“What size battery do I need?” is the question people arrive with, and the answer they usually get is a number pulled from the price list. Here is how to work it out from your own usage instead, using the anchors neutral sources actually publish — and why the biggest battery on the quote is rarely the right one.
Start with the two figures that decide it
A battery is worth exactly what it lets you avoid buying at the import rate. So the size question is really the overlap of two numbers:
- What you use when the panels are quiet — evening, overnight and early morning.
- What your panels can spare on an ordinary day, not on the best day in July.
If you have a smart meter, the first number is in your half-hourly data. If you do not, a year of bills and a sensible read of your routine gets close enough to size from.
For context, Which? puts the average home’s consumption at between 8 kWh and 10 kWh of electricity a day, and says new lithium-ion home batteries typically range from around 5 kWh to 15 kWh. The Energy Saving Trust, whose battery advice was updated on 19 August 2026, gives a typical home system of 10 kWh — and is refreshingly direct that “there’s no exact formula for working out what you need”.
The rule of thumb that actually holds
Which? states it as well as anyone: a battery should ideally cover your evening and night-time electricity use, ready to be charged again when the sun comes up — and you should check how much your solar panels can realistically produce before deciding.
Put the other way round: never buy more capacity than your array can fill. A 15 kWh battery behind a 3.5 kWp array in a shaded garden will spend most of the year part-empty, and the unused kilowatt-hours cost exactly as much as the used ones.
Why usable capacity is the number to compare
Quotes list nominal capacity. What you get is usable capacity, which is always lower — as Which? puts it, “usable capacity is usually less than stated capacity”, because batteries lose energy in charging and discharging and most are not designed to be fully discharged routinely.
Two batteries advertised at “10 kWh” can differ by more than a kilowatt-hour of real storage. Ask for usable kWh on every quote and compare those, alongside the round-trip losses: Tesla’s Powerwall 3 datasheet, for example, publishes 89% for solar → battery → home against 97.5% for solar used directly. Storing is never free.
Where the size question turns into a cost question
Capacity is the biggest single driver of price, which is why sizing is where most of the money is won or lost. The Energy Saving Trust’s national figures put home storage between £1,500 and £10,000, with a 5 kWh system around £4,600 — so a step up in capacity is rarely a rounding error.
Three things should move your number up rather than the salesperson’s enthusiasm: an EV charging at home, a heat pump, or a time-of-use tariff you intend to exploit overnight. The Energy Saving Trust makes the first two explicit, noting a battery saves more money running a heat pump than simply exporting surplus solar.
If none of those apply and your evenings are quiet, a smaller battery that cycles fully every day will beat a larger one that does not. Modular ranges make “start sensible, add later” a real option rather than a sales line, which is one reason we quote them often.
What we do at the survey
We look at your consumption pattern, your tariff, your roof’s realistic generation and whether backup matters to you, then size to the overlap — and we will say when the honest answer is a smaller system, or none. You can test the shape of it yourself first with our savings calculator.
For the wider picture, our solar battery storage hub covers how batteries pair with solar and what the grid paperwork involves, and the solar battery storage cost page explains what sits inside an installed price — including why online hardware prices look so much cheaper than a quote.
Sources: Energy Saving Trust, Battery storage (updated 19 August 2026); Which?, Solar panel battery storage (15 May 2026); Tesla, Powerwall 3 UK datasheet (2026). All read 25 September 2026.