Battery size calculator: how many kWh do you actually need?

Last reviewed: 5 September 2026. Default peak price is the Ofgem price cap for 1 October to 31 December 2026 (26.32p/kWh, direct debit, GB average). Every assumption is listed under the result. Nothing is sent anywhere: the whole calculation runs in your browser.

What your home actually uses

Annual electricity use (kWh)

It is printed on your annual statement. If you have no idea, the Ofgem typical domestic profile is around 2,700 kWh a year. Guessing badly here is the single biggest source of error, so it is worth digging out the real number.

Share used outside daylight hours 45% — someone home all day, daytime-heavy 55% — typical household 65% — out at work, everything happens evenings 75% — evenings only, plus overnight EV or heating

This is the part a battery can serve. Electricity you use at midday is either covered by solar or bought at the same price with or without a battery.

How you would charge it

Charging strategy Cheap overnight tariff (works with no solar at all) Surplus solar only

Peak/day price (p/kWh)

Off-peak price (p/kWh)

Daily solar surplus available to store (kWh)

On a time-of-use tariff the battery earns the difference between the two prices, every night, whether or not you own a single panel.

Cost, for the payback line

Installed cost per kWh (£)

Installers currently quote roughly £400–£700 per kWh on a 10 kWh system, VAT-free until 31 March 2027. See what home batteries cost in the UK and the VAT deadline.

Work out my size

How the sizing works

Battery sizing is usually done backwards. A salesperson asks how big your roof is, or how much you want to spend, and sells you the unit that fits. The number that actually determines whether a battery earns anything is much duller: how much electricity you use when the sun is not shining. Capacity beyond that sits idle, and idle capacity earns nothing while still costing full price.

So the calculation runs in this order:

1. Annual use ÷ 365 gives an average day.
2. The share you use outside daylight is the only part a battery can serve.
3. That figure is the usable energy you need per cycle.
4. Usable is not nameplate. We assume 90% usable depth of discharge and 90% round-trip efficiency, so nameplate capacity is the usable figure divided by 0.9, and you buy roughly 1.11 kWh from the grid for every 1 kWh you get back out.
5. Winter is not the design case. Sizing for the worst December night means buying capacity that is dead weight for eight months, so we size for the average day and accept that some winter evenings run out.

The assumptions, stated openly

Assumption	Value	Where it comes from
Peak electricity price	26.32p/kWh	Ofgem price cap, 1 Oct–31 Dec 2026, direct debit, GB average
Off-peak price	Your input, 7p default	Illustrative overnight rate. Check your own tariff
Usable depth of discharge	90%	Our own working assumption for a modern LFP unit
Round-trip efficiency	90%	Our own working assumption, AC-coupled system
Cycles per year	340	Our own assumption: not every night is a full cycle
Installed cost	Your input, £550/kWh default	Midpoint of the range installers quote

[VERIFICAR: the depth-of-discharge, efficiency and cycle-count figures are our own reasoned assumptions, not manufacturer-published averages for the UK market. We have not found a primary source that publishes a fleet average, and we are not going to dress an estimate up as one. Treat the output as an order of magnitude, not a quotation.]

What to do with the number

Round down, not up. If the calculator says 5.8 kWh, a 5 kWh unit will cycle fully almost every day; a 10 kWh unit will spend half its life half empty and cost nearly twice as much. The cheapest kilowatt-hour of storage is the one you never bought.

Then check the reasoning in how to size a battery properly and, if the numbers are marginal, read what happens after the VAT change before deciding when to buy.