What size solar battery do I need?

How do you work out what size battery you need?

Work out a solar battery size from four things, in this order. The Energy Saving Trust notes there is no exact formula and that your installer will be able to advise for your home [3]; these steps let you check that advice.

1. Your evening and night use. Your smart meter data or supplier app shows how much you use after the panels stop producing. Ofgem's typical figures below are for a whole day, not the evening.
2. What can refill it. Spare solar on a sunny day, or an off-peak window on a battery tariff. A battery bigger than your spare solar and off-peak charging can refill between them can sit partly empty.
3. Usable capacity and power. Compare usable kWh, the energy you can draw, and kW, how fast it charges and discharges [7][8].
4. Any backup reserve. Charge kept back for power cuts is not available for everyday use.

A worked example

Here is the method run for a made-up household. The inputs are round numbers chosen to show how the steps limit the size; they are not measured data and not a system design.

For context, Ofgem's typical figures below are for a whole day; only part of that falls after dark, which is why your own meter data matters more.

How much can solar fill a battery?

On average across a year, the batteries in MCS's worked examples added 3.3 to 4.4 kWh a day to the solar used at home, less than the capacity each used for solar, by our calculation [2]. MCS says its method can sense-check a size but was not designed as a battery sizing tool [2]. A battery can only shift the solar left over after daytime use, so a home that already uses much of its solar fills less of it.

5kWh vs 10kWh battery: which is better?

A 10 kWh battery is worth considering over a 5 kWh one if your home uses and can refill more than about 5 kWh of stored electricity in an evening, as in the worked example above. It can also suit a plan to export stored electricity at a peak rate [4]; otherwise the extra capacity can sit unused. At DESNZ's 2025-26 median cost per kWh of nominal capacity, a 10 kWh (nominal) battery costs more in total than a 5 kWh one but less per kWh, by our calculation [11]: prices are on solar battery costs, and whether either pays back is on is a solar battery worth it.

Solar battery sizes chart

Home batteries come in usable sizes from about 5 kWh to over 20 kWh, and several are modular, so you add units to go bigger. The chart shows usable capacity as each maker publishes it.

How big is a solar panel battery physically? The Energy Saving Trust puts a typical system at around 100 cm by 60 cm by 25 cm, slightly taller and wider than a washing machine but much less deep [3].

How long will a 10 kWh battery last?

A battery with 10 kWh usable running a steady 1 kW load lasts about 10 hours, by simple arithmetic: usable kWh divided by the load in kW. Losses and any backup reserve make the real figure shorter.

Power matters too. A battery that can discharge at 3.5 kW cannot run a 7 kW load on its own; the Huawei LUNA2000-7-S1, for example, is rated at 3.5 kW per module [9].

How much can a battery charge overnight?

A battery can fill overnight if its usable capacity divided by its charging power fits inside the off-peak window. On paper, each example below fills from empty in under three hours at its maximum charging power, which fits Octopus Flux's 2am to 5am window [4]. That is an idealised calculation, capacity divided by power before losses: real charging can be slower, depending on the inverter settings, the battery's own charging behaviour and what the house is using at the time. The Powerwall 3, at 2.7 hours on paper, has the least margin. Flux needs solar panels, a home battery, a smart meter and Octopus import and export tariffs [4]. Octopus Go gives five hours but needs an electric car charged at home [5], and Intelligent Octopus Go gives six but needs a compatible car or a supported smart charger [6].

Is a 10kW battery enough?

Check what "10kW" means first: kWh is how much a battery stores and kW is how fast it delivers it. A battery storing 10 kWh is about the Energy Saving Trust's typical home system [3]; a 10 kW power rating says nothing about storage. The battery cost guide explains the units on datasheets.

Should you keep a backup reserve?

A backup reserve keeps some charge for power cuts, which leaves less for daily use. Separately, the Energy Saving Trust says that when the battery gets down to a set level, "maybe 20% of the total storage capacity", the system stops drawing on it [3]. Check whether a datasheet's usable figure already allows for this before taking more off: Sigenergy and Huawei state their usable figures at 100 per cent depth of discharge [8][9]. Enphase says the 5 kWh usable figure of its IQ Battery 5P includes a 2 per cent safety-critical limit that protects the battery during a long grid outage, and that a further 3 per cent is kept for its electronics at night [10].