What Size Solar Battery Do I Need?

Short answer: size a home battery to the electricity you use after the sun goes down, not to your total consumption. Ofgem's typical annual figures for a single-rate electricity meter, from 1 July 2026, are 1,600 kWh (low), 2,500 kWh (medium) and 3,800 kWh (high). If a third to a half of that is used in the evening, the daily amount to store works out at roughly 1.3 to 5.2 kWh across those three homes. In practice, installed batteries are larger than that: the average retrofit batteries in the 2025/26 official statistics were 5.14, 9.80 and 15.48 kWh across three size bands. We checked the sources on 8 October 2026.

Step 1: find your annual use

Use your bills or smart meter for the last 12 months. If you have no data, Ofgem's typical domestic consumption values are a starting point. From 1 July 2026 they are 1,600, 2,500 and 3,800 kWh a year for single-rate electricity, and 1,900, 3,400 and 6,100 kWh for multi-rate meters such as Economy 7 (Ofgem decision, May 2026). They were higher before, so older guides quoting 2,700 kWh are out of date. They also describe average homes: an electric vehicle or a heat pump adds load that these figures do not reflect.

Step 2: estimate the evening share

A battery can only supply the demand that happens when it discharges. Find how much of your use falls between the time solar output fades and the time you go to bed. A smart meter app or the half-hourly data from your supplier will show it. We do not have an independent figure for the average evening share, so the worked example below tries 30%, 40% and 50% as labelled assumptions.

Step 3: a worked calculation

Assumptions, all for illustration: annual use is the Ofgem single-rate figure; daily use is annual use divided by 365; the evening share is as stated; no allowance has been made for seasonal variation.

Read the table like this: a medium home using 40% of its electricity in the evening needs about 2.7 kWh of stored energy per day. That is the energy to be delivered, so the battery must offer at least that much usable capacity. Usable capacity can be lower than the headline kWh on the box, so ask for it in writing, and ask the installer to state it on the quote. Add some headroom for winter evenings and unusual days if you want the battery to cover most days.

Step 4: check against your solar

With solar panels, the battery only fills from the surplus left after the home has used what it needs in the day. A battery bigger than your typical surplus will not fill. Two things to check:

• Panel output in winter. A summer surplus can disappear in December. If your array is small, a smaller battery may be enough.
• Export tariff. If your export rate is close to your import price, shifting power earns little. The gap between the two decides the value, as explained in our cost and payback guide.

Without solar, a battery charges from the grid. Then the limit is your cheap-rate window and the tariff, covered in batteries without solar panels.

Step 5: compare with what the market sells

DESNZ's statistics for MCS-registered retrofit batteries in 2025/26 give total capacity divided by installs of 5.14 kWh (1 to 5.99 kWh band), 9.80 kWh (6 to 10.99 kWh) and 15.48 kWh (11 to 29.99 kWh) (DESNZ). Those averages are our arithmetic from the published totals, and they mean larger batteries are common. They do not show that a larger one is right for you. Mean price per kWh falls as size rises (£1,280, £870 and £670), so a bigger battery can look like better value per kWh while costing more in total and rarely being filled. The question is not the cheapest per kWh but the smallest battery that covers your evening use.

Questions to ask an installer about size

• What usable capacity does this model deliver, and what is the maximum power it can output at once? A battery that cannot supply your evening peak will leave the grid to make up the difference.
• What evening use did you assume for my home, and what meter data was it based on?
• Can the battery be expanded later if my use rises, for example with an electric vehicle or heat pump?
• Does the quote show the same capacity as the datasheet, in kWh usable, with the warranty terms stated?

Sizing also affects funding

The Warm Homes Loan Scheme caps a battery at £15,000 (scheme rules), and the Local Grant guidance says a battery is funded only where it drives bill savings and complements solar PV (DESNZ guidance). A council or lender will want a battery matched to the home, not the largest available. See the loan scheme guide and Local Grant guide. Check any installer you are considering with our MCS guide, and try the savings calculator.

Frequently asked questions