Switching from a gas, oil, or LPG boiler to a heat pump changes both your running cost and your carbon footprint, and the two do not always move in the same direction. This calculator works out the real annual cost and CO2e of each system from your heat demand, fuel prices, and the pump’s efficiency.
How it works
Both systems must deliver the same useful heat; the difference is how much energy and money that takes:
boiler fuel used = heat demand / boiler efficiency
boiler cost = boiler fuel used × fuel price
boiler CO2 (kg) = boiler fuel used × fuel emission factor
heat pump elec = heat demand / SCOP
heat pump cost = heat pump elec × electricity price
heat pump CO2 (kg) = heat pump elec × grid emission factor
saving = boiler cost − heat pump costA heat pump delivering heat at a SCOP of 3.5 uses only a third of the input energy a boiler does, but each unit of electricity costs more than gas, so the result depends on the balance of efficiency and price.
Example
For example, a home needing 12,000 kWh of heat with a 90 percent efficient gas boiler burns about 13,330 kWh of gas. At 7p/kWh that is roughly 933 a year and 2.4 tonnes of CO2. A heat pump at SCOP 3.5 uses about 3,430 kWh of electricity; at 28p/kWh that is about 960 a year but only 0.7 tonnes of CO2. Running costs are similar, yet carbon falls by around 70 percent.
Getting the SCOP right
SCOP (Seasonal Coefficient of Performance) is the single most important number in this comparison, and it varies considerably in practice. Manufacturer datasheets quote SCOP at standard European climate zone test points; your real SCOP depends on:
- Flow temperature: Radiators designed for 75 °C flow temperature force the heat pump to work hard and reduce SCOP to around 2 or less. Lowering flow temperature to 45–55 °C — achievable with oversized radiators or underfloor heating — lifts SCOP toward 3.5 or higher.
- Climate: Colder climates reduce the temperature difference the pump can exploit and lower average SCOP.
- Cycling and oversizing: A heat pump that cycles on and off frequently due to oversizing loses efficiency. Correct sizing to the actual heat load is essential.
- Domestic hot water: Heating water to 60 °C for legionella control operates the pump at a low COP. Some calculations use a blended SCOP that includes hot-water production; others exclude it. Make sure you know which your datasheet quotes.
Maximising the economic case
The electricity-to-gas price ratio is the key lever you cannot control, but several things shift the economics in your favour:
- Time-of-use tariffs: Electric tariffs that offer cheap overnight rates allow you to run the heat pump and charge a buffer cylinder or thermal store during off-peak hours, effectively reducing the average price you pay for heat.
- Solar PV: Surplus generation used directly by the heat pump reduces your effective electricity price significantly and can make running costs clearly lower than a gas boiler.
- Underfloor heating or large radiators: Keeping flow temperature low is the single biggest driver of a high real-world SCOP.
- Insulation first: Reducing heat demand reduces both systems’ costs equally, but it also shrinks the volume of heat the pump must move, allowing a smaller, cheaper unit to be installed.
Carbon trajectory
The CO2 factors the tool uses are based on current grid emission intensity. As grid electricity decarbonises over the coming decades, the heat pump’s carbon advantage grows automatically — you have already installed the low-carbon infrastructure. A boiler burning gas or oil becomes progressively higher carbon relative to the grid over time, with no retrofit path without replacement.