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What Does a Heat Loss Report Include?
A professional heat-loss report should show more than one final kW figure.
- Last updated: September 2026
- Heat Loss Hub · Advice Centre
A professional heat-loss report should tell you how much heat your property needs at the chosen design conditions — but a useful report should show considerably more than one whole-house number.
You should normally be able to find the whole-house design heat loss, individual room heat losses, indoor design temperatures, outdoor design temperature, building dimensions and areas used, construction types and U-values, fabric heat losses, ventilation and infiltration losses, important assumptions, the calculation method or design basis, and enough information to understand how the result was reached.
Some reports may also include radiator assessments, heat-pump sizing or wider system design, but these are additional design tasks and should not automatically be assumed to form part of every heat-loss calculation.
KEY TAKEAWAY
A heat-loss figure is a result. A heat-loss report should show the evidence behind it.
You should not need to be a heating engineer to understand the important parts.
KEY TAKEAWAY
A good heat-loss report should let you follow the calculation without requiring you to redo it.
GET STARTED
Get an independent room-by-room heat-loss calculation with a transparent, readable report.
Start by Checking That the Report Describes the Right House
Before looking at the final heat-loss figure, check something much simpler. Does the report actually describe your property correctly?
Look for the correct property, number of floors, rooms, extensions, loft conversions, windows and external doors, known insulation improvements, significant alterations, and heated and unheated areas.
If your home has a rear extension but the calculation only represents the original building, the final result cannot accurately describe the whole property. The same applies if a loft conversion is missing, a room has the wrong dimensions or insulation improvements have not been accounted for.
A calculation can be mathematically correct and still produce the wrong answer if the information going into it is wrong.
KEY TAKEAWAY
Before checking the maths, check that the report describes the right house.
1. Whole-House Design Heat Loss
Most reports will contain a headline result for the property. For example: Whole-house design heat loss: 7.4 kW.
This represents the calculated heating power required by the property under the design conditions used in the calculation.
It does not mean the property will constantly consume 7.4 kW of electricity. It is also not annual energy consumption, annual electricity use, an annual running-cost prediction, or automatically the correct nominal size of heat pump to install.
It is a design load.
KEY TAKEAWAY
The whole-house heat loss is a design load, not an annual energy-use figure.
The number only has proper meaning when you also understand the conditions under which it was calculated. We will come to those shortly.
2. Room-by-Room Heat-Loss Figures
A professional room-by-room calculation should provide an individual heating requirement for each heated room. For example:
| Room | Design Heat Loss |
|---|---|
| Living room | 1,420 W |
| Kitchen | 890 W |
| Bedroom 1 | 680 W |
| Bedroom 2 | 610 W |
| Bathroom | 510 W |
These room figures are important because different rooms lose heat at different rates. A large living room with external walls and substantial glazing may need considerably more heat than a small internal bedroom.
KEY TAKEAWAY
The whole-house figure tells you how much heat the property needs. The room figures tell you where it needs it.
The individual room results can later be used when assessing radiators or other heat emitters.
If you have been given something described as a room-by-room heat-loss calculation, there is a very simple first check: if the calculation is described as room-by-room, you should be able to find the result for each heated room.
For a deeper explanation of why the distinction matters:
Understanding Watts, Kilowatts and W/m²
Heat-loss reports often use several similar-looking units. Fortunately, the basics are straightforward.
Watts — W
Individual room heat losses are commonly shown in watts. For example: Bedroom: 680 W. That means the calculated heating requirement for that room at the design conditions is 680 watts.
Kilowatts — kW
Whole-house results are often easier to display in kilowatts. 1,000 W = 1 kW. So: 7,400 W = 7.4 kW.
Watts Per Square Metre — W/m²
Some reports also show heat loss relative to the floor area of the room. For example: Bedroom heat loss: 680 W. Heat loss: 48 W/m².
The 680 W is the total room requirement. The W/m² figure gives context to how thermally demanding that room is relative to its floor area.
KEY TAKEAWAY
The watts tell you what the room needs. W/m² gives context to how demanding the room is relative to its size.
3. Indoor and Outdoor Design Temperatures
This is one of the most important parts of the report to understand. A heat-loss figure does not exist independently of temperature.
