Sunday, 3 March 2013

Should I use electricity from my PV panels to heat my hot water tank?

If you have PV panels, you want to make the best use of the free electricity you get from them and I have been asked several times if it is sensible to use it to heat the water in your hot water tank. Unfortunately there is no simple answer to this. You can't just turn the immersion heater on when the sun is shining, as I will explain shortly, so you need special equipment. This has a cost and the payback depends on how much spare energy you have and how much hot water you use. It is hard to get accurate estimates for these but I have explored some scenarios.

By the way, I will be estimating financial benefits based on the assumptions that you are a UK resident getting Feed in Tariffs for your panels - which means you get paid for the electricity you generate whether you use it or not. Although you get a little bit extra from the export tariff this is simply deemed to be 50% of what you generate and not measured. In this situation, if your aim is to save money the electricity you generate is free and you should use as much of it as you can.

I also make the assumption that you have a hot water tank to put some heat into it and this is normally heated with gas. If your goal is to save carbon emissions you should carry on using gas to heat your water, because by putting your spare electricity onto the grid you avoid the need to generate it from fossil fuel power stations. As far as heating your water is concerned a unit of energy is a unit of energy but the carbon emissions from gas are only a third that of electricity.

Mean solar PV yield and  household electricity use. The peak PV will be higher than this sometimes: up to 2500 W or a little more.
This chart shows how much power you could get from your solar panels in an average July day. It also shows an approximation of how much power an average household uses through the day. The overlap (darker) shows how much electricity you might save. Of the 14 kWh generated you might use 30%. You have cut your bills by a half.

But that was average household use. The actual use on any day won't look like that at all. Here is a sample of a real day, when the house is empty between about 8.30 and 6pm. There is very little electricity use during the day so all that free electricity goes onto the grid. The used fraction is only 13% and you have saved very little on your bills.

The picture is a bit different if you are in the house all day, perhaps at the weekend. I don't know exactly what this house was doing but there is higher power use all day - probably the TV is on or some computers - and you can see peaks which might be the kettle, washing machine, cooker, and so on. Now you can see that even in the middle of the day when sun is highest you are still drawing power from the grid for high loads. In this case the household is using 47% of the electricity generated.

Which appliances is it safe to use? Most kettles are 3 kW. Washing machines, dishwashers and tumble dryers may use 2 kW or more for some of the time, depending on the load. If you use a low temperature wash then there is much less heating involved.

If you don't have a full 4kWp system then the amount generated will be less. This means the high peak loads are even worse for you. This chart shows the same day with a 2kWp system. The good news is the use fraction is now 67%.

Now you can see why it is a bad idea to simply turn on your immersion heater when the sun is shining. Even with a 4 kWp system you will only get perhaps 2 kW at noon even in July, but your immersion heater probably takes 3 kW or even more. You can get controller systems which monitor the mains and connect to your immersion heater, supplying only just enough power to use your free electricity. I know of two products: Immersun and SolarCache+. I can't vouch for them personally as I don't know anyone who has them. I believe the costs is around £500.

To see if this is worth whole you have to estimate how much you will expect to save. This depends on two things
  • How much spare solar energy you have to put into the tank
  • How much you are currently using in hot water energy

The table below summarises what we have seen so far of how much spare energy you can expect to have in July. The solar power available is high through April, May, June, July and August and drops off in September. Let's say you have 6 months worth at the July level which is a little optimistic. If you have a 4 kWp system then you might have 10.7 kWh per day on average during that time - it depends hugely on your pattern of electricity use. The only way to be accurate about his is if you have an electricity monitor that logs your use through the day.

Table showing how much spare electricity could be used to heat your hot water on an average July day,
4kWp (13.8 kWh/day)2 kWp (6.9 kWh/day)
Weekday (demand 7.3 kWh)12.0 kWh5.2 kWh
Weekend day (demand 15 kWh)7.4 kWh2.3 kWh
Average day (demand 9.5 kWh)10.7 kWh4.4 kWh

The next question is how much energy you are using for hot water at the moment. In an earlier post (How to save on Hot Water) I reported that households are very variable - two person households could use anything from 30 - 120 litres/day. Let's assume you use 80 litres/day and you run your hot water tank at 60 C. That requires 4.2 kWh/day. Plus there will be losses from your hot water tank. In Combi boiler versus hot water tank I reported typical losses from a well lagged hot water tank as 800 kWh/year or 2.2 kWh/day: total 6.4 kWh/day. If you have an efficient condensing boiler you would be using around 7.1 kWh/day, with an older gas boiler it could be 9.1 kWh/day. You can probably supply this comfortably from the 4 kWp system but you won't get that much from the 2 kWp system.

How much do you pay for your gas? On my last bill it was around 8p/kWh (though its get cheaper when you use more). From the calculations below, a heating controller system costing £500 would pay back in just under 4 years with the larger system but it would be 8 years with the 2 kWp system.

Table showing savings from using the spare electricity to heat hot water, assuming a 70% efficient boiler

4kWp (13.8 kWh/day)2 kWp (6.9 kWh/day)
Heating gas saved9.1 kWh (limited by need)4.4 kWh (limited by availability)
Savings/day ££ 0.72£0.35
Savings over 6 months£131 £63

In summary:
  • If your aim is to save carbon emissions put your spare electricity onto the grid.
  • If your aim is to save money - your savings depend both on your hot water use and on how much spare electricity you have which in turn depends on the size of your system and how much of the day you are at home. Your situation may be very different from the scenarios I have used. However, if you have a 4 kWp system and you are out of the house on weekdays you could probably get all your hot water from the solar panels.
  • High power loads such as an immersion heater will be greater than your solar power even at noon in July so you must get a controller such as Immersun or SolarCache+.
Finally here is another thought: if you have solar panels and you like to make tea during the day you might get a 1 kW kettle. It will take longer to boil but you will use more of your free electricity.


PV Generation data from PV Watts (location London/Gatwick, array tilt 40 degrees)

Average electricity use from Household Electricity Survey A study of domestic electrical product usage (I read the numbers off the chart for average annual use and subtracted lighting so this is very rough).

Sample electricity use data from SN 6583 -One-Minute Resolution Domestic Electricity Use Data, 2008-2009 (I chose 2 sample days more or less at random).


  1. That is very interesting, someone I know was advised to get a immersion heater and controller to maximise her ROI. I wasn't convinced it was a sensible idea so it's very good to see someone has done the sums.

  2. I see that Germany is introducing an incentive for pv installations to have battery storage so that consumers can use more of their own electricity and reduce peak demand as well as daytime demand. They are subsidising 30% of the cost of the storage system.

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