Heat pump savings — methodology

What this calculator does

It compares the cost to run an air-source heat pump against your current heating system — gas furnace, oil boiler or electric resistance — sized so both deliver the same useful heat each year. It estimates your annual running-cost saving, an optional payback if you supply a net install cost, and the CO₂ you'd cut.

The formulas

Annual running cost

current_fuel_kwh    = annual_heat_demand_kwh / current_efficiency
current_heating_cost = current_fuel_kwh * current_fuel_price_per_kwh

heat_pump_kwh = annual_heat_demand_kwh / heat_pump_scop
heat_pump_cost = heat_pump_kwh * electricity_price_per_kwh

annual_saving = current_heating_cost - heat_pump_cost

Both systems deliver the same annual_heat_demand_kwh. The current system burns more energy than the heat it delivers (efficiency below 1, or exactly 1 for electric resistance), while the heat pump draws less electricity than the heat it delivers (SCOP above 1).

Emissions

co2_current   = current_fuel_kwh * current_fuel_co2_per_kwh
co2_heat_pump = heat_pump_kwh * grid_co2_per_kwh
co2_saved     = co2_current - co2_heat_pump

Payback (only when a net install cost is entered)

payback_years = net_install_cost / annual_saving

We guard the division: with no install cost there is nothing to pay back; if the heat pump costs more to run, the install never pays back on running cost alone and we report that honestly rather than showing a misleading number. A zero SCOP or zero efficiency yields “no result” rather than infinity.

Default values

Every default below is editable in the calculator and shown with its source in the calculator’s assumptions table.

• Heat pump SCOP 3.2 — a mid-range seasonal figure for a modern air-source unit (typical 2.5–4.0).
• Current fuel price $0.05/kWh — a representative natural-gas price per kWh of fuel energy. Gas is sold per therm, so this is a conversion — always replace it with your own.
• Current fuel emissions 0.18 kg CO₂/kWh — natural gas combustion per kWh of fuel energy.
• Electricity price $0.17/kWh and grid carbon intensity 0.39 kg CO₂/kWh — US representative averages.

How we keep it honest

The calculation logic lives in a small, pure function that is unit-tested against normal, edge and invalid inputs (for example, a zero SCOP returns “no result” rather than infinity). If you spot an error, tell us and we’ll fix it.