Hot water storage helps maximise PV electricity use

Storing solar energy in the form of hot water is an effective and low-cost way of maximising the use of generated excess PV electricity.

Australia’s net-metering arrangements and despicably low feed-in-tariffs typically mean that the large amounts of excess PV electricity generated during the middle of the day – when residential demand is close to its lowest levels – effectively goes to waste, at least from the PV owner’s end. We pay in the range of $0.30–0.60 per kWh to retailers at peak times, either as blended or as time-of-use rates, while only getting back on average something on the lower end of $0.05–0.10 per kWh of PV electricity we export at any time of the day.

This means that if you manage to export the same amount of electricity back to the grid as what you consume within a metered time period, as a PV consumer on this arrangement, you will still pay the difference between the retail rate and the $0.05–0.10 feed-in-tariff per kWh consumed.

Perhaps this is not a fair ‘apples-for-apples’ or ‘electron-for-electron’ comparison but, to me, it still looks like a dodgy, one-sided deal. There’s very little consumers can do to enforce a swift change in this unfair pricing structure – I don’t expect current policy-makers to show any initiative on this front – so it’s no wonder retailers and energy providers are paying PV consumers as little as possible for the services they could potentially be providing to the grid.

There are, however, two simple things PV owners can do to work around this pricing imbalance: (i) shift loads to peak PV generation periods of the day; and (ii) store that excess energy created in peak generating periods for use later (e.g. in batteries). This article will describe ways in which excess PV electricity can be stored in the form of hot water using both of the above-mentioned strategies.

Make hot water while the sun shines

The basic idea is to transfer excess generated solar energy to an electric* water heater when onsite generation is higher than the load. This effectively displaces the need to heat water using higher-priced electricity from the grid and ensures that all the generated solar electricity is used.

A simple timer, programmed to turn on a water heater in the middle of the day when the sun shines at its brightest, can perform this function quite well. It’s simple, relatively easy to use but has the disadvantage of not being able to respond accordingly to changes in onsite electricity use. Some estimates have this set-up costing between $200–250 installed and paying itself back within 1-2 years for average Aussie homes.

There are also much more sophisticated programmable solar power diverters, such as the SolarImmersion and SunMate units, currently available in Australia. They are packed with control gear which senses the flow of electricity into and out of the switchboard and diverts any excess electricity to a water heater, with grid export as a last resort. These units are responsive to changes in the net load but are more expensive, coming in at a figure of $1,000–1,100 installed and likely paying back in approximately 3–5 years at current rates. This option tends to make sense only for PV households with quite high hot water needs or a well-integrated load monitoring and control system.

How much energy can a hot water tank hold?

The amount of heat that can be stored in a hot water tank is related to the heat capacity of water. The equation below outlines how to calculate the amount of heat stored in water at a specific temperature:

Hot water heating equation


–        Q is the amount of heat energy added/stored in the tank (Wh);

–        c is the specific heat of water (1.163 × 10-3 Wh/°C.g);

–        m is the mass of water (g); and

–        ΔT is the change in temperature of the water (~ 20°C to 60°C).

A large 400L tank can effectively store ~ 19kWh of USABLE energy, which is more than double the NOMINAL capacity of Tesla’s new daily-cycle 7kWh Powerwall unit. And if the hot water storage system is fitted with a tempering valve, the USABLE storage capacity of the 400L tank increases to ~ 33kWh with water stored at 90°C.

I’d say this is something also worth getting excited about. The bottom line is that most PV owners don’t realise that they already have a multi-kWh energy storage sitting at home right now! Waste not, want not.

* For consumers with gas water heating, an upgrade to a heat pump system is a high-efficiency option which may be worth considering.

Top Image Credit: Pipe Safe Plumbing

© 2015 Solar Choice Pty Ltd 

John Rodriguez


  1. We have 5 kw of panels and a “town gas” hot water system. We want to do what Susan Caves did. Currently we use 80 cents worth of gas per day but pay over a dollar per day supply charge. The HWS is a Rheem Stella 300 L and has never let us (3 adults) down. It is 16 years old now and we want a new electric unit to save at least $360.00 per year gas supply charge, before it dies from old age.

