Room Heating Power Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/8/2026
Heating power is calculated with surface × 70 W/m² for a well-insulated home, or × 100 W/m² for a home with high heat loss. A well-insulated 20 m² room needs about 1,400 W, versus 2,000 W if insulation is poor.
Explanation
This simplified method applies a flat coefficient (in watts per m²) that depends solely on the home's insulation quality: about 70 W/m² for a well-insulated building (recent construction to RT2012 standards or renovated), versus 100 W/m² or more for an older home with high heat loss. This is a deliberately rough estimate, which does not account for factors that nonetheless strongly influence actual need: ceiling height, orientation (north or south), number of walls facing outside, window quality, or climate zone. For a more advanced insulation thermal resistance calculator, or before buying a radiator or heat pump, a thermal survey carried out by a professional remains the only method that gives a reliable room-by-room sizing. This estimate mainly serves as an initial budget benchmark, for example to compare quotes or anticipate the electricity consumption associated with heating.
Example: a well-insulated 20 m² bedroom
Inputs
Surface: 20 m². Insulation: good (RT2012).
Calculation
Power = 20 m² × 70 W/m² = 1,400 W, or 1.4 kW.
Result
You need about 1,400 W (1.4 kW) to properly heat this room.
Frequently asked questions
Does this estimate replace a real thermal survey?
No. This per-m² method gives a quick order of magnitude, but a professional thermal survey accounts for ceiling height, orientation, the number of walls facing outside, window quality, and climate zone — factors that can make the actual need vary by 30% or more from this estimate.
Why does the coefficient almost double between good and poor insulation?
Because insulation directly determines how fast heat escapes the home: a poorly insulated building loses heat much faster and must therefore continuously produce more of it to maintain the same indoor temperature, hence a significantly higher power requirement.
Should the power of all rooms be added up to size a boiler?
That is one possible first approach, but a central boiler rarely serves all rooms at full power simultaneously, and its sizing also depends on the distribution system (radiators, underfloor heating). For room-by-room electric heating (convectors, standalone radiators), however, this per-room calculation is directly usable.