Methodology and data sources

Every result is worked out from the sources below. Your plan's "How we worked this out" panel lists each figure used for your home.

How we work it out

1. Solar output. The size of the system times the MCS figure for your postcode zone, roof direction and pitch, times a shading factor (SAP 10.2 bands, from 1 for a clear roof to 0.2 for one almost fully shaded). The year's output is spread across the hours using typical-year sunshine for your zone.
2. Your electricity. What you enter, or the median for your type of home from DESNZ. It follows a typical household's half-hourly pattern through the year.
3. Your heat. The fuel you use times your boiler's seasonal efficiency, or your energy certificate's figure, or the median gas use for your type of home. 22% is hot water; the rest follows the outdoor temperature below 15.5°C.
4. A heat pump delivers 2.78 units of heat for each unit of electricity across the year.
5. Half-hour by half-hour. We run a whole typical year in half-hours, with and without each upgrade. Solar serves the house first, then charges a battery (13.5 kWh, 89% round trip, 5 kW), then exports. On a time-of-use tariff the battery also fills up in the cheapest hours.
6. Every tariff you could switch to, priced for your region, keeping only those you'd qualify for. Each option uses the cheapest.
7. Costs are median installed prices, less any grant you qualify for. Batteries are replaced after 10 years at today's price.
8. Over 20 years means the saving across 20 years less what you pay. Prices stay at today's levels, except that VAT on electricity is assumed to return to 5% from April 2027. Annual savings are shown at those later prices.
9. Confidence. "Firm estimate" is a tariff switch, priced from published rates. "Good estimate" includes installed prices, which vary from job to job. "Rough estimate" means we estimated your heat use from your type of home or floor area rather than your bills.