Heat one room, not the whole house.

Where these numbers come from

Gas and electricity prices are the Ofgem price cap for October to December 2026, for a direct debit dual-fuel customer on a standard variable tariff. A fixed deal could sit above or below this.

Boiler efficiency defaults to 82 percent, the measured average for combi boilers in UK homes from a Defra/DECC in-situ monitoring study, not the 88 to 94 percent figure printed on the box, which is a lab rating rarely seen in a working house.

SCOP defaults to 4.0. Air-to-air units that qualify for the UK government's Energy Technology List rate at 4.3 or higher under EN14825, a genuine seasonal test rather than a single mild-weather reading, so that figure is not marketing puffery. 4.0 sits a little below that qualifying bar, allowing some headroom for real-world fitted performance against the rated figure. If you know the rated SCOP of a specific unit, use it instead.

The carbon figures use the UK government's own 2026 greenhouse gas reporting factors: 0.183 kgCO2e per kWh of gas burned, and 0.131 kgCO2e per kWh drawn from the grid. The electricity figure fell by around a quarter on the 2025 factor, most of that from real decarbonisation of the grid rather than a methodology change, which is why the carbon case for electric heating has strengthened faster than the cost case in the last year.

The "BMW Mini Cooper in weight" comparison uses 1,205kg, the manufacturer kerb weight of a current-generation BMW Mini Cooper S (F56) — just for a sense of scale, not a precise unit.

Sources: Ofgem price cap, DECC in-situ boiler efficiency study, UK Energy Technology List, air-to-air heat pumps, UK GHG reporting conversion factors 2026. This is a planning tool built on public data, not a survey of your home or a quote.