Sizing a home battery What size solar battery do I need?

Work out your battery size in four steps

Solar battery sizing comes down to two key factors: the energy you use when the solar panels aren't producing, and the spare solar energy they make to fill the battery. Everything else is fine-tuning.

1. Find your evening and overnight use. Your smart meter app, or your supplier's, usually shows use by half hour. Add up roughly how much electricity you use from late afternoon until morning. If you only have the yearly figure on your electricity bill, divide it by 365 for your average daily consumption. Which? puts the average home's total at 8 to 10 kWh a day. Ofgem's typical figures from July 2026 are 1,600, 2,500 and 3,800 kWh a year for low, medium and high users: about 4.4, 6.8 and 10.4 kWh a day. A good share of that falls outside daylight hours.
2. Check your panels can fill it. On a sunny day, a 4 kWp system makes far more than a home uses in the day. In winter it may barely cover daytime use, so a battery only fills from solar in the brighter months. More on that below.
3. Allow for usable capacity. Batteries keep a reserve and never empty fully: the Energy Saving Trust says a system stops at a set level, maybe 20%. The share you can use is called the depth of discharge, so in that example you'd get about 80% of the total capacity. Compare models on the kWh you can actually use, not the label.
4. Think ahead. An electric car or heat pump can double your use. A modular battery you can add more storage to later avoids paying for storage capacity you don't need yet.

Common solar battery sizes by household

Solar battery capacity in UK homes is typically 5 to 15 kWh, according to Which?. Use the table as a starting point, then check it against your own numbers.

There's no single right size. It's the one that matches your use after dark and that your panels can fill.

Battery capacity and power: kWh and kW

Battery capacity, in kilowatt hours (kWh), is how much stored energy the battery holds. Its power rating, in kilowatts (kW), decides how much it can supply at once. If the oven, kettle and shower together draw more than the battery's output, the rest comes from the grid, even with a full battery. A home battery doesn't take a grid-connected house off grid: when it's empty, or demand is too high, the grid fills the gap. For busy households with high demand in the evening, a higher output can matter as much as extra capacity.

If you want backup power

A battery only runs the house in a power cut if the system includes extra equipment for it: see are solar batteries worth it? If backup matters to you, say so at the start and size for it. Decide what you'd want to keep running, such as the fridge, lights and broadband, and for how long. How long it lasts depends on those loads and how much charge is in the battery when the power goes.

Sizing for a cheap tariff and future energy needs

If you charge the battery cheaply overnight from grid electricity and run the house on it through the day, size it towards your daily electricity consumption rather than just the evening. That's where larger battery systems earn their keep, especially in winter when solar output is low. With the October 2026 price cap averaging 26.32p a unit, every unit you move to a cheaper overnight rate counts.

Plan for your future energy needs too. An electric car or heat pump adds use in the evening and overnight. Either size up now, or choose a modular system you can add batteries to later.

One thing to check: if a battery is also charged from the grid, your export supplier doesn't have to pay for electricity exported from it under the Smart Export Guarantee, though it may choose to. We work the sums out with your actual tariffs.

The limits on battery storage installed in a home

The Energy Saving Trust says batteries can go in garages, on external walls or in utility rooms, and a typical unit is roughly 100 x 60 x 25 cm. The safety standard for battery storage in homes, PAS 63100:2024, says they should go outdoors where practicable, and sets limits on how much can be installed:

• No more than 20 kWh in a single battery enclosure, so bigger systems use multiple units.
• No more than 40 kWh per home, or 80 kWh if it's outdoors or in a separate garage or outbuilding.
• Not in a loft, a bedroom or an escape route such as a landing or stairs.

Bigger batteries also count towards your grid connection. If your panels and battery together exceed 3.68 kW per phase, the network operator has to approve the system first. For home storage there's a fast track, which the Energy Networks Association says cuts the confirmation time from 45 days to 10 days or less.

Does a bigger battery cut energy bills more?