Heat Pump Size Calculator Floor Area × Insulation, Plus Hot Water
Enter the heated area, how well the building is insulated and how many people live there. You get the output the heat pump has to deliver.
Insulation Beats Size
The same 140 m² needs 7 kW as a modern build and 24.5 kW unrenovated. The envelope decides, not the floor plan.
An Estimate, Not a Design
A binding installation needs a room-by-room heat load calculation. This is the number you check a quote against.
What size heat pump do I need?
At a Glance
Take the heated floor area, multiply it by the specific heat load of a building like yours in watts per square metre, and add about 250 W per person for hot water. A fully renovated 140 m² house at 60 W/m² with three occupants needs (140 × 60) + 750 = 9.15 kW. The same house unrenovated at 170 W/m² needs 24.55 kW — nearly three times as much.
The full method — DIN EN 12831 — works room by room through transmission losses, ventilation losses and thermal bridges. The short-cut used here replaces that with one figure per building type, which is how installers do a first pass before the detailed calculation.
| Building | Specific load | 140 m², 3 people |
|---|---|---|
| Passive house | 15 W/m² | 2.85 kW |
| Low-energy build | 30 W/m² | 4.95 kW |
| Modern build, current standard | 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 |
If you cannot place your building on that list, your heating bill can. Divide last year's heating energy in kWh by the heated area: under 50 kWh/m² a year is a modern envelope, 80–120 is average, above 150 is unrenovated.
output = (heated area × specific heat load) + (occupants × 250 W)These are the values the calculator loads by default.
Count only the heated area
140 m² of living space. The unheated cellar and the garage stay out of it.
Pick the insulation standard
A fully renovated envelope — insulated walls, modern windows — sits at 60 W/m².
Work out the space heating load
140 × 60 = 8,400 W.
Add hot water
Three people × 250 W = 750 W. Total 9,150 W, or 9.15 kW.
The figure is a heat demand. Turning it into a model on a data sheet takes two more steps.
Read the output at the design temperature. An air-source heat pump's rated output is quoted at a mild outdoor temperature. At −7 °C or −10 °C — the design condition in much of central Europe — the same machine delivers noticeably less. Find the row in the data sheet for your design temperature and your flow temperature, and check that number against your result.
A lower flow temperature is worth more than a bigger pump. A heat pump running at 35 °C flow into underfloor heating uses far less electricity than the same machine at 55 °C into old radiators. If your result is high, insulation and larger radiators buy more than extra kilowatts.
Oversizing costs money twice. A pump too large for the building cycles on and off instead of modulating, which wears the compressor and hurts efficiency. Sizing to the calculated load — with the backup heater covering the few coldest days — is standard practice, not a compromise.
For cooling rather than heating, see the Air Conditioner BTU Calculator.
A first pass, not a heat-load calculation
This is the short-cut method: one specific load for the whole building. It cannot see which walls face north, how airtight the building is, how much glazing there is, or that one room is over an unheated passage. A real installation needs a room-by-room calculation to DIN EN 12831 or the equivalent national standard, and that is also what sizes the radiators and the pipework.
The specific loads are typical central-European figures. A colder climate zone shifts them up and a milder one down, because the design outdoor temperature is baked into them. Use the result to sanity-check a quote — if an installer proposes 16 kW where this says 9, ask what they know that this does not.