🔥 Radiator Power Calculator
Room by room, estimate the heating power needed from insulation and climate, then the electric or water radiators to install, including with a low-temperature heat pump.
⚠️ Indicative estimate. To choose a heat pump or boiler, or to apply for renovation grants, have a room-by-room heat loss calculation (EN 12831) done by a professional.
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About the Radiator Power Calculator
A radiator that is too small will not heat the room in a cold spell; one that is too large costs more to buy. This calculator estimates the power each room needs with the volumetric heat loss coefficient G, used for early design: the older and less insulated the home, the higher G. It then suggests standard electric radiator sizes and, for central heating, the nominal output to choose for your water temperature.
How to Use the Calculator
- Add your rooms with their floor area and ceiling height; the set-point follows the room type.
- Pick the insulation level from the building period or the work done.
- Enter the French climate zone and altitude, or type the design outdoor temperature directly.
- Water radiators: choose your boiler or heat pump regime to get the nominal output to order.
Formulas Used
- Power: P = G × volume × (set-point − design outdoor temperature) × (1 + margin).
- G coefficient (W/m³·K): 2 before 1975, 1.6 from 1975 to 1989, 1.3 to 2000, 1 from 2000, 0.8 from 2005, 0.6 from 2012, 0.35 for recent very efficient homes (French thermal regulations).
- Design temperature: from −2 °C (zone A) to −15 °C (zone I) at low altitude, lower in the mountains (NF P 52-612).
- Set-points: 19 °C in living areas, 17 °C in bedrooms, 22 °C in the bathroom.
- Water radiators: output = output at ΔT50 × (ΔT ÷ ΔT50)n, ΔT being the log mean temperature difference between water and room (EN 442). Example: at 45/35 °C in a 20 °C room, a radiator gives only about 30% of its nominal output.
Tips
- Heat pump: at low temperature you often need radiators two to three times larger, or underfloor heating.
- Large rooms: split the power over several radiators, ideally under the windows.
- Insulate first: every insulation upgrade lowers G and the power to install.
- Keep a reasonable margin (10 to 20%): heavy oversizing brings no extra comfort.
Common Mistakes and Limits
- Thinking only in W/m²: ceiling height and climate change the power a lot.
- Ignoring zone and altitude: across France the design temperature varies by more than 10 °C.
- Using a water radiator’s rated output as is with a heat pump: it is given at ΔT50.
- Simplified method: it does not replace a wall-by-wall heat loss calculation (EN 12831).
Privacy & Security
- 100% in your browser: no data is sent to a server.
- No sign-up: free and instant.
- Works offline once the page has loaded.