Heat Loss Calculator
Calculate the heat loss (power requirement in watts) through walls, windows, and roof, based on area, U-value, and the temperature difference between inside and outside.
How to use the calculator
- Enter area and U-value for walls, windows, and roof (find the U-value in building plans or product datasheets).
- Enter the desired indoor temperature and a design outdoor temperature (e.g. the coldest typical winter day where you live).
- See the total power requirement in watts.
How it's calculated
This is the standard building physics formula for transmission loss (heat leaking out through building components). Each component has its own U-value (lower is better insulated) and contributes to total heat loss in proportion to its area and the indoor-outdoor temperature difference.
Thermal bridges — what the U-value misses
A U-value describes heat loss through a notional, homogeneous surface. Real constructions are not homogeneous: studs in an insulated wall, joists through a roof, and junctions at windows, corners and foundations all conduct heat faster than the insulation around them. These thermal bridges are not included in the element's U-value, and together they can account for a noticeable share of the real heat loss.
The consequence is that a calculation based on U-values alone paints too optimistic a picture. It remains useful — particularly for comparing options, such as how much replacing windows is worth against adding loft insulation — but it should be read as a lower bound on heat loss rather than as a precise energy calculation. Air leakage, which is also excluded, pulls in the same direction.
Frequently asked questions
What is a U-value?
The U-value measures how much heat passes through a building component per square meter and per degree of temperature difference — lower U-value means better insulation. The unit is W/m²K.
How do I find my house's U-value?
Building plans, energy certificates, or insulation/window product datasheets often state the U-value directly. The calculator's default values are typical for well-insulated, modern buildings — older buildings often have higher (worse) U-values.
Can I use this to size a heat pump or electric heater?
It gives a good starting point for transmission loss, but remember the limitations above (air leakage and ventilation aren't included) — consider adding a safety margin, or have a professional do a full energy calculation.