    Incidentally immerSUN seems to be out of business and I cant find the web page for Sunnymate.

    1. Hi Colin,

      Thanks for pointing out the out-of-date info in the article (immerSUN are indeed out of business). I’ve updated it to have links to the SunMate and SolarImmersion.

      Good to hear your HWS is serving you so well!

  2. I’m not very technically aware of hot water system electricals, but can offer my experience with the Sunnymate device that we had installed when we replaced our natural gas HWS about a year ago. We had previously installed a 4kW pv system (Hunter region NSW). The idea is that instead of sending excess generated energy out to the grid during the day, it is directed to the HWS, which is a standard household off-peak one. The Sunnymate does the job of preferentially using generated pv energy for other household consumption, but any excess is then directed to the HWS. Therefore, on a good sunny day, our hot water system requires little boosting at night. The disadvantage is that it seems the boosting we need cannot be done on our controlled load (‘off-peak’) circuit – apparently Ausgrid do not permit the controlled load and the normal circuits to be in anyway connected to each other (this came from the Ausgrid-certified electrician). Which means that any boosting is done at our normal electricity cost of about 23c /kWh or so, which is ok for us as I am more interested in avoiding use of coal-generated power. The Sunnymate is a new technology that I wanted to try, and I’m happy I did. I think it will be an important step towards more efficient use of domestic pv power.

  3. To put things simply,
    A timer based system for hot water element heating must take 3.6 kwatt of power whether the solar PV export is 3.6kwatt or less.
    If the export power is less, the difference is made up by using peak rate energy from the grid.

    The Immersun and Sunny Mate devices only use what is currently being exported from the PV system down to less than 100 watts. No peak rate power is needed to make up the difference.
    The Immersun and Sunny Mate pulse the available export energy into the element. So if there is only a small export value, very narrow voltage pulses are produced and if there is 3.6 kwatt available for export, the voltage pulses will be almost continuous.

  4. I agree with Neil’s point on the timer method- it can work but it only if done correctly. Some individuals just won’t be able to do it or simply won’t have the time to look into this option/set it up and make sure everything is working correctly. Not to mention you have to go out and buy a few tools for the timer method so you won’t save very much money right away.

  5. Having just installed a hybrid system, because I have batteries my energy provider in Queensland will pay nothing for the excess.

    Is there a way to divert PV output that would otherwise go to waste (zero FIT), through my storage hot water system on the basis of “It gets only what is available”?

    In effect, even though the system would normally draw something like 3kW until the thermostat shuts the power off, it can only have what would otherwise be exported (for example 1kW) but for a longer period.

    One of the problems I am starting to discover is many appliances are too power hungry. Is this because we generally want things too quickly i.e. to boil the kettle quicker?

  6. Hi John,
    I appreciate your regular educated blogs most recently a blog regarding the REAL method of comparing different storage options, COS. In this case awareness that the most cost effective storage is already in the home in many cases is great to see being spread , however on this particular subject that I have spent a lot of time on over the years I have to issue a warning with the philosophy of using a timer and diverting the PV to the hot water unit. there are 2 possible draw backs with this method mainly due to the need to remove the HW unit from the off peak tariff and add to a higher tariff during the day.

    1, many hot water units have a high power element, often 3.6kW if your PV array is only has a excess operation of say 1kW you will be using 2.6kW from the grid when this turns on and paying a much higher rate for this energy than if this was being used at off peak times. the difference on cost per kWh would be approx. 10-15c per so in effect if the off peak rate is at approx. 12c per kWh this would create a higher cost of water heating.
    2, if there is no solar generation on an given day/s due to weather or other factors the same issue arises you are now paying a much higher rate to heat that water than if it was heated at night time on an off peak tariff.

    Mathematically the timer method can work but it does require a lot of education of the user as to the available generated power to heat the water against the demand of the heating element and a willing to manually manage the system on a daily bases according to conditions.

    I believe the diverters you mention would be safer method to utilise the hot water tank as a method of storing your excess generated energy.

    thanks for the continuing blogs

Comments are closed.