What Size Home Battery Do I Need? A UK Guide

Why battery size matters more than people think

Battery storage is sold in kilowatt-hours (kWh), and it’s tempting to assume more is always better. It isn’t. A battery that’s too small leaves you buying peak-rate grid electricity you could have stored; a battery that’s too big sits half-empty most nights, meaning you’ve paid for capacity you never actually use. The sweet spot is a battery sized around how much electricity you use in the evenings and overnight — the hours when your solar panels aren’t generating and grid power costs around 24p per unit.

Home batteries today are typically modular and stackable, anywhere from about 5 kWh up to 60 kWh for larger homes. Most UK households land somewhere in the 5–15 kWh range. Getting to the right number is less about guesswork and more about looking at three things: your daily consumption, your solar generation, and how you actually live.

Start with your evening and overnight usage

The job of a home battery is to shift electricity from when it’s cheap (or free, from your own solar) to when it’s expensive. So the number that matters most is how much power you draw when the sun’s down.

A rough way to estimate: take your annual electricity use from a recent bill in kWh, divide by 365 to get a daily average, then assume roughly half to two-thirds of that happens outside daylight hours. A typical home using around 3,500 kWh a year burns close to 9–10 kWh a day, with perhaps 5–7 kWh of that in the evening and overnight. That points to a battery in the 5–10 kWh bracket as a sensible starting point — enough to cover the peak-rate window without paying for capacity that never gets cycled.

Match it to your solar generation

If you’re pairing the battery with solar panels, the second question is how much surplus your array actually produces. There’s no point fitting a 15 kWh battery if your panels only ever generate 6 kWh of spare energy on a good day — the rest of the battery simply never fills from solar.

As a rule of thumb, a 460W-panel array of 10–12 panels on a decent roof can throw off a useful surplus in summer but far less in the depths of a North West winter. Sizing the battery a little ahead of your average summer surplus, rather than your single best day, usually gives the best year-round return. This is exactly why we don’t quote a battery size off the panel count alone.

Where a smart tariff changes the maths

If you’re on — or planning to move to — a smart tariff with cheap overnight rates, the sums shift. Now the battery isn’t just storing solar; it’s charging up on cheap off-peak electricity and discharging through the expensive peak hours, whether the sun shines or not. That makes a slightly larger battery worthwhile, because you can reliably fill it every night regardless of the weather. Households on this setup often justify 10 kWh or more where a solar-only home might stop at 5–7 kWh.

Don’t forget backup and future plans

Two things nudge people towards a larger battery. First, power-cut backup: if you want to keep essentials — or the whole house — running during an outage, you need enough stored capacity to see it through, which usually means sizing up. Second, future load: an EV on the drive, or a heat pump on the horizon, both add meaningful overnight demand. If either is likely within a few years, it’s often cheaper to size sensibly now than to add modules later.

Indicative costs

To give a feel for the numbers: a 6-panel array with a 10 kWh battery starts from around £6,500, 12 panels with 10 kWh from around £7,770, and 20 panels with 10 kWh from around £10,000. Because modern systems are stackable, adding a second battery module later is straightforward — but it’s rarely as cost-effective as specifying the right capacity at installation.

Getting the number right

There’s no universal “correct” battery size — only the right one for your usage, your roof and your tariff. Our free, no-obligation survey looks at your actual electricity bills, your generation potential and your plans, then recommends a capacity sized to pay its way rather than the biggest box we can sell you. If you’d rather not guess, that’s the place to start.