tanks and pumps compared to comparable brands. BUT not all things are equal. Take for instance Denmark, a coastal town in WA. Its water quality exceeds others in terms of shires and towns including Perth suburbs in terms of chloride (mean of 301.25 mg/L according to latest water authority ‘aesthetic’ data). This figure invalidates Sanden’s recommended water spec of chloride not to exceed 200 mg/L – so goodbye warranty! Albany Solar recommends the Chromagen Midea heat pump given these water conditions (anode lasts longer apparently; they ditched the Stiebel Elstrom because of corrosion issues). Guess it makes sense to also talk to local installers and plumbers when considering PVs and heat pumps as not all devices work across all conditions according to the product specs.

  16. Sorry if his wife s a rookie question, but is it possible to also use hot water produced by a heat pump to heat a house using wall mounted hydronic heating panels?

    • Ronald Brakels says

      Definitely possible, but quite expensive to set up. The lifespan of heat pumps is an issue, but if you are only using it for half the year that might stretch out its operating life.

  17. Tony Hardy says

    Would love to have a table / matrix of the available heat pumps in Australia, including COP, litre storage, $ cost, warranty, etc.

    I currently have the Sanden Heat Pump 315l since May 2015, I have it set to come on at around midday to use my solar pv. It’s very quiet and works fine in the middle of winter.

  18. David Morgan says

    I have a Sanden heat pump and solar panels, and am very happy with it after 2 years.
    It seems to me that there is an important argument missing here, especially when limited by suitable roof space. When there is more sunshine than needed with a straight solar hot water heater, the water reaches temperature early in the day, and then the excess solar radiation cannot be used, and must be dissipated somehow to avoid boiling. With solar-heat pump, once the water is “hot” the excess energy is sold to the grid. I have limited exposed roof space, so having fewer solar panels to make room for a solar water heater is not attractive.

  19. JOhn Denny says

    HI, I’m looking into getting a heat pump and there seems to be so many differing opinions out there that I don’t know what to think. We are looking at the Reclaim Heat Pump – does anyone have any experience with these? Cheers JOhn

  20. Richard Winter says

    Hi John

    We went with a Sanden heat pump watee heater with natural refrigerant (C02). Its got a high coefficient of performance of 4.5 and is quiet whwn the fan cones on (37 decibels). Our water quality is poor here in Denmark WA and Sanden insists on a filter being installed to take out salt. We have a UV filtration init from the rain tanks to help too. Check oit their warranty in your location and make sure your insatller is acctedited to fit thr pump and service it. Some plumbers havent a lue when it comes to thr filtration units.

  21. I have contacted Sanden regarding an installer and they said they don’t install in WA because of the water quality. Must be new. When did you get your unit?

    • Richard Winter says

      Sounds right. We got our Sanden pump October 2018 from Solargain in Albany. I believe they got it ordered from the eastern states given Sanden wont supply directly. I understand DSR Energy here in Denmark are Sandens accredited installers.

  22. Well, I am not in Denmark, I am in Goomalling. I guess there are enough other ones to choose from. I still have some time to do the research.

  23. Cam Muir says

    I have previously had a Sanden Heat Pump and have recently moved. I have noticed the Reclaim Heat Pump which looks very similar to the Sanden. Has anyone got a comparison on the too re performance/easy of use/price?

  24. Hi Ronald, we are building a grid-connected solar passive and active house in the countryside outside Canberra (therefore very cold in winter). PV panels will be installed on the ground on a hill and the number of panels is not constrained. Our FiT is likely to be small therefore our strategy will be to use as much power as possible while the sun is shining. Regarding hot water, we have options of putting a solar thermal system on the roof, installing a heat pump HWS or just using a cheaper twin element electrical HWS (on a timer or with a diverter). Any savings from the last option would be used to buy more PV panels. Any comments you have on the best strategy would be greatly appreciated. Comments from My Efficient Electric Home FB page contributors were varied but tended to favour a heat pump running on solar power when available. Thanks

    • Ronald Brakels says

      Installing more solar panels and using a standard hot water system should be the most cost effective option. As you will also be exporting any surplus from those panels it should also be the most environmentally sound approach. A “brute force” approach where you use a less efficient appliance but use the savings to generate more solar energy often works well given the current cost of solar, provided you are not constrained in how much solar capacity you can install.

