Case 1
- Heated floor area
- 140m²
- Insulation standard
- Fully renovated envelope — 60 W/m²
- People in the household
- 3
9.15kW
Open with these valuesHeat Pump Size Calculator
9.15kW
Result: 9.15 kWHeated floor area times the specific heat load of the building, plus 250 W per person for hot water. A fully renovated 140 m² home with three people needs about 9.2 kW. These are rules of thumb — a binding design still needs the room-by-room calculation of EN 12831.
The numbers at a glance
Held fixed: Heated floor area 140.0 m², Insulation standard Fully renovated envelope — 60 W/m².
| People in the household | Result (kW) |
|---|---|
| 0 | 8.40 |
| 1 | 8.65 |
| 2 | 8.90 |
| 3Your value | 9.15 |
| 4 | 9.40 |
| 5 | 9.65 |
| 6 | 9.90 |
Worked examples
Case 2
- Heated floor area
- 140m²
- Insulation standard
- Unrenovated older building — 170 W/m²
- People in the household
- 3
24.55kW
Open with these valuesCase 3
- Heated floor area
- 100m²
- Insulation standard
- Low-energy build — 30 W/m²
- People in the household
- 2
3.50kW
Open with these valuesHow it's calculated
kW = (area × W/m² + people × 250 W) ÷ 1000
- StepEnter only the rooms you actually heat; an unheated cellar does not count.
- StepPick the insulation standard — it moves the result more than anything else.
- StepAdd the people in the household; each one adds 250 W for hot water.
- ResultCompare the result with the output at your design outdoor temperature.
What this number means
Bigger is the expensive mistake
A pump above the building's load cannot modulate down far enough, so it cycles on and off. That wears the compressor and lowers seasonal efficiency. Standard practice is to size to the calculated load and let the backup heater cover the coldest days.
Insulation moves the answer more than area
The same 140 m² home with three people needs 7.05 kW as a modern build, 9.15 kW fully renovated and 24.55 kW unrenovated. That is more than a threefold spread from one dropdown, while the three occupants together only add 0.75 kW.
Let the heating bill pick the standard
Divide last year's heating energy in kWh by the heated floor area. Under 50 kWh/m² a year points to a modern or low-energy build, 80 to 120 to an average partly renovated one, and over 150 to an unrenovated one. That beats going by the construction year, because new windows and an insulated roof make an old house behave like a much newer one.
The W per m² figures are rules of thumb
They come from Central European building practice, not from a standard — a binding design uses the room-by-room method of EN 12831. Five of the seven sit inside the published bands for their building type, while the passive-house figure of 15 W/m² is above the certification limit of 10 W/m² and 45 W/m² is above the 36 to 38 W/m² a current new build reaches. Both deviations point the same way, towards a slightly larger machine than needed.
Commonly misread
Bigger is safer, then it will not run short on cold days.
Oversizing is the more common and more expensive error, because the pump then cycles instead of modulating. Size to the calculated load and let the built-in backup heater cover the handful of coldest days.
The pump is rated 9 kW and I need 9.15 kW, so it roughly fits.
A rated output is quoted at a mild outdoor temperature, and an air-source pump delivers least exactly when you need most. Look up the data sheet row for your design outdoor temperature and flow temperature, and compare that figure instead.
I entered the whole floor area, cellar and loft included.
Only the rooms you actually heat belong in that field. An unheated cellar carries no load and would inflate the result.
The house was built in 1960, so it must be unrenovated.
The construction year is a poor guide on its own. A 1960 house with new windows, a new roof and an insulated façade behaves like a much newer one, and the heating bill tells you which case you have.
Reference table
| Insulation standard | Specific load | 140 m², 3 people |
|---|---|---|
| Passive-house class | 15 W/m² | 2.85 kW |
| Low-energy build | 30 W/m² | 4.95 kW |
| Modern build | 45 W/m² | 7.05 kW |
| Fully renovated envelope | 60 W/m² | 9.15 kW |
| Average, partly renovated | 80 W/m² | 11.95 kW |
| Dated, minimal insulation | 120 W/m² | 17.55 kW |
| Unrenovated older building | 170 W/m² | 24.55 kW |
Questions
What size heat pump do I need?
Heated area times the specific heat load of the building, plus 250 W per person for hot water. A 140 m² home with three occupants needs 7.05 kW as a modern build, 9.15 kW fully renovated and 24.55 kW unrenovated. The insulation standard moves the answer far more than the floor area does.
How do I know which insulation standard applies?
Let the heating bill decide: divide last year's heating energy in kWh by the heated floor area. Under 50 kWh/m² a year points to a modern or low-energy build, 50 to 80 to a renovated envelope, 80 to 120 to an average partly renovated one, and over 150 to an unrenovated building. That beats going by the construction year, because an old house with new windows, a new roof and insulated walls behaves like a much newer one.
Why does the heat pump's rated output not match my result?
Because a rated output is quoted at a mild outdoor temperature, while an air-source heat pump delivers least exactly when you need most. Data sheets carry a table of outputs by outdoor temperature and flow temperature. Find the row for your design outdoor temperature and the flow temperature your heating needs, and compare that figure with the result here.
Is it safer to buy a bigger heat pump?
No — oversizing is the more common and more expensive mistake. A pump larger than the building's load cannot modulate down far enough, so it cycles on and off, which wears the compressor and lowers seasonal efficiency. Standard practice is to size to the calculated load and let the built-in backup heater cover the handful of coldest days.
Why does the number of people matter?
Because hot water is a separate load from space heating: showers, baths and taps draw heat regardless of the outdoor temperature. The usual planning figure is 250 W per person, so a household of four adds 1 kW on top of the space-heating requirement. If the heat pump handles space heating only, set the occupant count to zero.
How solid are the W per m² figures?
They are rules of thumb from Central European building practice, not a standard — a binding design uses the room-by-room method of EN 12831. Five of the seven sit inside the published bands for their building type; the passive-house figure of 15 W/m² is above the certification limit of 10 W/m², and 45 W/m² for a current new build is above the 36 to 38 W/m² such buildings reach. Both deviations point the same way, towards a slightly larger machine than needed.
Sources and last check
Information, not professional advice.