TM54:2022, Evaluating operational energy use at the design stage, is guidance developed by the Chartered Institution of Building Services Engineers (CIBSE) in the UK. It provides a methodology for estimating how much energy a building is likely to use when it is actually in operation.
If that sounds similar to Part L, there is some overlap, but the two are doing different jobs.
Simply put, a Part L calculation is primarily a compliance model. It uses standardised assumptions about things such as occupancy, operating hours and temperatures so that buildings can be assessed consistently against the Building Regulations. It is not intended to predict exactly how much energy that particular building will use once it is occupied.
TM54 is performance modelling. It uses assumptions that are intended to better represent the particular building, including how it is likely to be occupied, operated and managed. It also considers energy uses such as lifts, catering, servers, small power and specialist equipment.
TM54 was developed in response to the performance gap: the difference that can occur between energy use estimated during design and the amount of energy a building actually uses in the real world. The methodology is intended to produce a better-informed estimate of operational energy use and to test how that estimate changes when important assumptions change.
Below, we answer some of the common questions people have when they first come across TM54.
1. What does a TM54 assessment actually involve?
TM54 is not simply a calculation that gives you one number. It provides a step-by-step methodology for evaluating operational energy use.
The full guidance runs from Step 0 to Step 17. In simple terms, the process can be grouped into five stages:
- Choose how the building will be modelled: The modelling method and level of detail are established first
- Work out how the building is expected to be used: This includes occupancy, operating hours and how different parts of the building are likely to operate
- Estimate the different energy uses: This includes lighting, lifts, small power, catering, servers, hot water and other equipment, as well as heating, cooling, fans and pumps
- Represent the building services and how the building will be managed: HVAC systems, controls and operational management can all have a significant effect on energy use
- Test the result: Sensitivity analysis and different scenarios are used to understand which assumptions matter most. Results are then compared with project targets or benchmarks and reported with the assumptions behind them
A TM54 result should therefore not be treated as a guaranteed prediction. It is a detailed estimate of likely operational energy use, with uncertainty considered as part of the methodology.
2. When might you come across TM54?
TM54 can be used for more than simply estimating a building’s energy use. It can help a project team test design decisions, work towards operational energy targets and understand how different assumptions about the way a building is used could affect its future performance.
In practice, you may come across TM54 in connection with:
- Planning applications: In London, this can include operational energy modelling required under the GLA’s Be Seen energy monitoring requirements for relevant major developments
- Sustainability certification such as BREEAM: Operational energy modelling may be required as part of the relevant energy credits
- Net zero and operational energy targets: TM54 can be used to test whether a design is on track to meet a project energy target
- Design option testing: For example, a team might compare the likely energy impact of two different heating, cooling or ventilation strategies before deciding which option to take forward
- Energy forecasting before occupation: This can give an owner or occupier a better idea of the building’s likely energy use before it is in operation
- Future climate and operating scenarios: Scenario testing can be used to explore how changes in weather, occupancy, operating hours, controls or management could affect energy use
- Setting energy targets for the building in use: These can later be compared against actual meter readings once the building is occupied
3. Where does TM54 sit in a project?
Operational energy modelling is intended to be an iterative process. It can start with target setting and early briefing, then be updated as the design develops, through construction, commissioning, handover and eventually into the in-use stage.
For that reason, TM54 is particularly useful when it is started early enough to inform the design, rather than only being carried out at the end to describe a building that has already been designed.
The earlier a team understands that a design is likely to miss its energy target, the more options it still has to change the building, the services or the way it is expected to operate.
That does not mean a TM54 assessment can only be carried out at the beginning of a project. It may also be commissioned later. Once the building is occupied, assumptions can be updated to reflect how it is really being operated and the model can be compared with measured energy use.
4. Where can I get a copy of TM54?
The current document is CIBSE TM54:2022, Evaluating operational energy use at the design stage.
It is a copyrighted CIBSE publication and is available through the CIBSE Knowledge Portal.
Be careful with PDFs found elsewhere online. The original 2013 edition has been superseded and should not be used for current projects.
There is also a July 2024 corrigendum covering corrections to pages 35 and 36. Current CIBSE PDF downloads incorporate those corrections.
5. Where can I see a real TM54 assessment report example?
One of the easiest ways to understand what a TM54 assessment actually looks like is to read a completed report.
For London projects, TM54 reports and operational energy assessments can often be found in the public documents submitted with major planning applications.
A simple way to find them is to search a borough planning portal for terms such as TM54, operational energy or Be Seen.
Two public examples:
- TM54 Operational Energy Evaluation, Hillingdon planning portal
- TM54 assessment, Planning Inspectorate document service
CIBSE also provides worked case studies with TM54:2022, which are useful for seeing how different modelling approaches can be applied.
6. How do you learn to carry out a TM54?
If you have been asked to do one for the first time, a sensible order is:
- Read TM54:2022 itself: Understand what each step is trying to achieve rather than simply copying somebody else’s report
- Read real assessments: Look at how other teams record assumptions, results and scenarios
- Understand the modelling tool you are using
- Learn how the TM54 workflow differs from compliance modelling
- Have your first assessments reviewed by someone experienced
The difficult part is rarely just operating the software. It is deciding which assumptions are reasonable and making sure the model represents the project appropriately.
7. Is TM54 a legal requirement?
TM54 itself is guidance, not legislation. However, a planning condition, sustainability assessment, client brief or regulation may require an operational energy assessment or energy forecast, with TM54 being the methodology specified or accepted for producing it.
So the more useful question is usually: what has this particular project actually asked me to provide?
8. Do you need TM54 for Part L compliance?
No.
Part L compliance uses the National Calculation Methodology, such as SBEM or approved dynamic simulation software for non-domestic buildings.
TM54 is a separate performance-modelling exercise. It estimates likely operational energy use rather than demonstrating compliance with the Building Regulations.
9. Is TM54 the same as an EPC?
No.
An EPC provides an asset rating for the building. Part L determines whether the design complies with the Building Regulations. TM54 estimates likely operational energy use.
They may use some of the same building information or modelling software, but they are answering different questions.
10. Do you need dynamic simulation software to carry out a TM54?
Not always.
TM54 allows different modelling approaches, including spreadsheet-based or quasi-steady-state methods and dynamic simulation modelling. This does not mean a TM54 assessment is normally calculated entirely by hand. Even simpler approaches generally use spreadsheets or calculation tools to organise and calculate the different energy uses.
Choosing the modelling approach is actually Step 0 of the methodology. The tool and level of detail should be appropriate for the particular project.
Dynamic simulation provides a more detailed route. It can represent the building fabric, HVAC systems and controls and model operation hour by hour across the year.
12. Can you carry out a TM54 on a residential building?
Yes. The methodology can be applied to any type of building, new or existing.
Whether you actually need one is a separate question and will depend on the project requirements.
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