Installed power is not annual generation
Panel capacity is commonly stated in kWp under reference test conditions. Annual generation is measured in kWh. Multiply capacity by a location-specific annual yield in kWh per kWp for an initial estimate. Roof direction, tilt, shading and system losses matter. A national example yield cannot describe every roof or predict next year's weather.
Distinguish two useful percentages
Self-consumption is the proportion of solar generation used in the home. Solar coverage is the proportion of household demand supplied by that solar energy. If panels generate 4,000 kWh and the household directly uses 1,600 kWh from them, self-consumption is 40%. With household demand of 3,200 kWh, direct solar coverage is 50%.
Value imports avoided and exports separately
Multiply solar electricity used at home by the relevant import price, then add the value of exported electricity under your actual arrangement. Do not automatically value every generated kWh at the retail rate. Contract fees, settlement rules and time-dependent prices may change the result; check them for the market and year being considered.
Treat a battery as a separate investment
A battery moves available energy to another time; it does not create more solar generation. Estimate additional usable self-consumption after charging and discharging losses. Include the export income forgone for electricity diverted into storage. Capacity, power limits and the timing of demand restrict the benefit. A battery does not automatically provide backup during a power cut.
Use payback as a first comparison
Simple payback divides installed cost by positive annual net savings. It leaves out financing, changing prices, degradation and replacement costs unless you add them separately. Compare a cautious and an optimistic scenario. Obtain a site assessment and itemised quotation; the calculator does not size wiring, structural supports or protection equipment.
Data sources
- Ofgem · July–September 2026
- Ofgem: Understanding electricity and gas bills
- Ofgem: Price cap and standing charges explained
- Utility Regulator: Consumer information for Northern Ireland
- European Commission · PVGIS
Last reviewed: 2026-09-08