Whole-home heat loss calculator

Work out whole-home design heat loss from assessed fabric and ventilation heat transfer. Fabric heat transfer describes heat passing through parts of the building. Ventilation heat transfer describes heat carried out by air movement. Both inputs need to come from a property assessment or a suitable measured study.

Enter the two heat-transfer figures in watts per kelvin, shown as W/K. Add the indoor and outdoor design temperatures used in that assessment. The result is the heating power needed for those stated conditions. It is not an estimate made from floor area, the age of the home or a typical household energy bill.

Whole-home heat transfer = fabric W/K + ventilation W/K. Design heat loss kW = total W/K × (indoor − outdoor °C) ÷ 1000.

A heat-transfer coefficient is the number of watts lost for each degree of temperature difference. A one-degree Celsius difference is the same size as one kelvin. Subtract outdoor temperature from indoor temperature, then multiply by the combined coefficient. Divide watts by 1000 to express the answer in kilowatts.

The result is power, rather than energy used over a year. Kilowatts, written kW, describe a rate of heat loss at the entered temperatures. Kilowatt-hours, written kWh, include time. This tool does not multiply the design loss by every hour in a year or convert it into a heating bill.

Check the substituted working against the assessment. Make sure the fabric and ventilation figures cover the same building. Adding a whole-home measured total as fabric heat transfer and then adding ventilation again would count part of the heat loss twice.

These illustrative inputs are separate from your saved answers. Prices, use and performance are held constant for this worked scenario; they are not personal defaults.

The coefficients must come from a professional assessment or measured heat-transfer study with its stated uncertainty. No assumed wall, air-change or floor-area value is supplied. Blank fields stay empty; an example does not become a property measurement.

The chosen indoor and outdoor temperatures define this scenario. Changing the temperature difference changes the result while the entered coefficients stay fixed. That is an arithmetic comparison, rather than a weather forecast or proof of how the home performs in every condition.

This is not a room-by-room radiator design, a heat-loss survey or a heat pump sizing certificate. It does not check emitter output, equipment performance or installation suitability. Money and energy prices are not used. Display values are rounded, but the calculation uses unrounded numbers. An invalid edit keeps the last valid snapshot visible with a warning.

Use a documented assessment for the UK property being considered. Keep its units, building boundary and design conditions with the entered figures. A source explaining the heat-loss method does not supply the missing measurements for your home.

The checked dates identify the stored method records. They do not show that your property has been surveyed or that a particular design has been approved. If an assessment uses different terms, ask its author which figures match the two coefficient fields. Do not substitute an energy bill or equipment rating for a heat-transfer coefficient.

Ask a qualified heating professional when you do not have matching coefficients or need room-by-room design work. Bring the assessment, its assumptions and the chosen temperatures. Ask whether its fabric figure excludes ventilation so the two entries can be added safely.

Before choosing equipment, have an installer check the property, emitters and available heat output at the design conditions. This calculation can help explain the assessed total, but it cannot certify performance, safe installation or compliance with local requirements.