Solar batteries: sizing, cost and whether to bother

It changes the maths in both directions. A battery raises the share of your generation you use at home — the units worth about 27p instead of about 12p — but it also adds several thousand pounds to the cost and needs replacing sooner than the panels. Whether the first outweighs the second depends mostly on your tariff.

Almost always, yes. AC-coupled batteries fit alongside an existing array without replacing the solar inverter, which is what makes retrofit straightforward on systems installed years ago under the Feed-in Tariff.

Sometimes. On a time-of-use import tariff you can charge overnight at a low rate and discharge during expensive hours, which is a pure arbitrage play with no generation involved. It stands or falls entirely on the spread between your cheap and expensive rates.