What size solar battery do I need?

The short answer

Aim for a battery that covers your evening and overnight use, and no bigger than your panels can fill. The Energy Saving Trust says a typical home system might be 10kWh, and Which? says new batteries usually range from about 5kWh to 15kWh. In our worked example using MCS figures, almost all of the benefit came from the first 5kWh to 10kWh of usable storage.

What battery size actually means

Battery capacity is measured in kilowatt hours (kWh): how much electricity it can hold. A 5kWh battery could, in theory, run a 1kW appliance for five hours. Two figures are worth knowing:

• Stated capacity. The headline number on the box.
• Usable capacity. What you can actually use. Which? says this is usually lower, because some energy is lost when charging and discharging, and most batteries are not designed to empty fully. The Energy Saving Trust says a system will typically stop drawing from the battery at a set level, perhaps 20% of its capacity, to protect its life.

MCS, the body that certifies installers, tells installers to use the usable capacity from the product datasheet when they estimate your savings. If the datasheet doesn’t give one, MCS guidance assumes 90% of the stated capacity can be used for every battery type except lead acid, where it is 50%. If part of the battery is kept back for power cuts, only the rest counts. So always compare quotes on usable kWh, not the headline figure.

Start with how much you use after dark

A battery earns its keep by storing spare daytime solar power and releasing it in the evening and overnight, when your panels make little or nothing. Which? suggests aiming for a battery that covers your evening and night-time use, then recharges when the sun comes up.

Which? says the average home uses between 8kWh and 10kWh of electricity a day. Your own figure is on your bill or in your supplier’s app. To get a feel for your after-dark share, look at a smart meter breakdown by half hour if you have one. A home that is empty all day and busy in the evening will lean more heavily on a battery than one where someone is in and running appliances while the panels are generating.

The Energy Saving Trust is clear that there is no exact formula, and that your installer should advise based on your needs. The point of doing your own rough sums is so you can tell whether a quote’s battery size makes sense.

Match the battery to your panels

A battery can only store what your panels have spare. Which? warns against buying a battery bigger than your solar panels can fill. Which? also notes you may still need grid electricity on the longest winter nights even with a well-matched battery.

Worked example: what each extra kWh buys you

MCS publishes lookup tables (guidance note MGD 003) that installers use to estimate how much of your solar power you will use yourself, with and without a battery. We took the table for a home that is in for half the day and uses 3,000kWh to 3,499kWh a year (about 8kWh to 10kWh a day, in line with the Which? average). We picked the row for panels making 3,600kWh to 3,899kWh a year, and used 3,750kWh for the sums.

How we worked it out: we multiplied 3,750kWh by each percentage in the MCS table and rounded to the nearest 10kWh.

The pattern is the useful part. The first 5kWh of usable storage lifts the solar you use yourself by about 1,310kWh a year. Going from 5kWh to 10kWh adds only about 340kWh more, and going from 10kWh to 15kWh adds about 40kWh. Under the Ofgem price cap for 1 October to 31 December 2026, the average electricity unit rate is 26.32p per kWh for people paying by Direct Debit. At that rate, those extra 340kWh are worth at most about £89 a year, and less once you take off the export payment you would otherwise have earned for that electricity.

Two cautions. The MCS tables date from 2019 and, as MCS says, they don’t include electric cars or electric heating. And they only count solar self-use, not charging from the grid on a cheap overnight tariff, which can change the sums.

When a bigger battery can make sense

• Cheap overnight tariffs. The Energy Saving Trust says a battery can be charged when a smart time of use tariff is cheap, then used when prices are high. Here the size you need depends on how much you use in the expensive hours, not just on your panels.
• Heat pumps. The Energy Saving Trust says a battery will save you more money running a heat pump than if it is only exporting extra solar energy.
• Electric heating. The Energy Saving Trust says that to do the same with regular electric heating, the battery would need to be three times as big, and three times as expensive.

If one of these applies, tell installers when you ask for quotes so they size the battery for it.

Cost, space and lifespan

• Cost. The Energy Saving Trust puts batteries at £1,500 to £10,000, with a 5kWh system around £4,600. Batteries pay 0% VAT until 31 March 2027, then 5%, according to GOV.UK. For more on prices, see what solar panels with a battery cost.
• Space. The Energy Saving Trust says a typical battery is around 100cm by 60cm by 25cm, a little taller and wider than a washing machine but much less deep. It can go in a garage, utility room or on an outside wall, depending on the model.
• Lifespan. The Energy Saving Trust says batteries typically last about 10 to 12 years, and Which? says many come with a 10-year warranty. Panels can last 25 years or more, so expect to replace the battery at least once.

Already have panels? Our guide to adding a battery to existing solar panels covers the AC and DC options. If you are still deciding whether a battery is worth it at all, read is solar battery storage worth it.

What to do next

1. Find your yearly electricity use on a recent bill, and roughly how much of it is in the evening and overnight.
2. Use our solar calculator to estimate what your roof could generate.
3. Get at least three quotes from MCS certified installers, as the Energy Saving Trust recommends, and ask each one for the usable capacity, the self-consumption estimate with and without the battery, and the warranty.

When you are ready, compare solar battery quotes from installers who cover your area.