Get more out of your solar power system by using water as a battery

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Use your solar photovoltaic panels to heat your water too, and you could cut the amount of excess electricity you give away cheaply to the grid.

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The Conversation

Use your solar photovoltaic panels to heat your water too, and you could cut the amount of excess electricity you give away cheaply to the grid. zstock/


When the sun shines at its brightest, many of Australia’s 1.3 million homes with rooftop photovoltaic (PV) solar panels generate more electricity than they use. When this happens, excess electricity is exported to the grid.

In the old days (say, about 2009), most householders would be pretty happy with this, given that in Victoria, for example, it would have earned them up to 60 cents per kilowatt hour exported.

But these premium feed-in tariffs have now been slashed to as low as 6 cents or less. Meanwhile, householders might pay 30 cents per kilowatt hour or more to buy electricity from the grid when their solar panels are dormant in the evening.

Unsurprisingly, many solar panel owners are wondering how they can avoid giving electricity away cheaply to retailers. Maximising the use of electricity in the home while the sun shines is one strategy, but there are only so many times each day you need to run the dishwasher!

The alternative is to try and store energy for later use, by buying either a home battery system or an electric car. However these can be pricey, and on pure economic terms this tactic may not yet add up. There is, however, an economic energy storage option that many householders may not have considered.

Water works

A major energy demand in any home (often around 20%) is for hot water. Most Australian homes have gas-fired, electric-resistive, or rooftop solar thermal water heaters. But gaining in popularity, given its now favourable economics and lower environmental footprint, is the hot water heat pump.

Hot water and space-heating heat pumps work like fridges and air conditioners. They use a refrigeration cycle to pump heat “uphill”, from a colder zone to a warmer one. The benefit we enjoy with fridges and air conditioners is the cooling that occurs when heat is pumped from somewhere we don’t want it (such as from our lounge room on a hot summer’s day) to somewhere else (the air outside). For hot water heat pumps, it’s the other way around – we want to gather and use the ambient heat that is always present in the air outside our homes – the way that a reverse-cycle air conditioner does when it is on heating mode.

The best hot water heat pumps can capture up to 3.5 times as much heat energy from the air outside as they use in electrical energy, meaning that the resulting tank of hot water represents a net gain of free renewable energy.

Already in Australia, more than 184,000 homes have hot water heat pumps, which are eligible for renewable energy certificates.

Making hot water stack up

So, on to the economics for the homeowner. If you currently heat your water using gas, and you already have solar panels that generate excess electricity that you sell back to the grid for little benefit, a hot water heat pump could be a very worthwhile investment – especially if your current hot water system is due for replacement or has a poor energy star rating.

Tower of power: a hot water heat pump can capture more energy than it uses. R. Keech, Author provided

Having bought a hot water heat pump, your best strategy would be to activate it in the middle of the day when the sun is strongest. The system will heat water and store it in a tank for later use, not unlike how electricity is stored in a battery.

Now, rather than selling excess solar power back to the grid for a pittance, you are using that electricity to heat water more cheaply than you can by using gas or using electricity after dark. This change could boost the value of your excess electricity by at least five times.

How are these economics possible? As occurred with electricity, the price of gas is going up. According to my most recent gas bill, I now pay the equivalent of 8 cents per kilowatt hour (or A$22 per gigajoule, in gas industry terms).

Gas now costs more (per equivalent unit of energy) than what electricity retailers will pay you for excess solar power. More importantly, when we consider that a gas hot water heater might only be 70% efficient at transferring the heat of the burned gas to the water, whereas a heat pump can effectively be 450% efficient when capturing renewable heat from the outside air, this easily tips the balance.

How much spare solar electricity do I need?

One important question is how much excess solar power is needed to supply a home with hot water. According to a recent report by the Alternative Technology Association and earlier work by Beyond Zero Emissions and the University of Melbourne Energy Institute, the answer is around 3.5 kilowatt hours per day, for a typically sized home.

In many Australian locations, a 1 kilowatt solar system would be enough to provide this amount of electricity and heat your water. However, given that the average new rooftop solar system is now around 4.5 kilowatts, there may well be plenty of excess solar electricity generating capacity available on many homes.

