Battery size calculator
A battery is bounded twice: by what your roof has spare on a typical day, and by what you would otherwise buy that evening. The smaller of those is the capacity that will actually cycle. Anything above it is a bigger number on a quote.
Most battery sizing advice starts from the battery. This starts from your roof and your evening.
A battery earns money in exactly one way: it catches generation you would otherwise export cheaply and releases it into demand you would otherwise buy expensively. So its useful size is capped by two things — how much is spare on a typical day, and how much you actually use after dark.
The month-by-month table below shows both, using our own seasonal generation data. Watch what happens to the surplus column in December.
One thing this calculator deliberately will not tell you is payback. We have no sourceable UK price series for domestic storage, and inventing one to produce a satisfying number is precisely what we exist not to do.
Your surplus, month by month
How the sizing works
For each month we take our own modelled generation for your system at that location, divide by the days in the month, and subtract the share of your consumption that happens in daylight. What is left is the surplus available to store.
Separately, your evening and overnight demand is what a battery can discharge into.
The useful capacity is the smaller of those two figures on a typical day. Sizing above it buys capacity that never cycles.
What we could not establish
Battery prices. This is the honest reason there is no payback figure here. Our cost pages rest on an official statistics series for solar; we have found no equivalent for storage, and we will not publish a price we cannot source.
Round-trip efficiency in real use. Some energy is lost charging and discharging. Manufacturers publish figures; we have no independent UK data.
Degradation. Batteries lose usable capacity with cycles and age. Warranty terms state what the manufacturer will stand behind, which is not the same as observed life.
Your daylight consumption share. The calculator asks you to estimate it because nobody else can. If you have a smart meter, your half-hourly data will tell you.
This is information, not advice
Sizing is a starting point for a conversation with an installer who can see your consumption data and your consumer unit, not a specification.
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