Envelope · Heat pumps
Average heat loss of a house, and the heat pump it needs
There is no single average: a house loses heat in proportion to its insulation, its air leakage, its size and how cold it gets, so the honest answer is a calculation, not a number. What the published references give are fixed points. A Passivhaus may need no more than 10 W of heating per m² of floor on the coldest day [1]. In the UK government's largest heat pump trial, 742 heat pumps were installed, 7.7 kW on average, in a group of homes recruited at around 115 m² on average, and the report notes that installers often overestimated the heat loss [2]. The heat loss calculator's own house of 100 m² comes to 3.07 kW, 30.7 W/m², at −2 °C outside. That design heat load is also the heat pump: the UK heat pump standard asks for one that covers all of it at the design outside temperature [5].
Published figures, each with its source
| What | Figure | Source |
|---|---|---|
| Passivhaus limit | Heating load at most 10 W per m² of treated floor area, steady state, with nothing for warming up after a setback (the alternative to 15 kWh/(m²·a) of heating demand). | [1] |
| UK heat pump trial, Electrification of Heat | 742 heat pumps installed; homes recruited averaged around 115 m²; mean heat pump 7.7 kW; air source heat pumps most often 8.5 kW, from 5 to 16 kW. Installers and design tools often overestimated heat loss compared with the actual U-values. | [2] |
| Wall U-value an EPC assumes, England, cavity as built | 1.5 W/(m²·K) before 1976; 1.0 for 1976–1982; 0.60 for 1983–1995; 0.45 for 1996–2002; 0.35 for 2003–2006; 0.30 for 2007–2011; 0.28 for 2012–2022; 0.26 from 2023. | [3] |
| Doors and junctions an EPC assumes, England | Doors 3.0 W/(m²·K) up to 2006, 2.0 for 2007–2011, 1.8 for 2012–2022, 1.4 from 2023. Thermal bridging y = 0.15 W/(m²·K) up to 2002, 0.11 for 2003–2006, 0.08 from 2007. | [3] |
| Air | Minimum 0.5 air changes an hour in every room with an outside wall; where nothing is measured, an air permeability of 12 m³/(h·m²) at 50 Pa. | [4] |
| UK design temperatures | Outside, at 99 % and 99.6 % of the hours: London −1.7 and −3.0 °C, Manchester −2.7 and −4.5 °C, Glasgow −3.5 and −5.9 °C. Inside: living room 21 °C, bedroom 18 °C, bathroom 22 °C. | [5] |
An EPC (RdSAP) assumes these values when nothing better is known about a house; a survey with the real build-up replaces them.
Worked example: the house the calculator opens with
100 m² of heated floor and 250 m³ of heated volume, 21 °C inside and −2 °C outside, junctions at y 0.08 W/(m²·K) and 0.5 air changes an hour of leakage, with no mechanical ventilation. Its elements:
| Element | m² | U, W/(m²·K) |
|---|---|---|
| External wall | 90 | 0.3 |
| Roof, external | 55 | 0.16 |
| Ground floor | 55 | 0.25 |
| Windows | 18 | 1.4 |
| Doors | 4 | 1.8 |
| Result | Calculator |
|---|---|
| Design heat load | 3.07 kW: 3068 W · 10469 BTU/h at ΔT 23.0 K |
| Per m² of floor | 30.7 W/m² |
| Heat loss coefficient | 133.4 W/K: fabric 74.4 · bridges 17.8 · air 41.3 |
| Annual heat demand | 6403 kWh a year, 64 kWh/m²·yr at 2000 degree days |
| Reading | Moderate load. At 30.7 W/m² a heat pump works, but check every room individually: some emitters will need to grow before the flow temperature can come down to 45 °C. |
| Heat pump | at least 3.07 kW at −2.0 °C outside, at the flow temperature of the emitters |
| Hybrid with a boiler | at least 1.69 kW from the heat pump, the minimum share the standard sets MCS MIS 3005-D, 5.5.2 |
The biggest line is not a wall: it is ventilation and leakage, 41.3 W/K, 31 % of the house. The calculator's best single change is halving the infiltration to 0.25 ach, which takes the design load from 3.07 to 2.59 kW.
Worked example: the same house, age band by age band
The same house with the wall, the doors and the junction allowance an English EPC assumes for cavity walls as built in each age band RdSAP 10, Tables 1, 6, 21 and 26. Roof, floor, windows and air stay as the calculator opens, so the table shows what those three things do, not a whole house of that age.
| Age band | Wall U | Doors U | y | Design load | Per m² |
|---|---|---|---|---|---|
| D, 1950–1966 | 1.5 | 3.0 | 0.15 | 6.02 kW | 60.2 W/m² |
| G, 1983–1990 | 0.60 | 3.0 | 0.15 | 4.16 kW | 41.6 W/m² |
| K, 2007–2011 | 0.30 | 2.0 | 0.08 | 3.09 kW | 30.9 W/m² |
| M, 2023 onwards | 0.26 | 1.4 | 0.08 | 2.95 kW | 29.5 W/m² |
Each band needs less than the one before: from 6.02 kW for band D to 2.95 kW for band M, with the rest of the house unchanged.
The calculator takes the element rows by hand, not from a link: type the wall and door U-values and choose the y-value.
