What size solar battery do I need?

Updated 26 October 2026 · Birmingham Solar

Separate capacity from power.

Usable capacity, in kWh, is the energy available within the battery’s operating limits. Output power, in kW, is the rate at which it can supply appliances. Two batteries with similar capacity may support different power demands.

As a simple illustration, a battery delivering 2 kW for two hours supplies 4 kWh. Real operation must also allow for losses, reserve settings and the equipment’s limits. Check whether a quoted capacity is nominal or usable.

Look at the hours you want the battery to cover.

For a solar-led system, start with electricity use after generation falls away. Half-hourly smart meter data is more useful than annual consumption alone because it shows the pattern the battery would need to serve.

If most demand occurs while solar is generating, direct use may already be high. If evening use is low, a large battery can have capacity that is seldom used. Include realistic changes such as an EV, but do not assume its full annual demand should be met by a home battery.

Check how it will be charged.

Solar surplus varies by day and season. A battery sized to hold a large summer surplus may not fill from solar in winter. Grid charging can add another opportunity, but the tariff window, charge power and electricity purchased must be included in the assessment.

For example, a 3 kW charging limit over a two-hour cheap period permits at most 6 kWh at the charger input before losses and other constraints. A larger capacity cannot remove that charging-time limit.

Compare a smaller and larger option.

Ask for the additional annual benefit of the larger option, together with its extra installed cost. Check how often each option fills and empties, and whether the expected usage is compatible with the warranty terms.

Our calculator lets you change usable capacity and battery power to compare solar storage scenarios. It does not model grid charging or provide a product recommendation.

Keep backup requirements separate.

A battery does not automatically supply the house during a power cut. If backup is important, discuss the circuits, expected loads, switching arrangement and reserve requirement before selecting the equipment. Holding energy in reserve also changes the capacity available for normal bill savings.