Whether you're designing an HVAC system, auditing building energy performance, or specifying insulation, calculating heat loss accurately is essential. This guide explains the exact formulas used in our Heat Loss Calculator — including inputs, outputs, and a worked example.
What Is Heat Loss?
Heat loss refers to the rate at which thermal energy escapes from a building surface (wall, roof, window, floor) to the outside environment. It is driven by three factors:
- The area of the surface
- The thermal transmittance (U-value) of the material
- The temperature difference between inside and outside
Formula 1 – Temperature Difference (ΔT)
The temperature difference is simply the gap between indoor and outdoor temperatures.
Formula:
ΔT=Tindoor−Toutdoor
Formula 2 – Heat Loss in Watts (Q)
This is the core heat loss formula, derived from Fourier's Law of Heat Conduction:
Formula:
Q=U×A×ΔT
Where:
- Q = Heat Loss (Watts)
- U = U-Value / Thermal Transmittance (W/m²·K)
- A = Surface Area (m²)
- ΔT = Temperature Difference (°C or K)
Formula 3 – Heat Loss in Kilowatts (kW)
To convert watts to kilowatts:
Formula:
QkW=1000Q
Formula 4 – Heat Loss per Unit Area (W/m²)
This tells you how much heat is lost per square metre of the surface — useful for comparing different materials or construction types.
Formula:
q=U×ΔT
Where:
- q = Heat Loss per Unit Area (W/m²)
- U = U-Value (W/m²·K)
- ΔT = Temperature Difference (K)
Worked Example
Given:
- Surface Area: 25 m²
- U-Value: 1.8 W/m²·K (single-glazed window)
- Indoor Temperature: 21°C
- Outdoor Temperature: -5°C
Step 1 – Temperature Difference:
ΔT=21−(−5)=26K
Step 2 – Heat Loss (W):
Q=1.8×25×26=1170W
Step 3 – Heat Loss (kW):
QkW=10001170=1.17kW
Step 4 – Heat Loss per Unit Area:
q=1.8×26=46.8W/m2
In the above example, the indoor temperature is 26 K higher than the outdoor temperature, giving a temperature difference (ΔT) of 26 K. Using the heat loss formula (Q=U×A×ΔT ), the calculated heat loss is 1,170 W, which is equivalent to 1.17 kW. The heat loss per unit area is 46.8 W/m², meaning that each square meter of the surface loses approximately 46.8 watts of heat under the given conditions.
The U-value of a building component has a significant impact on heat loss. Typical U-values are approximately 5.0–5.8 W/m²·K for a single-glazed window, 1.2–2.8 W/m²·K for a double-glazed window, and 0.6–1.0 W/m²·K for a triple-glazed window. Insulated building elements generally have much lower U-values, such as 0.18–0.35 W/m²·K for an insulated cavity wall, 0.15–0.25 W/m²·K for an insulated flat roof, and 0.45–0.70 W/m²·K for a solid floor. Lower U-values indicate better insulation and reduced heat loss, resulting in improved energy efficiency.
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