Heat Loss Calculator

Estimate heat loss through one stated building element, then turn the result into a transparent heating-energy and cost scenario.

Use a whole-assembly U-value for the specific wall, roof, floor, window or other surface. Enter the average indoor-to-outdoor temperature difference for the period you want to screen.

Steady heat-loss rate600 W2,047.285 Btu/h
Heat loss over stated duration14.4 kWh49,134.845 Btu
Estimated heating input16 kWhAfter the entered heating-system efficiency
Estimated energy cost1.92 currencyAt your entered energy price

Important: this is conduction through one stated surface only: Q = U × A × ΔT. It excludes air leakage, thermal bridges, solar gain, internal gains, changing weather and interaction with other building elements. The input-energy estimate assumes a constant duty and stated efficiency; it is not a heat-pump COP, boiler fuel-use or whole-building energy model.

Method

The steady heat-loss rate is Q̇ = U × A × ΔT. Multiply the rate by a stated duration to estimate the associated thermal energy. The optional input-energy estimate divides that thermal energy by your stated heating-system efficiency.

Worked example

A 100 m² wall with U = 0.30 W/(m²·K) and a 20 K temperature difference loses 0.30 × 100 × 20 = 600 W. Over 24 hours at those constant conditions, that is 14.4 kWh of heat. At 90% heating efficiency, the corresponding input energy is 16 kWh.

Important limits

This is a surface-conduction estimate, not a whole-building heating-load or energy-use calculation. Air infiltration, thermal bridges, solar gains, weather variation, occupancy, system cycling, plant efficiency and tariff structure can materially change actual energy demand and cost.

Related tools

Thermal Resistance & U-value Calculator · Heat Conduction Calculator · Heat Transfer Calculators

Frequently asked questions

What heat-loss equation does this calculator use?
For steady conduction through one stated surface, it uses Q̇ = U × A × ΔT. Multiplying the rate by a stated duration gives thermal energy loss.

What U-value should I enter?
Use the U-value of the complete building element, not an individual material conductivity. An assembly U-value may include layers, surface films and other construction effects.

How is heating energy cost estimated?
The surface heat loss is divided by the heating-system efficiency you enter, then multiplied by your energy price. Both values are a transparent scenario assumption rather than a billing forecast.