UK Solar Battery Sizing Calculator
How much battery storage do you actually need? Enter your daily electricity use and solar array size and get a tailored kWh recommendation based on average UK generation.
Your home
Recommended battery
Why size a battery at all?
A solar battery stores the electricity your panels generate during the day so you can use it in the evening instead of buying it from the grid. In the UK this matters because the price gap is large: imported electricity costs many times more than what you are paid for exported solar under the Smart Export Guarantee (SEG). Every kWh you store and use yourself is a kWh you don't buy at peak prices — that is where the savings come from.
Time-of-use tariffs make batteries even better
Octopus-style time-of-use tariffs (such as Octopus Go or Agile) charge much less overnight than during the evening peak. With a battery and a compatible inverter you can charge the battery cheaply overnight and run the house on stored power through the expensive peak window — even on cloudy days or in winter when solar output is low. You don't need to quote exact pence-per-kWh figures for the maths to be compelling: the peak-to-off-peak gap alone usually justifies a battery.
How the calculator works
- Average daily generation = array kWp × annual yield ÷ 365. This is a yearly average — summer days produce far more, winter days far less.
- Daytime use is assumed at 40% of daily consumption (people at work use less during the day; adjust your daily figure if you work from home).
- Excess solar = generation minus daytime use — what is actually available to charge a battery.
- Night-time need = total daily use minus daytime use — what you need the battery to cover.
- Recommended size = the smaller of the two (no point buying storage you can never fill). This is mapped to a common retail size range, e.g. 5–10 kWh units, or larger systems like the 13.5 kWh Tesla Powerwall.
Common battery sizes in the UK
| Recommended (kWh) | Buy this size range | Example |
|---|---|---|
| Up to 3 | 3–5 kWh | Small modular units (GivEnergy 5.2 kWh run partially) |
| 3–7 | 5–10 kWh | GivEnergy 9.5 kWh, Fox ESS 10 kWh |
| Over 7 | 10–15 kWh | Tesla Powerwall 13.5 kWh, two stacked 9.5 kWh units |
Frequently asked questions
What size solar battery do I need in the UK?
Most UK homes with a 3–4 kWp array and 8–12 kWh daily use suit a 5–10 kWh battery. Run your own numbers in the calculator above — the right size is whichever is smaller of your daily excess solar and your night-time demand.
Is 900 kWh per kWp per year realistic?
Yes, 900 is a widely used UK average. Expect roughly 800 for north-facing, shaded or northern-UK roofs and up to 1,000 for unshaded south-facing roofs in the south of England. Enter your own figure if your installer quoted one.
Do solar batteries work in winter?
They still work, but there is far less to store — a UK array can produce under 1 kWh a day in December. Batteries paired with a cheap overnight time-of-use tariff (e.g. Octopus Go) still save money in winter by charging from the grid overnight and covering the evening peak.
Can I charge my battery from the grid overnight?
Yes, with a hybrid or AC-coupled inverter and a compatible tariff. This is exactly how time-of-use tariffs pay off: fill the battery at cheap overnight rates and avoid buying at peak prices. Check your inverter supports scheduled grid charging.
Do I need a hybrid inverter?
A hybrid inverter handles both solar and battery in one unit and is the simplest choice for new installs. If you already have a standard solar inverter, an AC-coupled battery (like the Tesla Powerwall) can be added without replacing it.
Last updated: September 2026.
Disclaimer: This tool is for information only and is not professional financial advice. Figures are yearly averages — installer quotes and actual generation vary by roof, location and equipment.