Heat Loss Calculator — Building Element U-value & Energy
Find exactly how many watts of heat escape through a wall, window or roof, and how many kWh you pay for per year. Enter the U-value from the building product datasheet or standard, the area, indoor and outdoor temperatures, and the number of annual heating hours.
W/m²K
m²
°C
°C
h/yr
Steady-state power lost through this building element at the given temperature difference
- 1
Temperature difference
20 − 0 = 20 - 2
Heat flux per m²
0.3 × 20 = 6U-value (W/m²K) multiplied by the temperature difference gives watts per square metre. - 3
Heat loss rate
6 × 20 = 120
How does this calculator work?
Heat loss Q = U × A × ΔT (watts), where U is the U-value (W/m²K), A is area (m²) and ΔT is indoor minus outdoor temperature (°C). Multiply by annual heating hours and divide by 1 000 for kWh/year. A lower U-value means better insulation and less heat escaping.
Formula
How this is calculated
The steady-state heat-loss model Q = U × A × ΔT describes how heat flows at a constant rate through any flat building element. U is the thermal transmittance in W/m²K — the inverse of the R-value (m²K/W) — and expresses how many watts escape per square metre per degree of temperature difference. A well-insulated wall might have U = 0.15–0.30, a double-glazed window around 1.1–2.0, and single glazing up to 5.7 W/m²K. The area A is the total surface, and ΔT is the indoor setpoint minus the outdoor design temperature.
For annual energy, multiply the instantaneous watt loss by the number of hours per year the heating system runs. A temperate-climate home with six months of heating accumulates roughly 4 380 heating hours; a cold-climate home with eight months around 5 840 h. Dividing watts × hours by 1 000 converts to kilowatt-hours. To estimate heating cost, multiply by your local electricity or gas tariff per kWh.
This model assumes steady-state conditions — that temperatures are constant and the element is uniform. Real buildings experience thermal bridging, air infiltration, solar gains, and thermal mass effects that are not captured here. Use the result as a design guide or budget estimate rather than a precise bill prediction.
Frequently asked questions
Manufacturers publish U-values in product datasheets and energy labels. For existing buildings, national building standards (e.g. EN ISO 6946 in Europe) give reference values for common construction types; a certified energy assessor can also measure the actual U-value in situ using a heat-flux meter.
Lower is better. Passive House walls target ≤ 0.15 W/m²K; modern Northern European building codes typically require walls ≤ 0.18–0.25 W/m²K and windows ≤ 0.8–1.2 W/m²K. Older single-brick walls can be as high as 2 W/m²K, and single-glazed windows around 5.7 W/m²K.
R-value (m²K/W) is simply the reciprocal of U-value: R = 1/U. It measures thermal resistance — the higher the R-value, the better the insulation. North American building codes (and insulation product labels in the US) cite R-values rather than U-values, sometimes in Imperial units (ft²·°F·h/BTU, where 1 SI R = 5.678 Imperial R).
Also known as
TG we-Calculate Editorial Team. (2026). Heat Loss Calculator — Building Element U-value & Energy [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/heat-loss-calculator
TG we-Calculate Editorial Team. "Heat Loss Calculator — Building Element U-value & Energy." TG we-Calculate. 2026. https://we-calculate.com/calculator/heat-loss-calculator.
TG we-Calculate Editorial Team, "Heat Loss Calculator — Building Element U-value & Energy," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/heat-loss-calculator
@misc{wecalculate_heat_loss_calculator, title = {Heat Loss Calculator — Building Element U-value & Energy}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/heat-loss-calculator}}, year = {2026}, note = {TG we-Calculate} }
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