Your report should identify the temperatures used for the calculation. For example: Outdoor design temperature: -3.1°C.
Individual rooms may then have different indoor design temperatures, such as Living room: 21°C, Bedroom: 18°C.
The calculation is effectively asking: how much heat must this room receive to maintain the chosen indoor temperature when it is at the chosen design temperature outside?
That is why the temperatures matter. If you calculate a living room to maintain 18°C, you will obtain a different heat loss from calculating it to maintain 21°C. Likewise, the same property calculated against an outdoor temperature of -5°C will have a different design load from one calculated against -2°C.
KEY TAKEAWAY
A heat-loss number without its design temperatures is missing part of its meaning.
You do not need to understand every detail of how those temperatures were selected. But you should be able to see what they are.
4. Building Dimensions and Areas
Heat-loss calculations depend on the size and shape of the building. The report or supporting calculation data may therefore include information such as room dimensions, floor area, external wall areas, window areas, door areas, floor construction areas, ceiling or roof areas, and adjacent spaces.
Why does this matter? Because a room does not lose heat equally through every surface. An internal wall separating two similarly heated rooms is very different from an external wall separating the room from winter conditions outside. A floor above a heated room is different from a floor above an unheated space. A wall adjoining a heated bedroom is different from one adjoining a cold garage.
KEY TAKEAWAY
Heat loss depends on what separates the heated room from the colder space around it. The calculation therefore needs to represent both the geometry of the room and what lies beyond its boundaries.
5. Construction Types and U-Values
You may see numerous U-values in a heat-loss report. You do not need to learn how to calculate them to make use of the report.
In simple terms, the U-value represents the thermal performance being used for a particular part of the building. You may see U-values for external walls, windows, external doors, floors, roofs, ceilings and other building elements.
The construction of the property matters because two walls of identical size can lose very different amounts of heat. An insulated modern wall and an uninsulated older solid wall should not simply be treated as thermally identical.
If a wall’s U-value is genuinely uncertain, our guides explain when testing is worth it and how in-situ measurement works.
KEY TAKEAWAY
The U-value tells you how much thermal performance the calculation has assumed for that part of the building.
This becomes particularly useful if you have two calculations that produce substantially different results. Do not immediately assume that one person’s arithmetic is wrong. Look at the inputs. Did they use the same wall construction? Did they account for the same insulation? Did they use similar window performance?
KEY TAKEAWAY
If two heat-loss reports disagree, the U-values are one of the first places worth comparing.
6. Fabric Heat-Loss Breakdown
A good report may show more than the total heat loss for each room. It may also show how that loss is distributed across different parts of the building. For example:
Bedroom — Total Heat Loss: 720 W
| Heat-Loss Source | Heat Loss |
|---|---|
| External wall | 190 W |
| Window | 105 W |
| Ceiling | 155 W |
| Floor | 85 W |
| Ventilation | 185 W |
| Total | 720 W |
This gives considerably more information than simply stating: Bedroom: 720 W. You can see which elements are contributing most heavily to the room’s heating requirement.
KEY TAKEAWAY
The total tells you how much heat the room needs. The breakdown tells you where the heat is going.
This can also make unexpected results easier to investigate. If one room has a surprisingly high heat loss, the breakdown gives you somewhere to start looking.
7. Ventilation and Infiltration Heat Loss
Not all heat leaves your home through walls, windows, floors and roofs. Heat is also lost when warm indoor air is replaced by colder outside air.
A heat-loss calculation therefore needs to account for relevant ventilation and air movement. Depending on the property and calculation method, this may include factors associated with general air changes, background ventilation, extract ventilation, chimneys, open flues, other ventilation openings, and infiltration through the building.
The exact treatment will depend on the property and calculation method. The important point when reading the report is that ventilation heat loss should not simply disappear from the calculation.
KEY TAKEAWAY
A house loses heat through its surfaces and through the air that moves through it. A useful report accounts for both.
Ventilation assumptions are also worth checking when two reports disagree. Two designers could use similar dimensions and fabric U-values but obtain different total heat losses because the building’s air movement has been treated differently.
