Last checked: 4 August 2026
The basic calculation
Annual bill saving ≈ annual generation × self-consumption rate × import price.
An illustrative 800W system generating 680kWh a year, with 70% used in the home and electricity at 25p/kWh, gives about £119 a year before maintenance or degradation. This is an example, not a forecast.
What changes the result?
- Direction, angle, shading and local weather
- Panel and inverter losses
- Downtime and seasonal variation
- Daytime household demand
- Electricity tariff and future price changes
- Purchase, mounting and professional-help costs
Can the Smart Export Guarantee pay you?
SEG pays eligible small generators for metered exports. Ofgem says the installation normally needs an export-capable half-hourly meter, and solar installations up to 50kW generally need MCS certification or an equivalent. A self-installed plug-in kit may therefore not qualify automatically. Ask the intended SEG supplier before including export income.
Simple payback has limits
Dividing cost by annual saving ignores financing, degradation, repairs, opportunity cost and future tariff changes. Use a low, central and high case rather than one precise number.