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SCOP is the seasonal average efficiency: 3.5 means 1 kWh of electricity delivers 3.5 kWh of heat. Real-world SCOPs of 3 to 4 depend heavily on low flow temperatures and good design. Heat-pump-specific tariffs and the GBP 7,500 Boiler Upgrade Scheme grant change the whole comparison. Hot water included in demand.
What The Heat Pump Running Cost Calculator Works Out
- Heat pump electricity per year: annual heat demand divided by the SCOP, priced at your electricity rate, which defaults to the current Ofgem price cap unit rate. A typical 12,000 kWh home at a SCOP of 3.5 buys about 3,430 kWh of electricity, around GBP 900 at the current cap.
- Same heat from a gas boiler: the demand divided by 85% boiler efficiency at the current Ofgem price cap gas rate, roughly GBP 1,125 for the same home, so the pump edges it at a SCOP of 3.5. A SCOP of 4 or a heat pump tariff tips it clearly the pump's way; a badly designed SCOP of 2.5 tips against.
- Verdict: the annual difference. SCOP is earned by design, not bought in a box: flow temperatures of 35 to 45 degrees, adequately sized radiators or underfloor heating, weather compensation and running long and low. Monitored UK installs regularly show 3.5 to 4 plus where design is taken seriously; poor performers trace to oversized pumps, high flow temperatures or meddling with the controls.
- Own heat demand, gas rate and boiler efficiency (advanced): use the kWh from your gas bills (adjusted for boiler efficiency) and your actual gas price. An old 70% boiler makes the heat pump look far better.
- Break-even SCOP (advanced): the pump wins on running cost when its SCOP exceeds the electricity-to-gas price ratio multiplied by boiler efficiency: around 2.8 at the current cap rates and closer to 2 on heat-pump-specific tariffs.
- Heat pump tariff (advanced): the same electricity at a dedicated heat pump tariff rate, typically 15p to 20p per kWh, with the break-even SCOP at that price.
- Saving going fully off gas (advanced): adds the gas standing charge you stop paying once the meter is removed, about GBP 108 a year at the current cap, to the running-cost difference on the heat pump tariff.
- Net install cost and payback (advanced): the install price less the GBP 7,500 Boiler Upgrade Scheme grant, compared with simply replacing the gas boiler, and the years the running-cost saving takes to recover the extra. Insulate before you size: fabric-first shrinks both the pump and the bill.
- Carbon saved (advanced): gas at 0.183 kg CO2 per kWh versus grid electricity at about 0.207 kg per kWh divided by the SCOP, so a heat pump cuts heating emissions by around 70% today and more as the grid cleans up. The MCS installer's heat-loss calculation, not any online tool, is the number the system should be built on.
Further reading: Energy Saving Trust: air source heat pumps.