Choosing a heat pump is an excellent decision for your comfort and your bills. But before comparing models, one question decides everything: what power? It is the most neglected step — and the one that separates a quiet, economical installation from a noisy, power-hungry heat pump worn out before its time. This guide explains how to think it through, with real orders of magnitude, without turning your project into a thermodynamics course.
Why power is the most important choice
The power of a heat pump, expressed in kilowatts (kW), corresponds to the amount of heat it can supply to your home. Too low, and it will not keep up on cold days and will constantly switch to costly electric backup. Too high, and it will run in fits and starts, wear out prematurely and consume more than necessary.
Correct sizing therefore consists in matching as closely as possible the real needs of your house. And those needs are measured above all by one quantity: heat loss.
Step 1 — estimate the heat loss
A house loses heat continuously through the walls, the roof, the windows, the floor and ventilation. It is this heat that your heat pump must replace to maintain the temperature. It is expressed in watts per square metre (W/m²), and this figure depends directly on the quality of the insulation.
Orders of magnitude by insulation level
- Passive or very well insulated house: 25 to 40 W/m². The sizer’s dream.
- Recent construction or high-performance renovation: 50 to 60 W/m². The most common case today.
- House from the 1980s-2000s, average insulation: 70 to 90 W/m².
- Old building with little or no insulation: 100 W/m² and beyond.
The difference is enormous: at equal surface, an old house may require a heat pump two to three times more powerful than a renovated home. This is why insulating before installing a heat pump is often the best investment — you reduce the power needed, and therefore the price of the machine and the consumption.
This is the typical range of heat loss in watts per m² in the Belgian residential sector, from the well-insulated house to the unrenovated old building. Multiplying by your heated surface already gives an order of magnitude of power.
Step 2 — convert into kilowatts
Once the heat loss is estimated, the basic calculation is simple:
Power (W) = heat loss (W/m²) × heated surface (m²)
Take a renovated 130 m² house with heat loss of 60 W/m²:
60 × 130 = 7,800 W, or about 8 kW.
For the same surface poorly insulated at 90 W/m², you climb to nearly 12 kW. Surface alone therefore tells you nothing: without the insulation level, no power figure makes sense. The volume (ceiling height), the region and the altitude — which set the reference outdoor temperature — refine the estimate further. In Belgium, sizing is generally done for a base outdoor temperature of around −8 to −10 °C depending on the zone.
This calculation gives an order of magnitude, useful for not being sold just anything. The definitive sizing goes through a room-by-room thermal assessment.
Step 3 — do not oversize (the classic trap)
Out of caution, there is a temptation to “go a bit bigger”. This is a mistake. An oversized heat pump reaches the setpoint temperature too quickly, then stops. It restarts shortly after, and so on: these are the short cycles (court-cycling).
The consequences are concrete:
- accelerated wear of the compressor, the most expensive part of the machine;
- degraded seasonal efficiency (SCOP), therefore higher bills;
- jagged comfort and increased noise nuisance at start-ups;
- extra cost at purchase, for no benefit.
Conversely, undersizing is not the solution: the heat pump will run constantly without reaching the setpoint in very cold weather, and the electric backup will kick in — ruining the expected savings. Correct sizing aims for the right point, slightly supplemented by a backup for the peaks, rather than oversized to cover them alone.
Step 4 — the bivalence point and the backup
Rather than sizing the heat pump for the coldest day of the decade, a bivalent approach is often adopted: the pump covers most of the year’s needs, and a backup (built-in electric resistance, or the existing boiler kept on) takes over during the few extreme days.
The outdoor temperature at which the backup begins to intervene is called the bivalence point — typically between −3 and −7 °C. Well set, it allows you to choose a more compact heat pump, cheaper and better suited to the rest of the year, without giving up comfort during cold peaks.
Step 5 — emitters and the flow temperature
Sizing does not stop at the machine: what distributes the heat in your rooms, the emitters, is just as decisive. A heat pump is all the more efficient when the temperature of the water it produces (the flow temperature) is low.
Underfloor heating: the ideal ally
Underfloor heating is content with water at 30 to 35 °C. At this regime, the heat pump shows its best COP (coefficient of performance), often above 4: one kilowatt of electricity returns more than four kilowatts of heat. It is the perfect match.
Radiators: aim for low temperature
Your existing radiators may be suitable, provided they accept a moderate flow temperature, around 45 to 50 °C, against 70 °C for a conventional boiler. Radiators that are too small force the heat pump to heat harder, which makes the COP drop and the bill climb. In that case, the tightest emitters are replaced with low-temperature models, larger ones, before installation.
Enter your surface, your insulation and your emitters: our simulator gives you a power range and an initial budget estimate, grants included.
Launch the simulator →And domestic hot water?
Most air-to-water heat pumps also provide domestic hot water via a tank. This function weighs little on the sizing of the heating, because production is occasional and stored. The point of attention is elsewhere: the capacity of the tank, to be matched to the number of occupants (on the order of 50 litres per person) so as not to run short of hot water at peak times. Serious sizing factors in this need from the outset.
Why a professional’s thermal assessment is essential
The orders of magnitude in this guide equip you to discuss and spot a fanciful proposal. But they do not replace a thermal assessment carried out on site. A professional measures the heat loss room by room, takes account of orientation, thermal bridges, ventilation and the real state of the insulation — all parameters invisible in a back-of-the-envelope calculation.
This assessment determines the right bivalence point, the right tank, the right emitters and, in the end, years of comfort with no nasty surprise on the bill. This is exactly what we carry out before every heat pump installation: a personalised study, not a catalogue.
The 30-second recap
- Power depends on heat loss (50 to 100 W/m²) × the surface, not on the surface alone.
- A well-insulated house needs a smaller heat pump — insulating before installing pays off.
- Neither oversize (short cycles, wear) nor undersize (costly electric backup).
- The emitters and the flow temperature make the COP: underfloor heating 30-35 °C, low-temperature radiators 45-50 °C.
- Only an on-site thermal assessment sets the definitive power — free at wendows.
Guide verified in May 2026 · updated every year