Heat Loss Calculator
Enter your floor area and pick the closest match for your walls and windows for an indicative peak heat-loss figure and heat-pump size to investigate.
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What is heat loss?
Heat loss is how quickly a home loses heat on a cold day, measured in kilowatts. A heating system must replace heat at least as fast. This estimate uses floor area, broad construction choices and your target temperature; it is not a room-by-room survey.
What real installations show
The figures below are pre-installation heat-loss calculations reported for British homes with monitored heat pumps. They are not heat loss measured by the monitoring equipment, so use them as context rather than a target for your property.
Reported design outdoor temperatures
Across 805 monitored installations, the median reported design outdoor temperature is -3.0 °C, and -3 °C is the single commonest choice, on 272 of them.
769 of 805 sit between -6 °C and 0 °C. The rest include entries as low as -25 °C and as high as 15 °C, and 26 report exactly 0 °C. The source does not distinguish a valid 0 °C choice from an unset/default value, so these records remain counted. The median includes them and is less sensitive to extremes than the mean; it is not described as unaffected.
Reported calculated heat loss by floor area
From 737 monitored installations that publish both a calculated heat loss and a floor area. This is the calculation entered in the system metadata, not heat demand measured from monitoring.
| Floor area (m²) | Reported calculated heat loss at design conditions | Homes |
|---|---|---|
| Under 100 | 4.7 kWmiddle half 3.5–5.6 kW | 128 |
| 100 to 150 | 5.9 kWmiddle half 4.8–7.3 kW | 244 |
| 150 to 200 | 7.4 kWmiddle half 5.9–8.8 kW | 197 |
| 200 to 300 | 8.0 kWmiddle half 6.7–10.5 kW | 121 |
| 300 and above | 11.8 kWmiddle half 8.6–14.1 kW | 47 |
Reported calculated heat loss varies widely within each floor-area band. Construction, insulation, ventilation and calculation method may all contribute; this read cannot attribute the difference to one cause.
These are homes whose owners chose to publish their own monitoring, so they are not a representative sample of British housing. 805 of 857 published installations are used here; the other 52 carry the publisher's own data_flag and are excluded.
These self-selected monitored installations carry no stored location, so they cannot show how design temperatures or calculated heat loss vary across Great Britain.
Heat pump performance data from HeatpumpMonitor.org, published by the system owners under CC BY 4.0 HeatpumpMonitor.org · CC BY 4.0
Before choosing equipment
An installer must replace this whole-home estimate with a room-by-room heat-loss calculation before specifying a system. An undersized pump may not keep the home warm; an oversized one can cost more and cycle inefficiently. Use the heat-pump running-cost calculator to compare likely energy costs separately.
Frequently asked questions
What is a good heat loss figure for a UK home?
There is no single good figure based on floor area alone. In this simplified model, the same 100 square metre home can move from about 3 kW to more than double that when its illustrative fabric assumption changes. A room-by-room survey must replace that scenario before equipment is specified.
What size heat pump do I need?
As a starting point, a heat pump needs to deliver at least your peak heat loss at the design outdoor temperature. This calculator rounds up to the next half kilowatt for that reason. It is only an indication: an MCS-certified installer must carry out a room-by-room calculation before specifying equipment, and their figure is the one to buy against.
Why does the calculator use -3 °C outside?
Minus 3 °C is a fixed simplifying assumption in this calculator, not a value selected for your postcode. MCS design temperatures are location-specific, so an installer must replace it with the appropriate local design condition.
Is this the same as an MCS heat loss calculation?
No. An MCS calculation works room by room against measured U-values, actual window areas and ventilation rates. This calculator uses a whole-house benchmark based on floor area, wall construction and glazing, so it gives an indicative figure for planning rather than a specification to buy against.
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