Batteries: common questions

It moves electricity in time. Solar generates at midday; a household uses most in the morning and evening. The battery catches the midday surplus and gives it back at teatime.

What it earns you is the gap between what you would have paid to buy that unit and what you would have been paid to sell it.

It roughly doubles how much of your own generation you use. For a typical household that is a jump from around a quarter to somewhere near two thirds.

The self-consumption calculator will do it for your own figures.

For a typical house with a 4 kW array, somewhere between 5 and 10 kWh of usable capacity.

Bigger is not proportionally better: there is only so much surplus in a day to catch, so the gains fall away quickly above about 10 kWh.

What the battery will actually give back, which is less than the figure printed on it. Some is held in reserve to protect the cells.

A battery sold as 10 kWh might have 8 usable, and the usable figure is the one to compare on.

Only if it was specifically built to. By default a solar system shuts down in an outage, for the safety of anyone working on the lines.

Backup needs extra equipment and has to be asked for before installation. See solar, batteries and power cuts.

Yes, and on a tariff with a cheap night rate this can be worth more than the solar shifting.

It works every day of the year, not only when the sun shines. Some people fit a battery on this basis with no solar at all.

Ten to fifteen years. It declines gradually, holding a little less each year, and does not fail outright.

Warranties are usually ten years, or a number of cycles, or a total throughput — whichever comes first, so check which of those limits applies to you.

Usually wall-mounted in a garage or utility room. It needs a sensible temperature range and some ventilation.

Domestic batteries are overwhelmingly lithium iron phosphate, a chemistry chosen partly because it is thermally stable. Lofts are the least good location: too hot in summer, too cold in winter.

Not planning permission, if it is inside a building. But it does need telling the company that owns the wires in your street, because a battery can export.

Adding one to existing solar can push the combined system over a threshold and turn a notification into an application, so establish that before ordering.

Generally no. Home batteries are not short of space, so the density improvements that drive phone and car batteries buy you little.

What matters is cost per usable kilowatt-hour, cycle life and safety. More in is silicon-carbon suited to home batteries?