How many kilowatts of heating does your home actually need?
You are buying a new furnace, a heat pump or an electric heater and you do not know what size to pick? Too small and it will not keep up on the coldest night. Too big and you are throwing money away on capacity you will never use.
This calculator shows how many kilowatts (kW) of heating your home or a single room actually needs. Type the floor area, the ceiling height and the quality of insulation. In a few seconds you get the answer plus an annual cost comparison for five fuels: gas, electricity, heat pump, wood pellets and heating oil.
Everything is calculated on a typical heating season for Central / Northern Europe and North America. You can also switch to advanced mode and enter your own fuel prices, your own design outdoor temperature or your own W/m² coefficient.
How to use it
- Type the heated floor area in square meters. Only count what you actually heat, leave out the garage or an unheated attic.
- Enter the ceiling height (usually 2.5-2.8 m in apartments, 2.7-3.5 m in houses). A higher ceiling means more air volume to keep warm.
- Pick the insulation quality from four options, from a passive house (excellent insulation, 15-30 W/m²) all the way to old construction from before 1980 that leaks heat like a sieve (150-200 W/m²).
- The calculator shows the required heating power in kW: that is how big your furnace, heat pump or radiators combined need to be to hold 21°C inside on the coldest day of the year.
- Below you get the annual heating cost for five fuels: natural gas, electricity, heat pump, wood pellets and heating oil. The cheapest one is highlighted green.
- Switch on advanced mode (toggle at the top) to override fuel prices, the design outdoor temperature (default −20°C) or your own W/m² value if you know it from an energy audit.
When this is useful
Six situations where the calculator gives you a real number instead of an installer's guess:
- Replacing a gas furnace with a heat pump. The installer says "you need 12 kW" but you want to check yourself. You type 150 m² house from 2015, good insulation, 2.7 m ceilings. You see: 7.5 kW is plenty. A heat pump that costs $8,000 instead of $13,000.
- Checking whether gas is still worth it. Gas prices jumped, your neighbor is bragging about her heat pump. You enter your house, you see: gas $900 a year, heat pump $600. Difference $300 per year. The heat pump pays back in 12 years, the decision is on numbers, not feelings.
- Picking an electric heater for one room. A 15 m² room in a 1990s building. Calculator says: standard insulation × 15 m² = 1.5 kW. You know a 2,000 W (2 kW) heater is plenty, you do not buy a 3 kW one "just in case".
- Designing a new house on a lot. The architect asks what size mechanical room to plan. You compute: 180 m² built to 2025 code, great insulation = 9 kW. A small air-to-water heat pump is enough, no need for a furnace the size of a closet.
- Checking whether pellets really beat oil. Your old oil boiler is going to scrap. The calculator shows side by side: pellets $700, gas $900, electricity $1,700. Pellets win, but you need storage space.
- Renting an apartment with electric heat. The landlord says "economical electricity". You compute: 50 m² 1990s standard × 1,800 hours × your electricity rate = $800 per year. You know whether you can afford the apartment before you sign the lease.
Once you know the heating power, pair it with the ventilation / HRV calculator to size mechanical ventilation, check live tariffs in the electricity prices feed, and if you're combining with PV, the solar + storage sizing tool shows the matching kWp / kWh.