What is the difference between kW and kWh?
kW is the instantaneous output needed at design conditions. kWh is energy used over time. This calculator estimates heating output, not annual heating cost.
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Heat demand calculator
Estimate design heat loss and the required heating output for a building or a single room.
Calculation scope
Location and temperatures
Select a Polish city
Warszawa
°C
Building geometry
m²
2
m
Detached
Typical — about 18% of walls
m²
Insulation and ventilation
New building — Polish WT 2021
Natural ventilation
Advanced settings
Values come from the selected presets. Change them if you know the actual envelope or system parameters.
U-values
W/(m²·K)
W/(m²·K)
W/(m²·K)
W/(m²·K)
W/(m²·K)
%
Air exchange
1/h
1/h
%
How is a building heating load calculated?
The calculator estimates the output required to maintain the selected indoor temperature under winter design conditions. The kW result is useful for preliminary heating-system sizing, while W/m² helps compare building performance.
Building mode estimates envelope areas from floor area, storey count and exposure. Room mode calculates a single room before radiator or underfloor-heating selection.
What the heat-loss calculator includes
Losses through walls, windows, doors, roof and floor use area, U-value and temperature difference. Ventilation losses are calculated separately from room volume and air-change rate.
Heat recovery reduces only the mechanical ventilation part; infiltration remains a separate loss. For natural ventilation, the calculator uses the larger of hygienic air change and infiltration; for mechanical ventilation, it adds controlled air change and infiltration. Every assumed coefficient can be edited in advanced settings.
A preliminary estimate, not a full EN 12831 calculation
A complete calculation needs exact geometry, construction layers, linear thermal bridges, ground coupling, required airflows and adjacent-zone conditions. Use this tool for early sizing and option comparison.
Frequently asked questions
Can I size a heat pump or boiler from this result?
It is a useful starting point, but final sizing should include a detailed load calculation, equipment performance, bivalent temperature, DHW and operating strategy.
Should I automatically add a 20% safety margin?
No. A large automatic margin can oversize the heat source. Improve the input data first and select any margin deliberately for the specific appliance.
Which outdoor temperature should I use?
For listed Polish cities the calculator selects a design temperature from −16 to −24°C. For any other location, enter the local design temperature manually.
How to use the calculated output
Use the result as an input to these calculators: