Solar and home battery calculator

Enter system size, annual yield per installed kWp, household consumption and a direct self-consumption share. Set your own installation costs, import price and export payment. These inputs describe a scenario; they are not a quotation or a site survey.

Formula

Annual generation = installed kWp × yield in kWh/kWp. Direct use is limited by household demand. Battery delivery is limited by surplus generation, capacity × cycles × round-trip efficiency, and remaining household demand. Savings value avoided imports and remaining exports.

Worked example

A 4 kWp array with an assumed yield of 1,000 kWh/kWp generates 4,000 kWh annually. At 40% direct self-consumption, 1,600 kWh supplies the home before adding a battery, provided demand is sufficient.

What the result means

Coverage is self-consumed energy divided by household consumption. Simple payback divides investment by annual savings. Battery payback uses only the extra savings attributable to storage, after the export income forgone.

Electricity price · United Kingdom

£0.2611/kWh is Ofgem’s average capped electricity unit rate for standard variable tariffs paid by Direct Debit in England, Scotland and Wales, valid from 1 July to 30 September 2026. It includes 5% VAT and excludes the separate standing charge. It is not an average across all contracts and does not apply to Northern Ireland. Use your own tariff where available.

Last reviewed: 9 September 2026 · Ofgem · July–September 2026

Enter the unit rate including any applicable tax. Check your bill rather than adding a tax percentage automatically. Standing charges, export payments and subscription fees are separate. Ofgem information applies to Great Britain; Northern Ireland has its own regulator and market arrangements.

Assumptions and limits

Annual totals cannot model hourly matching, winter shortages, roof shading, degradation, financing or future tariffs. Battery capacity is treated as energy charged per cycle.

Questions answered

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