Solar Battery Sizing

Battery sizing starts from two quantities that are often confused: evening demand, the electricity a household actually uses after solar production falls, and solar surplus, the electricity panels produce above daytime use that is available to charge the battery. A battery should be sized to the smaller of the two on a typical day, in usable terms, because a battery larger than evening demand will rarely discharge fully, so much of its capacity sits unused and degrades from calendar ageing, while a battery larger than typical surplus will rarely charge fully from solar, so it must be charged from the grid or sit half-empty, which changes its economics.

Usable capacity, not nameplate capacity, is the right figure for that comparison, since batteries reserve a margin to protect lifespan, and round-trip losses mean a kilowatt-hour stored is worth less than a kilowatt-hour used directly. Power matters too: a battery must be able to deliver enough kilowatts at once for the evening loads it is meant to cover, whether that is lights and a fridge, or an oven, a heat pump and a shower together, and a large-capacity battery with low power may not cover a peak load even though it holds enough total energy.

Seasonal variation is the second sizing factor, and it often surprises households. In summer, solar surplus is large and evening demand may be moderate, so a battery can cycle fully most days, while in winter surplus is small and evening demand is larger, because heating, lighting and cooking all peak in the evening, so the same battery may cycle only partially or not at all from solar. Sizing for summer alone therefore overstates winter benefit, while sizing for winter alone may oversize for summer, and a good sizing calculation should show monthly cycling, self-consumption and grid imports, not just an annual average.

Tariff structure is the third factor. A household on a cheap overnight tariff may charge a battery from the grid overnight and discharge at peak price, which can justify a larger battery than solar surplus alone would, while a household on a flat tariff with a generous export payment may gain little from storing at all, since exported electricity already earns a reasonable value.

Before sizing, record evening use for a typical summer week and a typical winter week, including which appliances run after sunset, and record daytime solar surplus where monitoring already exists, or use a production estimate matched to household daytime use. Ask for a sizing calculation that shows, month by month, expected charge, discharge, self-consumption and grid imports, with usable capacity, round-trip efficiency and degradation stated, and with the tariff structure used in the calculation named.

Those constraints should be checked before a capacity is finalised, not after ordering.

In the EU, battery sizing guidance and support rules are national, with member states differing on whether batteries qualify for support, on metering and self-consumption rules, and on fire and siting regulations that limit size or location. A battery size that is supported in one country may not be supported in another, and a size that is lawful in a garage in one country may be restricted in another, so national rules should be checked.

In the UK, MCS covers battery storage in certified solar installations, building regulations and fire guidance govern siting, and Ofgem tariff rules govern the import and export prices that make a battery worthwhile. A battery sizing calculation should therefore be dated, with the tariff and scheme rules at the time of sizing stated, since both can change and alter the optimal size.

A common mistake is sizing a battery to roof size or to a round number, such as 10 kilowatt-hours because it sounds complete, without checking evening demand or surplus, so the battery is either half-used or half-charged most days. Another is sizing to a single, exceptional evening, such as a party night with every appliance on, which oversizes the battery for the other 364 days of the year.

A third mistake is ignoring degradation: a battery that meets evening demand in year one may fall short in year ten as usable capacity declines, and a sizing calculation that uses only year-one capacity will overstate long-term benefit. Ask for year-one and year-ten usable capacity figures, and check whether the battery can be expanded later if demand grows.

Keep the sizing calculation with its monthly tables, the evening demand records, the tariff terms used, the battery data sheet with usable capacity and power, and the expansion options if any. Those papers let a later owner or installer judge whether the battery still suits the household, or whether demand has changed enough to justify expansion or replacement.

Ask a qualified installer: what evening demand and surplus did you use, what usable capacity and power result, what monthly cycling do you expect, and how does degradation and tariff change affect the case. Demand-based, monthly answers show a sized battery, not a round-number guess.