Here’s a bottom line: instead of paying around A$400 per year for gas to heat water, someone with a large rooftop solar system might instead use their excess home-generated electricity, which their retailer values at just A$80 per year. The result is that the homeowner is better off by A$320 per year.

This analysis ignores the up-front cost to replace a gas hot water heater with a hot water heat pump. But before your hot water system is on its last legs, it would pay to give this idea some thought.

The ConversationSource: The Conversation. Reproduced with permission.

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  1. finn 4 years ago

    Yes – we are getting heaps of emails from people who have the same idea 🙂

    I’ve just published this post which walks through the technical options for diverting PV to your hot water (and the costs involved):

    • Andrew 4 years ago

      Nice clear article on your site on solar hot water diversion products. A three year return is really good. Unfortunately most families can’t afford that sort of outlay.
      A while ago on whirlpool I found 2 seperate people working on wireless monitoring of excess solar piwer to divert that power to heat hot water.
      I hope one or both of them can bring out an affordable product soon.
      I think a price of 500 or less would bring in many more customers.
      PS. For those of you with solar hot water North of Vic, you can turn off your booster for most if not all of the year.

      • finn 4 years ago

        Agreed – it should not cost a consumer $1,000 to install an intelligent hot water diverter. As soon as someone hits the $500 installed price point they’ll be laughing. It would be an easy upsell for PV reatailers

  2. Simon Moss 4 years ago

    There are also some threads on Whirlpool Forums talking about the Immersun device including someone who is using it in Australia. Use immersun as the search term.

    • paul tait 4 years ago

      People who thought they were getting free electricity by spending a lot of money to install P.V panels, find they are paying Origin 15c per Kw. (they get 6c and pay 21c). This is not only unfair, it doesn’t make sense.

      • Sim 4 years ago

        Look at it like this. All the solar development scared the whatever out of the utilities companies and they have to go to every length to try to stop it. When they do this they annoy everyone and people look for ways to deal with it like battery systems.

  3. wideEyedPupil 4 years ago

    Thanks for the article Tim. Isn’t one of the real problems that the networks are carving out what are effectively transaction fees of 50% sale price on transfer of energy both in and out of homes? So the 6 cents we get is a lot less than what the retailer pays before they on sell the power for 30 cents. The grid could and should be a great asset to us, but seems captured, is it time for the community to buy back the grids?

  4. Tim Forcey 4 years ago

    Thanks for the info and links in these comments! Tim Forcey

  5. Dave F 4 years ago

    Thanks Tim, nice article and this obviously makes economic sense to anyone in my situation (I already have PV but I’m about to lose my Victorian TFiT).

    Unfortunately I can’t get past I the lack of ‘exergetic’ sense – relative to PV, a solar (thermal) water heater is the technical equivalent of putting a hose in the sun. Surely solar hot water is the better option from the perspective of complexity, efficiency, resource use and exergy!

    Any thoughts on how they stack up financially against PV+heat pumps? (and the million dollar question: why?)

    • Farmer Dave 4 years ago

      Hi Dave, I agree that using PV to heat water via resistive heating or via a heat pump makes no sense thermodynamically: one needs low grade heat and given a choice between collecting low grade heat in an elegant device like a set of evacuated tubes or collecting photons, converting them to electricity in a high tech PV system and finally converting that energy back to low grade heat, it should be a no-brainer to just collect the low grade heat.

      It seems that the trend to heat water using PV generated electricity has two causes: (1) people have excess PV generated electricity for which they are getting a miserly feed-in tariff, or (2) solar hot water is simply too expensive. I think cause (2) has a lot going for it: when I last looked at prices a brand name 30 evacuated tube set on stand with circulation pump, temperature sensors, cost about $5000, while equivalent equipment imported from China by a grey market importer costs about $1500. The main difference I can tell is that the brand name equipment has the Australian watermark standards compliance, while the grey market alternative complies with European standards. Wouldn’t it be nice if Greg Hunt got excited about the high cost of solar hot water in Australia?

