Free Heat Pump Payback calculator with clear step-by-step results.
Heat pump economics turn entirely on the ratio between the electricity price and the fossil fuel price relative to the efficiency gain. A seasonal COP of 3.4 against an 82% boiler is a four-fold efficiency improvement, which only saves money if electricity costs less than four times the fuel it replaces.
Existing running cost
Cost = heat demand / existing efficiency x fuel price
Heat pump running cost
Cost = heat demand / seasonal COP x electricity price
If electricity costs more than the efficiency ratio relative to gas, running costs rise. That is common where electricity is priced at four or five times gas per kilowatt-hour.
Around 3.0-3.5 for a well-designed air source system on low-temperature radiators or underfloor heating, dropping to 2.2-2.6 if it has to serve high flow temperatures in an uninsulated house.