      • Geoff Miell says

        You state:

        “Installing more solar panels and using a standard hot water system should be the most cost effective option. As you will also be exporting any surplus from those panels it should also be the most environmentally sound approach.”

        It would be interesting to see the numbers crunched (with stated assumptions) to compare the different scenarios. Perhaps a post could be worthwhile?

        Also, the “exporting surplus” into the grid may be constrained to a set maximum (e.g. 5 kW per phase). Would that have a significant influence on the results?

        • Ronald Brakels says

          If there are no constraints on solar capacity then a standard hot water system is a pretty clear winner as it might cost $1,000 vs. $5,000 for a good quality heat pump. And even then the standard hot water system is likely to last a lot longer. That $4,000 difference pays for a heck of a lot of extra solar panels. It’s worth it even if there are no solar exports.

          I could present this in an article, but I’m too busy with other stuff at the moment. I’m literally doing multiple hours of work every day at the moment. Oh, I have it tough!

          • Sheridan Roberts says

            Thanks Ronald and Geoff. I don’t think there are any constraints on what we can feed into the grid but I need to check that. Hopefully in the future we can use some of the surplus to charge an EV.

          • Vikki Utting says

            Hi Ronald
            I am a consumer and though I cannot understand your teck talk, you or someone seeing this may be able to help.
            We bought a pup obviously, not realising the issues with heat pumps. The Heat Trap Solar worked wonderfully for 17 months, but the heat pump just died. The 2-year warranty covered the pump, but not the labour.
            At $3.6K, we expected better, and when I asked whether the pumps normally give out after such a short time, the MD ‘assured’ me they should last 5 years, thus my feeling of being duped, as at well over $1K with every pump replacement, this turns out to be a very expensive system.
            Thus, next time the pump gives out we want to cut our losses and either put in a gas storage (our last went for 10 years without spending a cent on it) or another company’s heat pump.
            Can you recommend a ‘reliable’ heat pump manufacturer, or am I dreaming?
            With thanks.

          • Ronald Brakels says

            Sorry to hear about your problems. If you have been assured the heat pump should last 5 years by the by the company selling it then that is a guarantee that it will last 5 years regardless of what the written warranty says. You are protected by Australian consumer guarantees regardless of what you are told or what’s in the written warranty. I don’t think it’s reasonable to have to pay anything for repairs after just 17 months and I think it’s likely a consumer tribunal would agree with me.

            That said, Sanden is generally considered to provide reliable heat pumps. Hopefully there are other reliable heat pump hot water systems out there, but Sanden is the only one I’ve heard anything good about with regard to reliability.

            Rather than go back to gas, you may want to consider a standard solar hot water system plus rooftop solar. A simple timer that switches the hot water system on during the day can be cost effective.

      • Des Scahill says

        I’ve not had much success with heat pump hot water systems myself, after trying 3 of them. The last one worked, then karked it about 3 months after the warranty expired. That was after considerable research beforehand.

        As a rough guide, for 2 person family, somewhere between 3 and 4 kwh hrs a day seems to be needed to provide the needed quantity of hot water for dishwashing, laundry, showers etc. Although you might end up dropping that to 1 kwh a day with a heat pump, the extra cost of the heat pump tank vs a standard electric hot water system is hard to justify.

        Solar Hot water systems are over-priced too in my view. Evacuated tube systems are pretty efficient, but again, need to weigh up the economics

        So, first step is to try and work out the average daily kwh you’ll likely save if you get ‘latest and greatest’ HW system and weigh up cost saving vs that of simply installing ‘stock standard’ hot water system, preferably with a draw-down of less than 3600 watts

        • Ronald Brakels says

          The most economic option usually will be to get a standard hot water system and put the money saved towards more PV. If my parent’s hot water system died I’d just recommend another standard hot water tank as they simply do not use enough hot water in sub tropical Queensland for anything else to make economic sense. (They already have PV).

          Which brands of heat pump systems did you have trouble with? I know you may have just had bad luck and the manufacturer’s may have improved their products in the meantime, but I still think it’s useful information to know.