      • Sim 4 years ago

        solar thermal hot water systems cost up to Aus$5000 installed( On the roof). Heat pump hot water systems cost between Aus$2000 to $2500 installed. It you already have solar panels may as well heat the water during the daytime.

    • solarguy 3 years ago

      It is Dave. See my post at top.

  6. Tim Forcey 4 years ago

    Hi Dave F: Thanks for the comment and question (and there are lots more comments and questions where this article originally appeared at The Conversation.

    Beyond Zero Emissions in the Zero Carbon Australia Buildings Plan (supported by the University of Melbourne Energy Institute) analysed solar thermal hot water vs PV and heat pumps. In regions like the NT, solar thermal won out. In Victoria, PV and heat pumps were favoured. The Alternative Technology Association’s (ATA) “efficient hot water buyers guide” refers to the BZE work and also info from the Clean Energy Council and elsewhere (See “Renew” magazine Issue 129 from Oct – Dec 2014 (pretty current!) or ATA or “Renew” magazine website.

    It comes down to how much boosting (from gas or electricity) is required with the best solar thermal systems. In the NT you won’t need much boosting, whereas in Victoria you will need a fair bit of boosting on certain winter days. The good heat pumps these days will keep heating water and capturing some heat from the outside air (perhaps not as effectively) even when that outside air is rather cold (such as even at night in winter).

    There is an argument also for those with limited good roof space, that if you devote that limited space to solar thermal you can’t use the same roofspace for PV. (Well, there are those hybrid thermal + PV systems that may become more common one day.)

    To get best performance say from a evacuated tube solar thermal system, it may help to mount it at an angle higher than what your roof presents.

    There is another argument about how solar thermal systems sometimes have to “dump” hot water when it gets too hot. Something to look out for / ask vendors about.

    Richard Keech makes the point that you can look at what level of renewable energy certificates a system can earn for one measure of comparison.

    Of course doing what you can to minimise hot water use (excellent shower heads, wash laundry in cold water, insulate your hot water pipes well, set the water heater temperature at the right spot) reduces the amount of energy that you have to devote to heating water.

    Is this enough info for you or not enough? If not enough it points out that possibly more analysis, publication, communication is needed about these options!


    • Dave F 4 years ago


      thanks for your response – you’ve given a great description of the current market situation.

      I used to run one of the labs which undertook compliance testing of both solar and heat pump water heaters, so I’m intimately familiar with their technical benefits and limitations.

      My central point is that the cost of solar water heaters is too high!

      If you’re working with limited roofspace, a solar thermal collector will operate at 5-7 TIMES the efficiency of PV. I’d have said that thermal is the obvious solution in this case.

      The overheating/dumping issue is a problem, but one that is addressed by good design.

      For systems that need a lot of boosting (e.g. in Vic or Tas), there are new products emerging on the market that combine heat pumps with solar water heaters to give you the best of both worlds (and they don’t cost much more than a regular solar water heater).

      In response to Farmer Dave: it is true that the more stringent Australian Standards add to the cost of products in our market. However, having seen first-hand the failures of some imported products when measured against our requirements (e.g. metals leaching into the water, bursting pressure vessels and fittings), I would never recommend looking towards grey imports!

      I like to hope that we are on the cusp of a technological leap in solar thermal. I’m no longer working in this area – I don’t have any idea what this leap might be – but I think the solar thermal industry ought to have a much stronger foothold (globally).

      Costs need to come down, or the “simple and elegant” option will soon become obsolete. What kind of analysis, publication, communication or support is needed to make this happen?

      • Raahul Kumar 4 years ago

        Zhonghua has already mandated solar hot water heaters everywhere. It’s simple, the Government has to mandate solar hot water heaters and it will happen.

        And a lot of countries are doing exactly that, so I do expect the solar thermal industry to bounce back on the back of solar hot water manufacturers from Beijing.

      • nyerr 4 years ago

        High on cost and low on returns – would can such hybrid systems be really ideal for the 1.3 million UK homes? The thermal hot water stack system might be a little too much for a regular UK home perhaps. How about an alternate system involving a power switching device like solarimmersion and 2 immersion tanks or 1 immersion tank plus underfloor heating?

