Heat pump sizing: how many kW your house needs
The “100 W per m²” rule ignores your climate and insulation. Here is the output your house needs during the coldest days where you live.
A heat pump’s output is chosen to cover the house’s heat losses when it is coldest. It is worked out from the home’s heat-loss coefficient (G, in watts per cubic metre per degree of difference), the heated volume and the gap between 19 °C indoors and the reference outdoor temperature. The site takes as its reference cold the average of the three coldest consecutive days of each winter, measured by Météo-France from 2015 to 2024 at the station of the département’s main town: 5.2 °C in the Alpes-Maritimes, -2.7 °C in the Doubs. An average 100 m² house then needs 3.8 kW in Nice and 6.0 kW in Besançon. This is an order of magnitude: the installer sizes with the standard’s design temperature, which is lower, and must give you a heat-loss study. An oversized heat pump costs more and wears out cycling on and off; an undersized one leaves the backup heater doing the work.
What heat pump output do I need?
Output needed in cold spells
7.3 kW
| 3 coldest days (Strasbourg - Botanique) | -3.2 °C |
| Heated volume | 300 m³ |
| Air-to-water price guide (median per kW) | €12,432 |
Estimate: assumed ceiling height 2.5 m, the site’s insulation coefficient, 19 °C set point; the installer sizes with the standard design temperature, which is lower.
Checked by Radif Partners · Editorial policy · Method and sources
The calculation, step by step
The output needed equals G × volume × (19 °C − reference cold). Volume is the heated floor area times the ceiling height, 2.5 m by default. The G coefficient sums up insulation: it is how many watts each cubic metre loses per degree of difference between inside and outside. The site uses four usual values:
| Insulation | G coefficient (W/m³·°C) | Example |
|---|---|---|
| Poorly insulated | 1.60 | Pre-1975 house never renovated |
| Average | 1.10 | Insulated loft, double glazing |
| Well insulated | 0.75 | Walls, loft and windows renovated |
| Low-energy | 0.50 | Low-energy building, RT 2012 |
These coefficients are assumptions, not measurements of your house. The only way to get the real figure is a room-by-room heat-loss study, which the installer should carry out before proposing an output. If your quote states an output with no calculation at all, ask for it.
Output by département
The reference cold varies a lot between départements, and the output with it. For a 100 m² house:
| Département | Typical cold | Poorly insulated | Average | Well insulated | Low-energy |
|---|---|---|---|---|---|
| Alpes-Maritimes (06) | 5.2 °C | 5.5 kW | 3.8 kW | 2.6 kW | 1.7 kW |
| Gironde (33) | 2.0 °C | 6.8 kW | 4.7 kW | 3.2 kW | 2.1 kW |
| Paris (75) | -0.1 °C | 7.6 kW | 5.3 kW | 3.6 kW | 2.4 kW |
| Rhône (69) | -1.1 °C | 8.0 kW | 5.5 kW | 3.8 kW | 2.5 kW |
| Cantal (15) | -2.3 °C | 8.5 kW | 5.9 kW | 4.0 kW | 2.7 kW |
| Doubs (25) | -2.7 °C | 8.7 kW | 6.0 kW | 4.1 kW | 2.7 kW |
| Bas-Rhin (67) | -3.2 °C | 8.9 kW | 6.1 kW | 4.2 kW | 2.8 kW |
The seven départements in the table run from mildest to harshest. All 96 mainland départements, with their station and typical cold, are on the heat pump by département page.
Output and price
A heat pump’s price depends first on its output. Across the 22 air-to-water quotes published by the price observatory in 2026, the median is €1,697 per kW installed; across the 148 air-to-air quotes, €1,030 per kW. An average 100 m² house in the Rhône, at 5.5 kW, therefore comes to about €9,380 for air-to-water at that median price, an estimate to compare with your quotes. The air-to-water sample is small: the real range is wide.
What heat pump output do I need?
Output needed in cold spells
7.3 kW
| 3 coldest days (Strasbourg - Botanique) | -3.2 °C |
| Heated volume | 300 m³ |
| Air-to-water price guide (median per kW) | €12,432 |
Estimate: assumed ceiling height 2.5 m, the site’s insulation coefficient, 19 °C set point; the installer sizes with the standard design temperature, which is lower.
Output and consumption are not the same thing
Output (in kW) is what the heat pump must deliver at the worst moment of winter; consumption (in kWh per year) is what it uses over the whole season. A house may need 8 kW in a hard frost yet use only 4,000 kWh of electricity over the year, because most days are much milder. Consumption is worked out with degree-days, explained on the climate and degree-days page, and costed on the heat pump running cost page.
Insulate before changing your heating
Going from a poorly insulated to a well insulated house more than halves the output needed. If you plan to insulate the loft or replace windows in the coming years, do it first: you will buy a smaller, cheaper heat pump that runs in better conditions. MaPrimeRénov’ also funds insulation work, per item or within a whole-home renovation.