  25. I’m about to pull the pin an an iStore (SolarGain) heat pump. Our existing 16yo Vulcan 400L is rusting (anode) and has been like that for the past few years (but still going strong!).

    I’ve done the maths, and although not as expensive as Sanden (~$2400 for the iStore) – I think we will be better off in our situation.

    We will have a new 8.2kW inverter for two adults who are not at home M-F 9-5, so the heat pump will be a nice way to consume some excess power that we couldn’t sell back to the grid (5kW export).

    They haven’t been around long enough to generate too many reviews – but I found an average 4.8 – 4.6 out of 5 star reviews. Apparently it’s based on the EvoHeat model.

    Warranty is what you would expect:

    5yr parts and labour on tank 2yr
    parts and labour on refrigeration
    and electrical 1yr all other
    components ask me in 2-5 years time if I regret buying a heat pump (I hope I won’t!).

  26. Andrew Griffiths says

    Can an electric tankless instantaneous hot water system running of solar PV and Tesla powerwall 2 battery x2 be even better? It will certainly save more water?

    • Ronald Brakels says

      Hi Andrew

      The drawback of that is it will put wear and tear on the Tesla batteries and the cost of that will far outweigh energy losses from hot water storage. So economically it won’t work and since instant hot water heaters use resistance heating instead of heat exchangers it won’t come out ahead on efficiency either.

  27. Great website with heaps of useful information!

    I’ve got a heat pump hot water system and am looking to get a PV system installed this year. I’m considering getting a hot water timer or relay to help maximise self-consumption of the PV generated electricity.

    1) How long would you expect a typical hot water timer or relay to last? 5 years or more?
    2) Could using a timer or relay cause issues to the heat pump components by interrupting it part way through its cycle?
    3) My understanding is that most power from a PV system is produced between around 9AM-3PM. Is 6 hours enough run time for typical heat pump hot water systems to keep up with a typical household?

    Thanks in advance.

    • Ronald Brakels says

      Hi Brendo

      1) I’d be pretty disgusted if a timer didn’t last at least 10 years, but the warranties they come with often aren’t very impressive so that makes it seem as though their manufacturers aren’t very confident in them. On the bright side, failed timers aren’t something I can recall hearing people complain about.

      2) To avoid any possibility of problems I would recommend using the heat pump hot water system’s built in timer. If it doesn’t have a timer than I’d suggest checking with the manufacturer to be sure. (Or maybe getting a different heat pump as no timer makes it seem like it is lacking in functionality.)

      3) Generally speaking most household will have enough hot water if a storage hot water system is run from 9:00 to 15:00. (Although you might want to have it turn on at 10:00 when there will be more sun.) But it all depends on the size of the storage tank and hot water consumption. If you can switch it on when you start running out of hot water that can help get around the problem. If you find yourself running out of hot water in winter you can extend the heating period during cold months.

  28. Derek Harbison says

    3) The ability to refill the hot water tank is called it’s ‘refresh rate’. The Sanden heat pumps that have been mentioned in this post take around 4 hours at their longest to refresh. That means a stone cold tank that has been completely emptied of hot water will take about this long for a 315lt tank.

    I will often hear from plumbers installing these units report back that by the time they have completed installing the unit, and put their tools back in the truck they have hot water coming out of the unit: inside an hour when the unit is switched on. This does not mean the tank is full of hot water. It means that the hot water at the top of the unit is at operating temperature and the water below this is still being heated.

    These units use less than a kW at full clip so they should easily be able to operate even on a low solar day. They do not use a lot of power and are a very good fit for solar PV systems.

    • Geoff Miell says

      Derek Harbison,
      You state:
      “These units use less than a kW at full clip so they should easily be able to operate even on a low solar day.”

      It’s true the earlier Sanden Eco® model units use up to a maximum 1.0 kW of electrical power input.

      The newer Eco® Plus model units are rated to use up to 2.3 kW electrical (per spec) and output up to 6 kW of thermal heat into the water. I presume the 2.3 kW power draw only occurs at low ambient air temperatures for faster heat recovery (compared with the earlier model), and at higher ambient air temperatures the power draw is much less – the fan, water recirculation pump and compressor are apparently all inverter driven.

