Heat Loss Calculator

A heat loss calculator estimates the rate at which thermal energy passes through part of a building envelope. The steady-state transmission equation multiplies surface area by U-value and by the temperature difference between indoors and outdoors. Area is measured in square metres, U-value in watts per square metre-kelvin, and a Celsius temperature difference is numerically equal to a kelvin difference. Their product is watts of continuous heat flow. U-value describes overall thermal transmittance: a lower number indicates better resistance to heat flow. Use an assembly U-value that includes all relevant layers and surface resistances, not merely the conductivity of one material. Windows, walls, roofs, floors, and doors normally have different values, so calculate each element separately and add the heat-loss rates for an envelope total. Thermal bridges around edges, framing, balconies, and penetrations may require additional calculations. The energy result multiplies the steady heat-loss rate by the selected duration and converts watt-hours to kilowatt-hours. It assumes temperatures and thermal properties remain constant for that entire period. Real weather, solar gain, occupants, appliances, thermostat setbacks, and heating-system cycling all change actual consumption. Air leakage and intentional ventilation are also absent from this transmission-only calculation. A complete heating-load assessment includes infiltration, ventilation, thermal bridging, design temperatures, and appropriate safety factors. For preliminary comparisons, the calculator clearly shows why insulation improvements matter. Halving the U-value halves conductive heat loss under the same area and temperature difference. Reducing exposed area or indoor-outdoor temperature difference has the same linear relationship. This makes the result helpful when comparing replacement windows, insulation upgrades, or alternative envelope specifications. Do not size a boiler, heat pump, or electrical service solely from this simplified result. Equipment selection should follow local design standards and account for the complete building. Use measured dimensions, verified product or assembly U-values, and a recognized local winter design temperature. With those inputs, this calculator provides a transparent component-level estimate that can be checked, discussed, and combined with other envelope elements before a qualified designer completes the final heating-load calculation.