    • Raahul Kumar 4 years ago

      Excellent post. I would say that the hybrid thermal + PV systems are the best way to go. It would also be smarter to mandate that these be installed as Building Integrated Photovoltaic from the start in all new homes, rather than retrofitted as an add on.

      That would slash costs considerably, via mass production and replacing existing infrastructure in a home, like the shingles on the roof with solar shingles.

  7. Barri Mundee 4 years ago

    Speaking of batteries, another encouraging development is achieving better efficiency and reliability is reported here:

  8. Miles Harding 4 years ago

    I was about to say “here we go with the great Australia Ripoff”, except that the pricing on the heat pump units has become a lot more reasonable as of late. It would seem that most are $2995.00 for 300 litres.

    There are some interesting facts to be gleaned from the spec sheets…
    The Stiebel 300e data sheet tells me the following:

    Current draw is about 2.3 amps (about 550 watts)
    The COP (Coefficient of Performance) varies:
    about 1.5 at 0C air temp to
    3.0 at 15C air temp to
    5.0 at 35C.
    This means that the water heating power is about 770 watt at 0C and about 1500 watts at 15C. It pays to run during the warm part of the day.

    The amount of energy needed to heat 300 L of water form 15C to 60C is approximately 16kwh, so the heater is going to take 10 hours to heat the water from cold on a average day.

    Of relevance here is the power draw and likely run time. For an average day, the run time on the heater (compressor) is likely to be 5 to 6 hours at 550 watts, or about 3kwh of electricity. Definitely the best time to run is during the warm part of the day and a timer will suffice to start the heater at some reasonable time, say 10 AM.

    For an off-grid application, the power draw and run time is very beneficial, the battery probably won’t participate in the heating operation and the modest load on the inverter is during a typically low period of the day.

    It may be time to turn the evacuated tube collector into a solar cooker and fill the roof space with few more panels to run the water heater. I estimate the area needed will be similar to the existing tube heater.

  9. Roger Brown 4 years ago

    My old gas heater at the time was using a 45kg bottle per month($50) for heating water / stove and a gas heater in winter (Qld). When it died , I purchashed a SolarHart Hot water system (frost model) in about 1994-95 , today its still going , with only 2 Hot water valves being replaced(self) and its the same things that need replacing in any hot water system. Very rarely do I put the electric heater on . Has been cleaned twice in its life , other than rain fall. Longterm , they do pay for themselves , many times over . I think its a good idea for people trapped with only getting 6-8 cents a Kwh to do this and heat your own water . More than 1 way to skin a retailer .

  10. solarguy 3 years ago

    Tim, I have some interesting facts on using PV to run a Heat Pump for heating water. Firstly 1kw of PV would not be enough to run a HW HP as the losses would bring it’s output down by up to 20%, 800w. A Sanden and some other brands of HW HP have a 1kw compressor input. So an additional 1.5kw of PV would be a better choice, however this would require 9.9 sqm of roof area. Where an Evacuated tube collector would need only 4.66 sqm.
    As PV output is much more affected on cloudy days, (anything up to 75% less output) compared to an ET collector which is able to produce usable heat on such days, even in Melbourne if correctly sized for the load. And yes there are days that even ET won’t work, just not many.
    Cost comparison Sanden 315lt HW HP $4,800 + an extra 1.5kw PV over planned array size $2,500 = $7,300.
    ET SHW system with 315lt stainless tank $4,579 which get 39 STC’S v’s Sanden HW HP 32 STC’s. A saving of $2,721 and far less in running cost.
    ET SHW system performance isn’t effected by ambient temperature, where the COP of a HW HP is. Better to use PV for other loads, like A/C as it takes less energy to heat air than to heat water.

  11. MaxG 3 years ago

    The only way to get rid of the price gougers is by getting rid of them — otherwise it will always be loose for the customer.

  12. Tim Forcey 1 year ago

    For more discussion, see the group My Efficient Electric Home on Facebook, now with more than 2,000 members. New members welcome!

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