      On 25 Aug 2015, an inherited ageing, gas-fired, pilot-lit, storage tank hot water system was replaced by a Sanden Eco® split-system heat pump unit (HPU) coupled via insulated water recirculation copper tubes with a stainless 250 litre capacity hot water tank at my residence. This HPU model draws a maximum rated 1.0 kW and was programmed to switch on at 10am (Standard Time) to take advantage of the electricity generated by the (3.2 kW) rooftop solar-PV system as much as possible. Depending on hot water consumption and ambient air conditions:
      – in summer, the reheat run-time varies from about 1½ to 2¾ hours, and
      – in winter, from about 2¾ to 3¾ hours, to recharge the tank once per day.

      The system has frost protection to counter winter ambient overnight temperatures that have been as low as -12°C, more generally down to circa -5°C, and occasional snow. The HPU faces East, where it is exposed to the morning sun when starting-up. This unit has a Coefficient of Performance (CoP) of 4.5 at higher ambient air temperatures, but apparently drops to 2.76 at 0°C ambient air temperature. There’s no auxiliary heating.

      On 19 Feb 2020, my original HPU model failed to start the heating cycle and on the following day, was replaced under warranty (thankfully with no cost to me) with a later Eco® Plus model unit (coupled to the original piping and tank). The CoP at 32.45°C ambient air temperature and 18.74°C water inlet temperature is specified at 5.96, for the newer model.

      Hopefully, the new HPU model has a significantly longer service life and improved performance – we’ll see.

      For those considering it, the Eco® Plus model spec includes a 316-stainless steel tank with 10-year full replacement plus further 5-year pro-rata warranty (except WA) and the HPU has a warranty of 6 years, with stated operational design life of 15 years.

      In my circumstances, despite the recent HPU failure, Sanden have apparently stood by and supported their product. IMO, Sanden do make a well-made, high-performance product that they apparently fully support, and seem to be continually improving their products, but does require a premium price for it.

      I’d suggest that without the patronage of adopting early technologies, it’s unlikely solar-PV would have rapidly progressed to the large scale, higher efficiencies and lower costs that we see, enjoy and benefit from today. Patronage drives development and progress, leading to lower costs.

      I think it’s worthwhile supporting technologies that demonstrate affordable, significant improvements in energy efficiencies and that drastically reduce GHG emissions. Our futures depend on it.

  29. David Keightley says

    Hi Ronald,

    Your site and this thread is very educational, thank you to you and the other contributors. We have done something with our BRIM system to help address many of the issues that have been discussed here. I am an ex-CSIRO research engineer with matching industry experience in systems ranging from aerospace to cancer treatment, road accident statistical analysis, electronics and of some use oddly enough in this field, signal processing. (techie jack of all trades I guess). All fields though were reliability, simplicity and results are critical.

    We have taken our commercial scale smart sensing-control system and, with the guidance of a principal engineer from the power industry who now sells energy systems, PV in particular, scaled it to deliver straightforward control and optimisation for a residential customer. We have tested the reduced system on a Canberra residence and with excellent results are now expanding the test sites.

    Our approach has been to keep it simple where possible and to show that with smart consumption and a reasonable PV system, you can knock most of your electricity charges out with smart energy measurement and addressing the big ticket energy consuming devices. We use both control, analytics and feedback mechanisms to make this work.

    I was particularly interested to read the discussions on hot water systems, but also have enjoyed and hope to continue to enjoy your straight-forward analysis of the pros and cons and grey areas (geographic and other constraints) with energy and power systems. I have seen some very poorly installed systems where the owners have spent a significant amount of money only to find in some situations, after buying state of the art heat pumps, hydronic heat, etc, that their electricity bills went up, despite extending their PV power collection capability significantly.

    Thank you for the excellent site. If you have a little spare time. I’d be happy to discuss what we have done in straight forward feedback and control to demonstrable drive electricity use down in a residential setting. If not, no problem, I will continue to enjoy your site.

    Best regards,

    David Keightley
    Founder Ecospectral Pty Ltd

  30. As it is now December 2023, could we have an update of your views taking in to account the available products please.

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