Heat Pump Size Calculator
This heat pump size calculator estimates the heating capacity a heat pump needs for a home. Work from the heated floor area and the building standard, or from the gas or oil the house uses today, add hot water for the people in the home and any utility blocking time, and the heat pump size calculator returns the capacity in kW and BTU/h, the next standard size, the annual heat demand, and the electricity the heat pump should use in a year.
Results
The Heat Pump Size Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the heat pump size calculator does
A heat pump must cover the heat a house loses on the coldest days, plus the energy for hot water, and it must do so in fewer hours when the utility switches it off for peak periods. Unlike a boiler, a heat pump that is far too large cycles on and off and loses efficiency, so sizing it close to the real load matters. The heat pump size calculator estimates the building heating load, adds a hot water allowance of 0.25 kW per person, and scales the total for blocking time.
Heat pump sizing formula
The heating load comes from one of two methods:
Full load hours are the hours a heater would run at full output to deliver a year of heat, about 2,000 for a typical home in central Europe and fewer in mild climates. The hot water allowance adds 0.25 kW per person, and the blocking factor 24 ÷ (24 − h) raises the capacity so the heat pump can deliver a full day of heat in the hours it may run. The annual heat demand is the load times the full load hours plus about 700 kWh of hot water per person, and dividing it by the seasonal efficiency (SCOP) gives the electricity use.
Heat pump size calculator example
A renovated 150 m² house (70 W/m²), four people, and a tariff with 2 hours of blocking time a day:
The house needs about 23,800 kWh of heat a year; at a SCOP of 3.5 the heat pump uses about 6,800 kWh of electricity. A home that burns 20,000 kWh of gas a year in a 90% boiler has a heating load of 9 kW, so with hot water it needs a 10 kW heat pump.
How to use the heat pump sizing calculator
Choose the area method for a new or renovated house: enter the heated floor area and pick the building standard. Choose the consumption method when you replace an old boiler: enter the gas or oil used per year in kWh (1 m³ of natural gas is about 10 kWh, 1 liter of heating oil also about 10 kWh) and the boiler efficiency. Add the number of people using hot water, any daily blocking hours from your electricity tariff, and the seasonal efficiency of the heat pump. Press Calculate.
Reading the heat pump capacity result
The capacity is the output the heat pump must deliver at the coldest design temperature for your area, not at mild test conditions. Air source heat pumps lose output as it gets colder, so compare the result with the capacity in the data sheet at your design temperature, for example −10 °C (14 °F), not with the headline rating at 7 °C. An inverter heat pump modulates down on mild days, so sizing to the design load does not make it oversized most of the year. For a binding figure, have a room-by-room heat load calculation done to EN 12831 or ACCA Manual J.
When to use a heat pump capacity calculator
- Replacing a gas or oil boiler with a heat pump
- Checking an installer quote for the size offered
- Planning a heat pump for a new build
- Estimating the yearly electricity use and running cost
- Comparing the effect of insulation upgrades on the size
Common heat pump size calculator mistakes
- Using the rated output at 7 °C. Check the output at your design temperature.
- Counting hot water twice. Old fuel use often includes hot water already.
- Forgetting blocking time. Off periods need extra capacity.
- Sizing from rules of thumb alone. Confirm with a room-by-room load calculation.
Tips for heat pump sizing
- Insulate and seal before you size, it can shrink the unit.
- Lower the flow temperature as far as the radiators allow.
- Use a buffer or hot water tank to bridge blocking times.
Heat pump size for house heating: radiators and flow temperature
A heat pump is most efficient at low flow temperatures. Underfloor heating works at 30 to 35 °C, while old radiators sized for a boiler may need 55 °C or more on cold days, which lowers the efficiency and may call for larger radiators in some rooms. The flow and the temperature difference across each circuit decide the pump and pipe sizes, which the hydronic flow rate calculator works out. In older US homes with ducted systems, a heat pump replaces the furnace and air conditioner together; the furnace size calculator gives a quick climate-based comparison.
Heat pump running cost and SCOP
The seasonal coefficient of performance tells you how many kWh of heat you get per kWh of electricity over a year. Air source heat pumps typically reach 3 to 4, ground source units 4 to 5. With 16,000 kWh of heat, the difference between a SCOP of 3 and 4 is about 1,300 kWh of electricity a year. Multiply the electricity use by your tariff, often a cheaper heat pump tariff, with the energy cost calculator to compare it with the fuel bill you have today.
Heat pump sizing chart by floor area
Capacity including hot water for 4 people, no blocking time:
| Building | Heat load | 100 m² | 150 m² | 200 m² |
|---|---|---|---|---|
| Passive house | 15 W/m² | 2.5 kW | 3.25 kW | 4 kW |
| Very efficient new build | 30 W/m² | 4 kW | 5.5 kW | 7 kW |
| New build, good insulation | 45 W/m² | 5.5 kW | 7.75 kW | 10 kW |
| Renovated older house | 70 W/m² | 8 kW | 11.5 kW | 15 kW |
| Unrenovated older house | 100 W/m² | 11 kW | 16 kW | 21 kW |
Air source heat pump size and blocking time
| Blocking time per day | Factor | Capacity (150 m², 70 W/m², 4 people) |
|---|---|---|
| 0 h | 1 | 11.5 kW |
| 2 h | 1.091 | 12.55 kW |
| 4 h | 1.2 | 13.8 kW |
| 6 h | 1.333 | 15.33 kW |
Heat Pump Size Calculator FAQ
What size heat pump do I need?
Multiply the heated area by the heat load per square meter, add 0.25 kW per person for hot water, and scale for blocking time. A 150 m² new build at 45 W/m² with 4 people needs about 7.75 kW, so an 8 kW unit.
How do I size a heat pump from my gas use?
Multiply the annual gas use in kWh by the boiler efficiency and divide by about 2,000 full load hours. 20,000 kWh in a 90% boiler gives a 9 kW heating load.
Is it bad to oversize a heat pump?
Yes. An oversized unit starts and stops often, wears faster, and runs less efficiently. Size it to the design load and let an inverter unit modulate.
How much electricity does a heat pump use?
The annual heat demand divided by the seasonal efficiency. 16,000 kWh of heat at a SCOP of 3.5 takes about 4,600 kWh of electricity.