Heat pump size calculator
A heat pump should match the heat loss of the house on a cold day: too small and it can't keep up, too large and it cycles on and off. The calculator gives a first estimate from the floor area and the insulation.
Your measurements
Result
- Heating power needed
- 8.4 kW
Correct sizing needs a room-by-room heat-loss calculation by a qualified installer.
How the calculation works
- Power needed (kW) = heated area × demand per m² ÷ 1,000: about 100 W/m² uninsulated, 70 W/m² with standard insulation and 50 W/m² with high-performance insulation.
Worked example
A 120 m² house with standard insulation needs about 120 × 70 = 8,400 W — a heat pump of about 8.4 kW.
What the calculator assumes
- The demand per m² is for ordinary ceilings and the design winter temperature of a moderate continental climate (from our Romanian calculator).
- The power is the one needed at the coldest design temperature; an air-to-water heat pump delivers less at −15 °C than its catalogue figure at 7 °C.
When to adjust the result
- Final sizing needs a room-by-room heat-loss calculation by the installer — an oversized heat pump cycles often and wears faster.
- Domestic hot water usually adds 1–2 kW or a dedicated cylinder.
- With old radiators that need water at 60–70 °C efficiency drops; underfloor heating is the ideal partner.
Getting the heat-pump size right
The safest basis is a room-by-room heat-loss calculation, not a rule of thumb per m². An oversized heat pump cycles more and costs more; an undersized one relies on its backup heater on the coldest days. Old fuel consumption is a useful cross-check for existing homes.
Heat pumps work best at low flow temperatures. Check whether the radiators can heat the rooms at 45–55°C — upgrading a few undersized radiators is often cheaper than a bigger heat pump. Also plan hot-water storage, the outdoor unit's position and noise distance to neighbours.
Frequently asked questions
What size heat pump do I need for 120 m²?
Roughly 6–12 kW depending on insulation and climate — around 8.4 kW with standard insulation by this rule of thumb. The installer's heat-loss calculation decides.
Is a bigger heat pump better?
No. An oversized unit starts and stops frequently, which lowers efficiency and shortens its life.
Does a heat pump work in very cold weather?
Yes, modern air-source heat pumps keep working well below freezing, but their efficiency drops, so correct sizing and low flow temperatures matter.
Do I need underfloor heating for a heat pump?
No. Radiators work if they are large enough for a lower flow temperature; underfloor heating simply allows the lowest temperatures.