Sizing a Home Battery From Your Own Data: Evening and Overnight Use, Not Averages
- Ofgem puts a typical British household at about 2,900 kWh of electricity a year, around 8 kWh a day.
- The part a battery can serve is the share used when solar is not generating, or when power is dear.
- The Energy Saving Trust says there is no exact formula and the installer should advise.
- Planned EVs and heat pumps should be in the sizing, not added later.
- Compare usable capacity, not nominal capacity.
A battery sized from an average household is a guess. Your own half-hourly data shows how much you use in the evening and overnight, and that window, not your total bill, is what a battery can serve.
Why averages mislead
Ofgem estimates that a typical household in Britain uses 2,900 kWh of electricity a year (Ofgem). Divided by 365, that is about 8 kWh a day. But a battery cannot serve all of it. Whatever you use while the sun is on your panels, or while power is cheap, does not need storing. The useful number is how much you use in the hours the battery is meant to cover.
The Energy Saving Trust says size and cost depend on your current use and the size of any generation equipment, and that planned purchases such as an electric vehicle or heat pump should inform sizing (Energy Saving Trust). It also says there is no exact formula and your installer should advise (Energy Saving Trust).
Find your half-hourly data
If you have a smart meter, your supplier's app or website can usually show use in half-hour steps. Ask for at least a few weeks, ideally including a cold spell. Our guide to smart meters and half-hourly settlement explains why that data exists. Without a smart meter, you can read the meter at night and in the morning on several days to estimate overnight use, though it is cruder.
Work out the window the battery covers
Decide what the battery is for. With solar, it is the period from when generation falls away until the next morning. On a time of use tariff, it is the expensive window, plus any use you want to shift out of the peak. Add up your use across that window for each day, then look at the typical and the heavy days, not only the average.
Mark the window
Choose the hours the battery should cover, for example early evening to the next morning, or the peak-rate period on your tariff.
Add up each day
Total the kWh used in that window for each day of the sample. Keep the totals in a list.
Look at the spread
Note the middle value and the heavy days. A battery sized to the heaviest day will sit under-used most of the time.
Add planned loads
If you plan an EV or heat pump, add their expected use to the window rather than waiting until after the install.
Convert to usable capacity
Compare your window total with the usable capacity on the datasheet. Our battery capacity guides show typical sizes.
Illustration and limits
As an illustration only: if your typical evening and overnight use is 5 kWh and a battery lists 5 kWh of usable capacity, it would cover that window on a typical day, with the heavy days partly covered. Larger batteries cost more but only help on days when you use more. The Energy Saving Trust adds that running electric heating instead of a heat pump would need a battery about three times as large, which shows how much heating choices change the result.
Overnight use is also the hard ceiling on what a battery can save. A small, steady base load means a small battery is enough. Use the calculator to test your own numbers with conservative assumptions.
Common questions
Is a bigger battery always better?
No. Extra capacity only helps on days you use enough to fill it. Size to your typical window and check the heavy days.
What if I do not have a smart meter?
You can read the meter at set times over several days. It is less precise, so leave more margin and ask the installer how they sized it.
Should I size for an EV I might buy?
If a purchase is planned, include it, as the Energy Saving Trust advises. If it is only a maybe, ask whether the system can be expanded later.
Related guides
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