Estimate steady heat loss through a fabric from area, U-value, and temperature difference.
| Heat loss (W) | – |
Use this calculator for a first watts figure through a wall, roof, or window. Enter the area, the U-value in W/m²·K, and the indoor–outdoor temperature difference in kelvin (or °C difference).
Watts = area × U × ΔT. Area can be entered in square metres (m²) or square feet (ft²). U-value stays in W/m²·K. ΔT stays in K; do not convert a temperature difference as if it were a thermometer reading.
This is fabric loss only. It does not add ventilation, thermal bridges, or solar gains.
Area: Element area, in m² or ft².
U-value (W/m²·K): Thermal transmittance from a table or a certificate.
ΔT (K): Indoor minus outdoor, in kelvin (same number as °C difference).
Heat loss (W): Steady fabric loss in watts.
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