Solar Battery Storage Calculator

This calculator estimates the optimal battery size for your solar panel system based on your electricity consumption patterns, system size and tariff rates.

How battery sizing works

The recommended battery size is based on the rule of thumb:

Battery size ≈ Daily usage × (1 - Daytime usage fraction) × 0.8

This is a common installer rule of thumb (not a published standard) — no British Standard prescribes home-battery sizing. It sizes the battery to cover roughly 80% of your evening and overnight electricity needs from stored solar. The result is then capped by your actual daily solar surplus and by your evening/overnight consumption — there's no point having a battery larger than what your panels can fill or larger than the energy you use after dark.

Solar generation

As a working average, UK solar panels generate around ~2.5 kWh per kWp per dayacross the year (more in summer, less in winter). A 4 kWp array therefore generates roughly 9–10 kWh per day on a well-sited, south-facing roof in southern England; the conservative UK average is nearer 3,400 kWh/year (~9.3 kWh/day), and northern or shaded roofs less again (MCS MIS 3002 irradiance / Energy Saving Trust). Adjust the per-kWp figure in Advanced Options for your own location and orientation.

Self-consumption

• Without battery: Typically 30-50% of generation is self-consumed
• With battery: Typically 70-90% of generation is self-consumed

Savings calculation

Daily saving = Stored energy × (Import rate - Export rate). Each kWh you store and then use yourself saves the difference between what you would have paid to import it and what you would have received for exporting it. “Stored energy” here is capped by three limits: the battery's usable capacity, your daily solar surplus, and your actual evening/overnight consumption — a battery cannot save you anything on energy you never use after dark. If your export rate is higher than your import rate, storing solar is no longer worthwhile (exporting pays more), so the tool reports payback as not applicable.

With time-of-use tariffs, extra savings can come from charging the battery from the grid at the off-peak rate and discharging at peak. This only applies to spare cycle capacity not already filled by solar — the same kWh cannot be both solar-charged by day and grid-charged overnight — and the share of cycles where this pays is an installer rule of thumb, not a published figure.

Important notes

This calculator provides estimates based on annual averages. Actual performance varies significantly by season, weather and individual usage patterns. Battery savings are highest in summer when solar generation peaks and lowest in winter.