What you could save with a battery to go with your PV panels

The day is coming when batteries are cheap enough that you can get one to go with your solar electricity panels. With this you can use your own energy in the evening as well as during the day. There are several companies developing products, for example Tesla have been working with Solar City to trial solar panel/battery combinations and intend to bring out their own product soon [1]. CCL Components are working with SolarMax [2] and RWE have announced intentions to bring out a home solar storage product. Using a battery (and some clever control devices) you can store the excess energy from your panels during the day and use it through the evening. This saves you more money and is also better for the grid, because it helps to even out electricity demand through the day. I have used data from the HES[4] to see what difference this could make in a typical household.




In the UK, using more of your own power does not affect your feed in tariff payments because the export tariff is 'deemed' rather than measured. In other countries this is not the case. For example, most of the US has net metering which effectively means your meter runs backwards when you export to the grid and in Germany you are only paid for the power you export. However, under the UK Feed in Tariffs scheme you are paid for the power you generate whether you use it or not, and the additional payment for export is simply deemed to be half the amount you generate.

The chart below shows average household electricity demand and average power from a 4 kWp solar system and a battery in which you can usefully store 4 kWh. (Since you can't discharge most batteries fully without damaging them it will actually be a bit bigger than that.) Also I have assumed the battery is 80% efficient - so 1 kWh of charging energy gives you 0.8 kWh from the battery later.

Solar panels, 4kWp and battery 4kWh. Total generation is 12 kWh and use is 10 kWh. The line indicates power generated by the battery and the bars indicate power being used. Solar power data is from PVWatts[5] for London and the household energy consumption data comes from the HES [4]. Assumes battery efficiency is 80%.

In this chart, during the day, all of your electricity needs come from the solar panels and most of the extra (in the white area below the line) goes to charging the battery. During the evening you use this energy (blue) and it even lasts through some of the night. If you didn't have the panels you would only use the green bit which is 42% of what you have generated but with the battery too you can use 75%.

The usefulness of your battery depends on a number of factors and varies through the year. In the winter time you probably use most of your output directly anyway, as shown in this chart for January. At that time you get only a third as much from your panels but you use 40% more, mainly in the evening. Quite a lot of this is extra lighting. The battery isn't much help then.

Solar panels, 4kWp and battery 4kWh. Total generation is 3.8 kWh and use is 14 kWh.

In the summer time, however, with a big enough battery you can get all your power from your panels, although 4 kWh isn't quite enough for that. The chart below shows savings through the year.

Typical daily savings through the year with 4kWp solar panels and 4kWh battery. The thin line shows what your panels generate on an average day.

With this battery and this use pattern your electricity bill would be down to 1930 kWh/year from 2850 kWh without the battery. In financial terms you have saved £130/year [6]. A larger battery doesn't save much more - 5 kWh gets you only another 150 kWh (about £21).

In practice your savings will be more than this because without the battery you will find you use some power from the grid even during the day. For example, if you panels are delivering 2kW and you switch on a kettle taking 3kW then you will still end up drawing 1 kW from the grid, even though a few minutes later you have plenty of power to spare. The battery can supply the extra, as long as it has charge, so it helps to smooth out these peaks. There are examples of this in Should I use electricity from my panels to heat my hot water tank?

Battery salesmen will tell you that a typical household uses only 25% of their panel's electricity and with a battery storage system that could be 80%. I think the 80% is about right but the 25% is rather low. Unfortunately most households don't have the equipment to measure this factor - you need an export meter as well as a generation meter. If the salesmen are right, then with a 4 kWp system giving you 3400 kWh generated per year you could save 1870 kWh/year, about £260 worth.

The charts and analysis I have shown here are based on average electricity use and average weather. If you use less electricity, your savings could be less too. You could get away with a smaller battery, but this may not reduce the overall system cost that much. (I don't have actual prices yet.) Also, if you have a smaller PV system then your savings will be less.

While you are saving money you are also helping the grid by smoothing out the loads, though you are not reducing the peak loads in the winter. This means we still need the same amount of generating capacity overall, and the grid has to be strong enough to deliver that capacity. You are helping to reduce the generation costs because power stations run more efficiently when they aren't having to ramp up and down all the time - but you aren't making any difference to the network costs. At the moment, about a quarter of your bill is for the network and the wholesale electricity cost is a bit less than half [7].

This post was edited on 7/May to add the paragraph on battery salesmen quoting 25% as the usual self-consumption rate.

[1] Tesla gears up for charge into home battery storage market (PV Magazine) Feb 2015
[2] SolarMax and CCL begin UK Partnership (CCL Components)
[3] Solar Energy Storage(RWE)
[4] Household Electricity Survey (DECC)
[5] PVWatts version 1
[6] assuming 14p/kWh
[7] Understanding electricity bills (OFGEM)