Methodology and sources

Every figure on this site comes from one data file. Each has a source and the date we last checked it.

Solar savings formula

Generation = system size (kW) × yearly output per kW for your location and roof direction, from PVGIS.

Saving = electricity you use from your panels × unit price + electricity you export × SEG (Smart Export Guarantee) rate.

Without a battery we assume you use 40% of what you generate. Standing charges are not included. We turn your postcode into a location with postcodes.io, then ask PVGIS for the yearly output of 1 kW of panels at a 35° pitch facing your roof direction, with 14% system losses. If you don't give a postcode, or PVGIS doesn't answer in time, we use a regional estimate instead and say so on your result.

Sources

• Electricity 26.32p/kWh and gas 7.97p/kWh: Ofgem energy price cap, Oct–Dec 2026. Next update 25 Nov 2026.
• Solar generation: PVGIS v5.3 (EU Joint Research Centre), PVcalc (API docs), checked 25 Oct 2026. Location from postcodes.io. Fallback: regional estimate table (South 950, Midlands/Wales 880, North 830, Scotland/NI 780 kWh per kW).
• Home details: EPC register (England & Wales), planned.
• Heat pump costs and uptake: Boiler Upgrade Scheme statistics (heat pump costs).
• Grants: GOV.UK, Boiler Upgrade Scheme: what you can get; Ofgem, Energy Company Obligation (ECO); Ofgem, Great British Insulation Scheme.
• VAT: HMRC VAT Notice 708/6.
• Export rates: 4.1p and 12p, as of 25 Oct 2026 Example. Ofgem SEG page.
• Install costs: illustrative ranges Example, not yet sourced.

Figures updated 25 Oct 2026.