Worked example: the same house in four UK design temperatures
The house the calculator opens with, unchanged, at the outside design temperatures the UK heat pump standard takes from CIBSE Guide A MCS MIS 3005-D, Table 2.
| Place, hours | Outside | Design load and heat pump | Per m² | Hybrid share |
|---|---|---|---|---|
| London, 99 % | −1.7 °C | 3.03 kW open | 30.3 W/m² | 1.67 kW |
| Manchester, 99 % | −2.7 °C | 3.16 kW open | 31.6 W/m² | 1.74 kW |
| Glasgow, 99 % | −3.5 °C | 3.27 kW open | 32.7 W/m² | 1.80 kW |
| Glasgow, 99.6 % | −5.9 °C | 3.59 kW open | 35.9 W/m² | 1.97 kW |
Each degree colder adds the same: the heat loss coefficient, 133.4 W for every kelvin. Two calculations for the same house with the same U-values but different design temperatures will not give the same heat pump.
From the heat loss to the heat pump
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Work out the design heat load
It is the heat the house loses on a design day, in kW: every element's area times its U-value, plus the junctions, plus the air that leaves, times the difference between inside and outside. The UK heat pump standard asks for it to BS EN 12831-1, with an inside temperature for each kind of room [5]. The calculator does the whole-house version, which is the figure a heat pump is sized on; the radiators need it room by room.
A rate in kW, not a yearly kWh. The annual figure is for the bills; the design load is for the plant.
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Choose the design outside temperature
Not the record low: the temperature the air stays at or above for 99 % or 99.6 % of the hours. MCS takes it from CIBSE Guide A, and with the 99.6 % column no extra allowance for intermittent heating is needed [5]. In the United States the load for sizing equipment is calculated to ACCA Manual J, which building codes and most jurisdictions require [7].
A colder design temperature is a bigger load. In the example above, every kelvin is the heat loss coefficient again.
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Cover all of it with the heat pump
A heat pump shall provide at least 100 % of the calculated heat load at the flow temperature it will run at, without help from the supplementary electric heater, and the manufacturer's and the emitter designer's performance data should back the choice [5]. A hybrid that keeps a boiler is the exception: the heat pump must give at least 55 % of the load, rated at 55 °C flow at the design outside temperature [5].
100 % at your outside temperature and your flow temperature, not at milder conditions.
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Read the output at your conditions
The rated heat output (Prated) in a heat pump's EU product information is declared at the reference design conditions of the ecodesign regulation, a design temperature of −10 °C [6], which is probably not yours. The figure to compare with the design load is the manufacturer's output at your design outside temperature and your flow temperature.
Ask for the output table, and find your two temperatures in it.
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Size the hot water on its own
The standard designs hot water separately: the daily demand sets the cylinder, sized together with the heat pump's capacity, and the reheat time is agreed with the customer [5]. The heat loss figure is space heating only, and the calculator adds nothing for hot water.
The design load is heating only.
Questions
What is the average heat loss of a house?
There is no single figure that fits, because insulation, air leakage, size and climate all change it. The published fixed points: a Passivhaus may need at most 10 W/m²; in the UK's Electrification of Heat trial the homes recruited averaged around 115 m² and the heat pumps installed 7.7 kW. The calculator's 100 m² example comes to 3.07 kW, 30.7 W/m², at −2 °C outside. [1] [2]
How do I calculate the heat loss of my house?
Add up every element's area times its U-value, add an allowance for the junctions, add the air that leaves (0.33 × volume × air changes an hour), and multiply the total by the difference between the inside and the design outside temperature. The heat loss calculator does exactly that and shows each line.
What size heat pump do I need for my house?
One whose output at your design outside temperature, at the flow temperature your emitters need, covers 100 % of the design heat load. For the example house that is at least 3.07 kW at −2.0 °C outside. [5]
Why do heat loss figures often come out too high?
Default values. In the Electrification of Heat trial, installers and design tools often overestimated heat loss compared with the actual U-values. Check the U-values and the air change rate before anything else: in the example the air, not the walls, is the biggest line. [2]
Does the heat pump also have to cover hot water?
Hot water is designed on its own: the daily demand sets the cylinder, sized with the heat pump's capacity, and the reheat time is agreed with the customer. The heat loss figure is for space heating only. [5]
Sources
- Passive House Institute, Criteria for Buildings, version 10c (January 2023), Table 1. univention.passiv.de/nextcloud/s/version10c-2023
- BEIS / Energy Systems Catapult, Electrification of Heat Demonstration Project: Home Surveys and Install Report (2022): section 4.3.3 (page 27), section 5.2 (page 35), section 9.1.4 (page 64). es.catapult.org.uk/wp-content/uploads/2022/12/BEIS-Electrification-of-Heat-Home-Survey-and-Install-Report.pdf
- BRE, RdSAP 10 Specification (9 June 2025): Tables 1, 6, 21 and 26. bregroup.com/documents/d/bre-group/rdsap-10-specification-10-06-2025
- MCS Heat Load Calculator, documentation, Ventilation Rates: CIBSE Domestic Heating Design Guide (2026) Table 2-18 and BS EN 12831-1:2017 Table B.6. heatloadcalculator.mcscertified.com/docs/reference-sources/ventilation-rates
- MCS, MIS 3005-D The Heat Pump Standard (Design), issue 2.0 (2 December 2024): clauses 5.5.1, 5.5.2 and 5.6; Tables 1 and 2. mcscertified.com/wp-content/uploads/2025/02/MIS-3005-D-Issue-2.0-Final-.pdf
- Commission Regulation (EU) No 813/2013 (ecodesign of space heaters), Article 2(7) and Annex III, Table 4 (text at legislation.gov.uk). www.legislation.gov.uk/eur/2013/813/annex/III
- ACCA, Manual J, 8th edition (the page of the standard). www.acca.org/standards/technical-manuals/manual-j
Read on 27 October 2026. The clause, table or page is given so each figure can be checked.