Heat Loss Calculator
Heat loss is the single number that determines what heating system your home needs. It is how fast your home leaks heat to the outside on a cold day, measured in kilowatts, and any boiler or heat pump has to be able to replace it at least as fast.
Enter your floor area and pick the closest match for your walls and windows. The calculator estimates your heat loss at design conditions and suggests the heat pump output to start looking at.
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What is heat loss?
Every home loses heat through its walls, roof, floor, windows and through air leaking in and out. The colder it is outside, the faster that happens. Heat loss is the rate of that leak at a chosen design condition, and it is expressed in kilowatts because it is a rate rather than a quantity.
This matters because heating systems are sized in kilowatts too. If your home loses 6 kW on a cold day, a 5 kW heat pump cannot hold temperature no matter how long it runs.
How this calculator works
The calculator uses a heat loss parameter: a typical number of watts lost per square metre of floor area, for each degree of difference between inside and outside. That parameter changes with construction, so the calculator varies it by wall type and adjusts it for glazing.
The estimate is then:
floor area × heat loss parameter × temperature difference
The temperature difference is your target indoor temperature minus this calculator’s fixed illustrative design outdoor temperature of −3 °C. It is not selected from your postcode. At a 21 °C target that gives a 24 °C difference.
What real installations show
The two assumptions above — a −3 °C design temperature, and heat loss scaling with floor area — can be compared with metadata from heat pumps that have actually been installed and monitored, published by their owners through HeatpumpMonitor.org. Its heat-loss field is the reported pre-installation calculation, not heat demand measured by the monitoring equipment.
Reported design outdoor temperatures
Across 753 monitored installations, the median reported design outdoor temperature is -3.0 °C, and -3 °C is the single commonest choice, on 262 of them.
719 of 753 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 687 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 | 120 |
| 100 to 150 | 6.0 kWmiddle half 4.8–7.4 kW | 232 |
| 150 to 200 | 7.3 kWmiddle half 5.8–8.8 kW | 179 |
| 200 to 300 | 8.1 kWmiddle half 6.8–10.5 kW | 112 |
| 300 and above | 11.4 kWmiddle half 8.4–13.8 kW | 44 |
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. 753 of 805 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
Why heat pump sizing matters
Sizing errors go wrong in both directions, but not symmetrically. An undersized heat pump cannot keep the house warm on the coldest days, which is the failure people notice. An oversized one costs more to buy and cycles on and off rather than running steadily, which is less comfortable and less efficient.
Because being short is the worse outcome, this calculator rounds its suggestion up to the next half kilowatt rather than to the nearest.
What this cannot tell you
This is a whole-house benchmark, not a survey. It does not know how much of your wall is window, whether your loft insulation is at current depth, how draughty the house is, or how any individual room behaves. Two homes with identical floor areas and wall types can differ substantially on all four.
Before anyone specifies equipment, an MCS-certified installer must carry out a room-by-room calculation. Treat this figure as a way of knowing roughly what to expect and whether a quote looks sensible, not as a specification.
If you want to know what a heat pump would cost to run rather than what size it needs to be, the solar and heat pump data page carries observed UK deployment figures.
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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Written by
Rob Gibbs
Hi, I'm Rob, and I run Energy-Review.co.uk. I initially started this project in 2018 when I was looking to switch energy suppliers and found there wasn't a website that provided simple, data-backed reviews on all the suppliers available. Since then, I have spent a lot of time (too much, some may say!) looking at all publicly available data about each supplier and writing reviews using this information. These reviews are updated as regularly as possible, and any data is backed up by a source where necessary. I have also started writing guides on various energy-related topics, which hopefully you will find useful. If you find any issues, please use our contact form to let us know.