8. What Was Measured, Evidenced and Assumed?
Not every piece of information about an existing house can be known with absolute certainty. A professional report should make important uncertainty visible rather than quietly turning every unknown into a fact. It is useful to think about the information in three categories.
Measured
Information directly recorded from the property. For example: room dimensions, window dimensions, door dimensions, ceiling heights.
Evidenced
Information supported by reliable documentation or other evidence. For example: architectural drawings, building specifications, window specifications, insulation records, renovation documentation.
Assumed
Information selected where definitive evidence is not reasonably available. For example, the precise construction or thermal performance of a concealed part of an older building may not always be directly verifiable without destructive investigation. An appropriate assumption may therefore be necessary. That is not automatically a problem.
KEY TAKEAWAY
A good report should distinguish what was measured, what was evidenced and what was assumed.
And importantly: an assumption is not automatically a weakness. A hidden assumption is much harder to evaluate.
If somebody later obtains better information, a visible assumption can be reviewed. A hidden assumption is much harder to identify.
9. Where Did the Information Come From?
Closely related to assumptions is traceability. Suppose the report uses a particular U-value for a window. Where did that value come from? Possibilities might include manufacturer information, architectural specification, known installation records, site inspection, reference data, or an engineering assumption.
The same principle applies throughout the calculation. Information might originate from site survey, customer measurements, architectural drawings, EPC information, construction specifications, manufacturer data, renovation records, reference data, or professional assumptions.
Not every report needs pages of references for every individual figure. But significant inputs should be sufficiently transparent that important assumptions can be understood and challenged where necessary.
KEY TAKEAWAY
The report should show not only the number, but where the number came from. This becomes particularly important when two calculations disagree.
10. Existing Property vs Proposed Improvements
Some heat-loss calculations model more than one version of the property. For example:
Existing Property
The house as it currently stands.
Proposed Property
The house after planned measures such as wall insulation, loft insulation, floor insulation, replacement windows, replacement doors, an extension, or wider renovation work.
This can be extremely useful when planning improvements. But the two cases need to be clearly identified. If a proposed heat-pump design depends on insulation that has not yet been installed, the homeowner should be able to see that.
KEY TAKEAWAY
If the report depends on future insulation or glazing improvements, the existing and proposed cases should be clearly separated. Otherwise, a lower future heat-loss figure could easily be mistaken for the performance of the house today.
11. Calculation Method and Design Basis
A professional report should identify the basis on which the heat-loss calculation was produced. Depending on the report, this may include calculation methodology, calculation software or tool, relevant standard, design conditions, and reference information.
You do not need a long technical explanation of every formula. But there should be enough information to identify the basis of the calculation.
KEY TAKEAWAY
A professional report should make clear which calculation method and design basis were used. Simply producing a final kW number without any visible context makes the result much harder to evaluate later.
12. Report Date, Project Reference and Revision
Basic document information can become surprisingly important. A professional report should ideally make it clear which property it relates to, when the calculation was completed, which project it belongs to, and which version or revision is current.
Why does the revision matter? Imagine that the first calculation assumes an uninsulated wall. The homeowner later provides evidence that insulation was installed. The calculation is revised. There are now two different reports containing two different heat-loss figures. Both may have been correct according to the information available when they were produced. But only one represents the latest project information.
KEY TAKEAWAY
If the inputs change, the report should make it obvious which calculation is current.
What May NOT Be Included in a Heat-Loss Report?
This is an important distinction. Providers package their services differently, and the phrase heat-loss report does not automatically mean complete heat-pump system design. A typical distinction might look like this:
| Core Heat-Loss Information | May Be Additional Design Scope |
|---|---|
| Room heat loads | Radiator sizing |
| Whole-house heat load | Heat-pump selection |
| Design temperatures | Flow-temperature design |
| U-values | Cylinder sizing |
| Fabric losses | Pipe sizing |
| Ventilation losses | Hydraulic design |
| Assumptions | Controls strategy |
| Calculation basis | System schematics |
Some providers may bundle several of these additional services together. Others may offer heat loss as a standalone calculation. Neither approach is inherently right or wrong. The important thing is knowing which service you are buying.
KEY TAKEAWAY
A heat-loss report describes the load. A full design decides how the heating system should meet it.
