What this calculator does
The Heat Pump Calculator answers the two questions that matter before buying a heat pump: how big must it be, and will it be cheap to run. Enter your heated floor area, the insulation quality of the building, the indoor temperature you want, and the cold outdoor "design" temperature your heating must cope with — the calculator returns the design heat load, the recommended heat pump size, and the coefficient of performance (COP) you can expect at those conditions.
It then compares what an hour of heating would cost with the heat pump, with a gas boiler, and with ordinary electric (resistance) heating, using your local energy prices. It runs entirely in your browser and is available in English, Arabic, and Spanish.
How to use the Heat Pump Calculator
- Enter the heated floor area in square meters or square feet — all rooms that the heating serves, not just one.
- Choose the insulation quality. "Good" is a modern, well-insulated building; "average" is a typical home; "poor" means thin walls, single glazing, or high air leakage.
- Enter the indoor temperature you aim for (21 °C / 70 °F is typical for living areas).
- Enter the outdoor design temperature — the low temperature your climate reaches on the coldest winter nights (but not an all-time record). If the heating must still work at −5 °C, enter −5.
- Enter your electricity price (and optionally the gas price) per kWh, in any currency — the results are in your currency's units.
- Click "Calculate" to see the heat load, suggested heat pump size, COP, and a running-cost comparison per hour of full-power heating.
Formulas
The calculator uses widely accepted engineering rules of thumb, scaled by the actual temperature difference of your design day:
The heat load formula assumes an average building loses 70 W per m² at an indoor–outdoor difference of 25 °C, and scales that loss proportionally at other temperature differences. The COP estimate is a Carnot-limit calculation with a realistic efficiency factor of 0.35, slightly pessimistic on the evaporator and condenser sides (5 K each) — this matches what real air-to-air heat pumps deliver at low outdoor temperatures. The gas boiler is assumed 90% efficient.
Examples
- A 120 m² home with average insulation, held at 21 °C on a −5 °C design day, with electricity at 0.25 and gas at 0.10 per kWh: heat load 8.7 kW (about 29,800 BTU/h), suggested heat pump 9 kW, COP ≈ 2.9, so about 3.0 kW of electricity. Cost per hour: 0.75 (heat pump) vs 0.97 (gas boiler) vs 2.18 (electric heater) — about 23% cheaper than gas and 66% cheaper than electric heating.
- Imperial: a 2,000 ft² well-insulated house held at 68 °F on a 14 °F design day, electricity at 0.15, gas at 0.10 per kWh: heat load 11.7 kW (about 39,900 BTU/h), suggested 12 kW, COP ≈ 2.6, drawing 4.5 kW. Cost per hour: 0.67 vs 1.30 (gas) vs 1.76 (electric) — about 48% cheaper than gas.
- A small 60 m², well-insulated flat at 20 °C indoors, −5 °C outside, with expensive electricity (0.30) and cheap gas (0.08): heat load 3.2 kW, suggested 3.5 kW, COP ≈ 3.0, drawing 1.1 kW. Cost per hour: 0.32 vs 0.28 (gas) vs 0.95 (electric) — with this price ratio the heat pump costs about 13% more than gas, but still 66% less than resistance heating.
Types of heat pumps
- Air-to-air (mini-splits): the most common and cheapest type; delivers warm air directly. The COP figures here describe it best.
- Air-to-water: feeds radiators or underfloor heating and domestic hot water. Works best with large, low-temperature emitters (underfloor); radiator systems on 60–70 °C water lose several points of COP.
- Ground-source (geothermal): draws from soil that stays 8–12 °C all winter, so the temperature lift is smaller and the COP typically 0.5–1 higher than an air-source unit at the same conditions. The sizing part of this calculator applies unchanged.
Frequently asked questions
What are COP and SCOP?
COP (coefficient of performance) is the ratio of heat delivered to electricity consumed at one moment — a COP of 3 means 3 kW of heat for 1 kW of electricity. SCOP (seasonal COP) is the same ratio averaged over a whole heating season; because winters are milder than the design day, a unit with a COP of 2.7 on the coldest days may have an SCOP of 3.5–4 over the season.
How do I know the heat pump will be cheaper than gas?
A heat pump beats a 90%-efficient gas boiler whenever your electricity price is below the gas price multiplied by COP ÷ 0.9. At COP 3 the break-even electricity price is about 3.3 × the gas price (in the same units per kWh). This is exactly what the running-cost lines show for your own prices.
Why does the outdoor design temperature matter so much?
Two reasons. First, the heat loss of the building grows with the temperature difference, so the load you must cover depends on how cold it gets. Second, an air-source heat pump both loses capacity and loses efficiency as outdoor air gets colder — sizing and COP must be judged at the design temperature, not at an easy autumn temperature. Choose a temperature your climate reaches a few times per winter, not the absolute record low.
Should I buy exactly the calculated size?
Buy the nearest standard size at or slightly above the suggested figure. A modest margin is healthy, but a heat pump much larger than needed short-cycles, loses efficiency, and costs more. For air-source units, check the manufacturer's capacity tables at your design temperature — rated capacity is usually quoted at a mild 7 °C (44 °F) outdoor temperature.
Does this include hot water?
No. The heat load calculated here is space heating only. Domestic hot water is a separate demand (often an additional 0.5–1.5 kW averaged over the day), which an air-to-water system must also cover.
Can gas prices be entered per therm or per m³?
This calculator compares prices per kWh. Convert first: 1 therm = 29.3 kWh, and one cubic meter of natural gas is roughly 10.5 kWh (check your bill's calorific value). Enter the converted price per kWh.
Is my data saved or sent anywhere?
No. Every calculation runs in your browser, and nothing you type is stored or sent to a server.
Related calculators
Other free calculators commonly used with this tool: Cooling Load Calculator, Wet Bulb Calculator, and Water Pressure Pump Calculator.
Want a deeper walkthrough? Read our heat pump sizing guide.