Solar panels
Choosing a home battery: size, power and warranty
Compare batteries on the energy you can actually use, the power they can supply at once, and what the warranty promises at the end, not on the headline capacity alone.
Updated 3 October 2026 · energy.org.uk editorial team
Capacity: usable, not total
A battery's total capacity is what it can hold; its usable capacity is what the system lets you draw. Energy Saving Trust notes that systems stop discharging at a set level, perhaps 20% of total capacity, to protect the battery. Compare quotes on usable kWh.
Power: how much at once
The continuous power (kW) is how fast the battery can charge or discharge. A battery with plenty of capacity but low power can't run an oven, kettle and shower together; the rest comes from the grid. Check the continuous figure, not just the peak.
Efficiency
Some energy is lost charging and discharging the battery. The round-trip efficiency tells you how much you get back. Our battery calculators allow for this.
Warranty terms
- Years. Energy Saving Trust says most lithium-ion batteries carry a 10-year warranty.
- Retained capacity. Many warranties promise a share of the original capacity at the end of the period, such as 70%.
- Throughput. Some warranties end after a set amount of energy has passed through the battery, which can come before the end of the years if you cycle it hard.
- Who provides it: the manufacturer or the installer, and whether labour is covered.
How big a battery?
A battery only saves money if it is filled with cheaper electricity, from your panels or a cheap overnight rate, and emptied when you would otherwise buy expensive electricity. A battery larger than your spare solar or your evening use will often sit part-empty. Use the solar battery calculator or, without panels, the home battery calculator.
Check
What to check first
- Usable capacity and continuous power on the specification sheet.
- Warranty years, retained capacity and any throughput limit.
- Whether it can provide backup in a power cut; most can't. See home batteries in a power cut.
- Where it will go, and whether that space stays within the manufacturer's temperature range.
Do
Practical next steps
- Estimate the value of different sizes in the solar battery calculator.
- Ask each installer for usable kWh, continuous kW, round-trip efficiency and the full warranty terms.
- Compare quotes on price per usable kWh, with the battery priced separately from panels; see the solar quote checker.
- Check the export and import tariffs you would use with it.
Take care
Important limitations
- Battery savings depend heavily on your tariffs and how you use electricity.
- We don't rank battery brands; check specifications and warranties in each quote.
- Warranty terms vary; read the document, not just the summary.