For a complete system design rather than heat loss alone:
Explore: Independent Heat Pump Design
Does a Heat-Loss Report Tell Me Whether My Radiators Are Big Enough?
Not automatically. Suppose the heat-loss calculation establishes: Living room heat requirement: 1,420 W. That tells you what the room needs under the design conditions.
It does not automatically establish whether the existing radiator can provide 1,420 W at the water temperatures proposed for the heat-pump system. That requires an emitter assessment.
The radiator’s type, height, length, construction and operating temperatures all affect its available output.
KEY TAKEAWAY
The room heat loss tells you what the radiator needs to deliver. It does not automatically tell you whether the radiator can deliver it.
Some heat-loss services include radiator assessment as an additional item. Others do not.
Explore: Heat Pump Radiators & Heat Emitters
Does a Heat-Loss Report Tell Me What Size Heat Pump I Need?
Again, not automatically. The whole-house design heat loss is one of the most important pieces of information used when sizing a heat pump. But equipment selection is another design decision.
The designer may need to consider actual manufacturer output, outdoor design temperature, required water temperature, performance at those conditions, modulation, system characteristics, and hot-water strategy.
A property with a calculated heat loss of 7.4 kW does not automatically mean that the correct answer is simply: buy an 8 kW heat pump.
KEY TAKEAWAY
Heat loss tells you the load. Equipment selection is the next design decision.
Does a Heat-Loss Report Predict My Electricity Bill?
No. This is an important distinction. Design heat loss represents the heating power required under defined design conditions. It is not an annual energy calculation.
Annual electricity consumption will depend on many additional factors, including weather throughout the year, indoor temperatures, heating schedule, heat-pump efficiency, flow temperatures, hot-water use, controls, and occupant behaviour.
A heat-loss report is therefore an important design document, but it should not be mistaken for an annual electricity-bill forecast.
KEY TAKEAWAY
Design heat loss is a heating-power requirement at defined conditions, not an annual electricity-use prediction.
More Pages Do Not Automatically Mean a Better Report
Some calculation software can produce extremely detailed documents. That can be useful. But page count is not a measure of engineering quality.
A 40-page report containing large amounts of unexplained software output is not automatically better than a shorter report that clearly shows the design conditions, the room loads, the whole-house load, the important inputs, the U-values, the assumptions, and the sources of important information.
KEY TAKEAWAY
A report is complete because the design basis is visible, not because the PDF is long.
The homeowner should be able to find and understand the important information without having to reverse-engineer the calculation software.
KEY TAKEAWAY
Software can produce the calculation. A good report explains the calculation.
The Five-Minute Heat-Loss Report Check
You do not need to recalculate the house to carry out a useful first check of a heat-loss report. Open the report and answer these eight questions.
1. Property
Does it describe the correct house? Check the rooms, extensions, alterations and known improvements.
2. Rooms
Can you find an individual heat-loss result for each heated room? If it is described as a room-by-room calculation, the room results should be identifiable.
3. Temperatures
Can you find the indoor and outdoor design temperatures? Without them, the final heat-loss figure lacks important context.
4. Fabric
Can you see the building construction or U-values used? You should be able to understand what thermal performance has been assumed.
5. Ventilation
Has ventilation or infiltration heat loss been accounted for? The property loses heat through both its fabric and air movement.
6. Assumptions
Can you identify important information that was assumed rather than measured or evidenced? Unknowns should not quietly become facts.
7. Whole-House Result
Is there a clearly stated whole-house design heat load? You should not have to hunt through pages of software output to find the headline result.
8. Scope
Can you tell where the heat-loss calculation ends and additional system design begins? Radiator sizing, heat-pump selection and hydraulic design may be included, but they are not automatically part of the heat-loss calculation.
If you can answer those eight questions, you already understand considerably more about the quality of the report than somebody who looks only at the final kW figure.
How to Judge Whether a Heat-Loss Report Is Useful
A useful report should be:
Room-by-Room
Individual heated spaces have identifiable results.
Defined
The indoor and outdoor design conditions are visible.
Transparent
Important inputs and assumptions can be seen.
Traceable
Significant information can be connected to its source.
