Battery systems: how to choose one

The two numbers

kWh is how much it stores. It decides how long your appliances run. A home using 3,000 kWh a year uses about 8 kWh a day, much of it in the evening.

kW is how fast it delivers. It decides what can run at once. A 3 kW output will run a kettle. It will not run a kettle and an oven together.

Check both. A large battery behind a small inverter stores plenty and still trips when the kettle goes on.

Three questions

1. What is it for?

• Lower bills. Size it to hold your evening and overnight use: often 5 to 10 kWh for a typical home.
• Power cuts. Size it from the appliances you must keep on and the hours you want. Use the system finder.
• Off-grid. Size it for two or three days of winter use.

2. Will it work in a power cut?

Many do not. Backup needs an inverter with a backup output and a way to isolate the grid. If it matters to you, get it written into the quote. See power-cut protection.

3. What does it connect to?

• All-in-one units include the inverter. Simple to buy, one warranty, higher price per kWh.
• Brand-matched batteries work only with the same maker's inverter.
• Standard 48V batteries work with many inverters. Lowest price per kWh, but you or your installer must match the parts.

Chemistry

Almost every battery sold for homes, boats and vans today is lithium iron phosphate, written LiFePO4 or LFP. It is stable, tolerates daily deep cycling and typically carries a 10 year warranty. Lead-acid still has a place where cost today matters more than cost over time.

Saving money with a battery

A battery saves money in two ways. It stores solar you would have exported for about 12p and lets you use it instead of buying at 26.32p. On a time-of-use tariff it can also fill up on cheap overnight electricity in winter, when there is little solar.

Run both cases in the ROI calculator: tick the battery box and compare the payback.

Judging a warranty

• How many years, and how many cycles or how much energy throughput?
• What capacity is promised at the end: 70% or 80%?
• Who pays for removal and refitting?
• Who stands behind it if the maker stops trading?

That last point is not theoretical. GivEnergy entered administration in April 2026, leaving owners with working batteries and uncertain warranties.

Installation and VAT

Batteries installed in homes are charged at 0% VAT until 31 March 2027. Supply-only purchases are charged at the standard rate. To be paid for exported electricity, the installation normally needs MCS certification, so check this before choosing a DIY route.