Battery capacity is where money quietly gets wasted. Underbuy and you're importing expensive evening electricity with an empty battery; overbuy and you've spent thousands on kilowatt-hours that sit unused most nights. The right size comes from one number most people never measure: what your home uses between sunset and sunrise.
Count the evening, not the day
A battery earns its keep by covering the hours when your panels aren't generating and grid electricity is at its dearest, around 26p per kWh under the July 2026 price cap. So the capacity you need is driven by your evening and overnight consumption, not your total daily usage.
A typical UK home uses 8 to 10 kWh per day, and roughly half to two thirds of that lands between 4pm and 8am. That puts most households' useful battery range at 5 to 10 kWh. If you have a smart meter, your supplier's app will show half-hourly usage; add up a few evenings and you have your number. Without one, a reasonable shortcut is 60 percent of your daily average.
A worked example
Take a four-bed semi using 11 kWh a day, with about 7 kWh of that after 4pm. A 9 to 10 kWh usable battery covers essentially every evening, charged from solar surplus in summer and cheap overnight electricity in winter. At a 19p spread between off-peak and peak rates, cycling most of that capacity daily is worth £550 to £850 a year, on a battery costing £4,500 to £6,500 installed. Payback lands around 6 to 9 years.
Now give the same house a 16 kWh battery instead. The extra 6 kWh only earns on the minority of days with enough surplus solar or unusually heavy evening use. The marginal capacity might add £100 a year against perhaps £2,000 of extra cost. That's the overbuying trap: the first kilowatt-hours pay back fast, the last ones barely pay back at all.
When a bigger battery genuinely pays
There are three situations where stretching beyond your evening usage makes sense. An EV charged at home shifts a large chunk of load into the cheap overnight window, and a bigger battery can serve the house all day on that cheap rate. A time-of-use export tariff changes the maths too, letting you charge cheaply overnight and sell back at a premium in the evening peak, where spare capacity becomes a small trading asset. And households planning a heat pump should size for the consumption they're about to have, not the consumption they had last year.
If none of those apply, be sceptical of quotes that jump straight to 13.5 kWh or more. It's a popular capacity because of one famous product, not because most homes need it.
Modular batteries hedge the decision
Several of the systems in our catalogue are modular: the FoxESS Energy Cube stacks from 4.32 kWh upwards, Sigenergy's SigenStor grows in similar steps, and Growatt's APX builds from 5 kWh modules. With these you can start at 5 to 10 kWh and add a module later if an EV or heat pump arrives, paying for capacity only when you have the load to use it.
Fixed-capacity units like the Tesla Powerwall 3 take the opposite approach: one 13.5 kWh size, expandable only in big jumps. Excellent hardware, but make sure the capacity fits your usage rather than the other way round.
Rules of thumb by household
As a starting point: a one-to-two-person flat or small terrace with 5 to 7 kWh daily usage suits 5 kWh of storage. A three-to-four-bed family home around 9 to 12 kWh a day suits 8 to 10 kWh. A large or all-electric home, or one with an EV, justifies 13 kWh and up. Treat these as openers for a conversation with your installer, not gospel.
Our battery finder tool does this arithmetic for you from your monthly bill and circumstances, and shows which products in our catalogue match the capacity it recommends. However you get there, insist that any quote states the usable capacity, not the gross number, since some batteries reserve 5 to 10 percent that you can never discharge.