Complete
The calculation considers both building-fabric and ventilation heat losses.
Understandable
You can identify the important results without having to reverse-engineer the software that produced them.
KEY TAKEAWAY
A good heat-loss report should let you follow the calculation without requiring you to redo it.
Frequently Asked Questions
A professional heat-loss report should normally include the whole-house design heat loss, individual room heat losses, design temperatures, relevant building-fabric information, U-values, ventilation losses and important assumptions. It should provide enough information to understand the basis of the final result rather than presenting only one headline number.
If the service has been sold as a room-by-room heat-loss calculation, you should be able to identify the calculated heating requirement for each relevant heated room. A whole-house figure remains important, but it does not replace the individual room results.
A transparent report should make the important thermal assumptions visible. U-values show the thermal performance used for walls, windows, floors, roofs and other relevant building elements.
Yes. The heat-loss result depends on the temperature difference being calculated. Without knowing the indoor and outdoor design temperatures, the final heat-loss number is missing important context.
Ventilation heat loss is the heat lost when warm indoor air is replaced by colder outside air. A complete calculation therefore considers both heat passing through the building fabric and heat lost through relevant air movement.
Important assumptions should be visible. Existing buildings often contain information that cannot be directly verified, so assumptions are sometimes necessary. The important point is that they should not be hidden.
W/m\u00b2 means watts per square metre. It expresses the room’s calculated heat loss relative to its floor area. The room’s total watts tell you its heating requirement, while W/m\u00b2 gives additional context about the heating demand relative to room size.
Not necessarily. Heat loss establishes what each room needs. Radiator sizing determines whether an existing or proposed emitter can provide that amount of heat at the intended operating temperatures. It may be included as an additional service.
Not automatically. The whole-house heat loss is an important input into heat-pump sizing, but actual equipment selection requires further design work and consideration of manufacturer performance data.
No. Design heat loss is a heating-power requirement under defined conditions. Annual energy use depends on weather, operating temperatures, efficiency, heating schedules, hot-water use and many other factors.
No. An EPC assesses and rates the energy performance of a property using a different methodology and for a different purpose. A room-by-room heat-loss calculation is used to establish heating requirements under defined design conditions.
No. Heat loss establishes the building and room heating loads. Full system design may additionally cover heat-pump selection, emitter sizing, operating temperatures, pipework, hydraulics, hot water and controls.
You cannot establish accuracy simply from the final kW figure. Start by checking whether the property information is correct, whether the design temperatures are visible, whether the U-values and construction assumptions appear reasonable, whether ventilation has been considered and whether important assumptions are identified. A transparent report makes those checks much easier.
Start by comparing the inputs rather than the final numbers. Look for differences in property dimensions, rooms included, indoor design temperatures, outdoor design temperature, U-values, construction assumptions, insulation, window performance and ventilation assumptions. Two different results do not automatically mean that somebody has made an arithmetic mistake. The difference may have started with different information or assumptions.
A Useful Report Should Explain the Result, Not Just State It
A professional heat-loss report should leave you with more than one number. You should be able to understand what the whole property needs, what each room needs, which temperatures were used, what building construction was assumed, how fabric and ventilation contributed to the result, which information was known, which information was assumed, and what the report does and does not design.
A heat-loss figure is a result. A heat-loss report should show the evidence behind it.
Before checking the maths, check that the report describes the right house.
A heat-loss number without its design temperatures is missing part of its meaning.
A good report should distinguish what was measured, what was evidenced and what was assumed.
A report is complete because the design basis is visible, not because the PDF is long.
A good heat-loss report should let you follow the calculation without requiring you to redo it.
GET STARTED
Get an independent room-by-room heat-loss calculation with a transparent, readable report.
Explore: Room-by-Room Heat Loss Calculations
Explore: Independent Heat Loss Calculations for Homeowners
Explore: How Much Does a Heat Loss Calculation Cost?
Explore: Room-by-Room vs Whole-House Heat Loss Calculations
Explore: What Information Is Needed for a Heat Loss Calculation?
Explore: Heat Pump Sizing
Explore: What Size Heat Pump Do I Need?
Explore: Heat Pump Radiators & Heat Emitters
Explore: Independent Heat Pump Design