Solar battery size calculator UK

Use consumption outside solar hours, likely surplus, reserve and efficiency to explore a sensible battery-capacity starting point.

WHAT THIS CALCULATOR TELLS YOU

Battery size should be driven by the energy you can realistically shift, not simply by buying the largest battery available. Consumption timing, solar surplus, tariffs, usable capacity, power and backup requirements all matter.

Method reviewed: September 2026Assumptions are visibleIllustration, not a quotation

Outputs are simplified illustrations using the values you enter. They are not a prediction or guarantee of generation, savings, battery performance, charging cost or investment return.

HOW THE CALCULATION WORKS

Transparent methodology

Daily demand

Annual electricity use ÷ 365.

Shifting need

Daily demand × the share used outside solar-generation hours.

Indicative capacity

The tool considers your shifting need, available surplus, reserve, cycles and entered round-trip efficiency.

HOW TO INTERPRET IT

Use the result as a scenario.

A home using a lot of electricity overnight may have more energy to shift than a home whose consumption already aligns with daytime solar. But a large battery can still be under-used if there is not enough solar surplus or tariff opportunity to charge it.

For a real project, replace planning assumptions with site-specific design data, current tariffs, actual equipment specifications and a formal quotation.

CALCULATOR QUESTIONS

What the tool can — and cannot — tell you

Is nominal capacity the same as usable capacity?

Not necessarily. Battery specifications distinguish total/nominal capacity from the amount intended to be usable. Check the actual product specification.

Can I use a battery without solar?

Some systems can charge from the grid and work with time-of-use tariffs. The economics depend on tariff spread, losses, cycling and system cost.

Does battery power matter as well as kWh?

Yes. Capacity describes stored energy; power describes how quickly the battery can charge or discharge. Both matter to system design.