CIBSE TM54 Explained: Why Part L Does Not Tell You How Much Energy Your Building Will Actually Use
Imagine finishing a new commercial building.
The project passed Part L.
The energy model looked good.
The EPC was strong.
Then twelve months of utility bills arrive.
Actual consumption bears little resemblance to what the project team expected.
What went wrong?
Possibly nothing went 'wrong' with the compliance calculation at all.
It may simply have been asked to answer a question it was never designed to answer.
This is where CIBSE TM54 becomes important.
TM54 provides a methodology for evaluating operational energy performance at the design stage and helps project teams investigate the gap between expected and realised energy consumption. CIBSE itself describes this difference as the performance gap.
What is CIBSE TM54?
TM54 is CIBSE guidance for evaluating the likely operational energy performance of buildings during design.
In plain English:
It helps estimate how much energy the real building might actually use.
That sounds like something every energy model should already do.
But there is an important distinction between:
compliance modelling
and
operational-energy modelling.
They have different purposes.
Why Part L is not an energy-bill prediction
Part L answers an important question:
Does this design meet the energy requirements of the Building Regulations?
To do this consistently, calculations use standardised assumptions.
That allows different buildings to be compared using the same rules.
But real occupants do not behave according to standard assumptions.
A real office may operate:
12 hours rather than eight
at different heating setpoints
with more computers
with large server loads
on weekends
with catering equipment
with extended cleaning periods
with tenant-installed equipment
The building may also have controls that are operated differently from the design intention.
A Part L calculation can therefore be completely valid while actual energy consumption is substantially different.
What does TM54 do differently?
TM54 encourages designers to evaluate operational energy using assumptions that are relevant to the actual building.
That means investigating things such as:
realistic occupancy
actual operating hours
small power
server and IT loads
catering
lifts
external lighting
heating
cooling
ventilation
domestic hot water
controls
renewable generation
CIBSE's TM54 methodology also considers scenario testing, sensitivity analysis, targets, benchmarks, assumptions and limitations.
This is crucial.
Rather than pretending that one predicted number is definitely correct, the analysis can ask:
What happens if our assumptions change?
A simple example
Suppose an office model assumes occupancy from 8am to 6pm.
The tenant later announces that the building will support international teams and operate from 6am until 11pm.
The ventilation runs longer.
Lighting runs longer.
IT equipment runs longer.
Heating and cooling run longer.
Even if every system performs exactly as designed, annual energy consumption may rise significantly.
A compliance model may not have been intended to capture that operational reality.
A TM54-style analysis can.
The problem with one-number energy predictions
Clients often want a simple answer.
How much energy will the building use?
Unfortunately, buildings do not work like that.
The result depends on uncertain variables.
Take heating setpoint alone.
An office operated at 19°C will behave differently from the same office operated at 23°C.
Now combine that with different occupancy, weather, equipment loads and working hours.
The range expands quickly.
A better operational model therefore provides:
a central prediction
clearly documented assumptions
alternative scenarios
sensitivity analysis
That tells the client much more than one impressive-looking number.
TM54 should influence the design
This is the part that matters most.
TM54 should not become another report produced for the project folder.
The model should reveal opportunities.
Imagine the assessment shows that fan energy is unexpectedly high.
The team can investigate:
lower duct pressure drops
larger ductwork
more efficient fans
different air-distribution strategies
demand-controlled ventilation
Or perhaps cooling demand dominates.
The team can look at:
glazing
orientation
solar shading
equipment gains
setpoints
natural ventilation
thermal mass
Operational-energy analysis is most valuable when it creates a design conversation.
MEP systems need realistic loads
Mechanical systems are frequently designed around peak conditions.
That is necessary.
But buildings spend relatively little time operating at absolute peak load.
The rest of the year, plant may operate at partial load.
This means the project team should ask more than:
Is the equipment efficient at its rated condition?
They should also ask:
How will it perform during the thousands of hours when the building requires much less output?
This is particularly important for:
heat pumps
chillers
pumps
fans
boilers in existing systems
variable refrigerant systems
ventilation plant
Good operational modelling helps these questions emerge early.
Small power is not always small
Plug loads are easy to underestimate.
Computers.
Monitors.
Printers.
Chargers.
Kitchen equipment.
Specialist tenant equipment.
Servers.
Displays.
They can make a significant contribution to total electricity use.
They also generate heat.
That means one assumption can influence two parts of the model:
electrical consumption
cooling demand
A higher equipment load therefore does not just add electricity.
It may increase the amount of electricity required to remove the heat that equipment produces.
This is why tenant information is so valuable.
Controls can make or break the prediction
A system might have excellent theoretical efficiency but operate unnecessarily.
Examples include:
ventilation running while the building is empty
heating starting several hours before occupants arrive
cooling running during winter
heating and cooling operating simultaneously
pumps remaining at full speed
temperature setpoints being overridden
occupancy sensors being disabled
These issues are difficult to represent accurately if the design team treats controls as a late-stage BMS specification.
The operational-energy model should help develop the controls philosophy.
Use TM54 before the design is fixed
Running operational-energy analysis during Stage 4 may still identify problems.
The issue is that many solutions will already be expensive.
If excessive cooling demand is identified during Stage 2, the team can reconsider glazing or shading.
At Stage 4, the likely solution may be larger cooling equipment.
Both approaches can make the calculation work.
Only one reduces the underlying demand.
This is why energy modelling is most useful as a design tool rather than a final compliance check.
Operational energy and net zero
As the industry moves towards genuine net-zero outcomes, predicted operational energy matters more.
You cannot credibly plan a net-zero building without understanding the demand you are trying to reduce.
A strategy that says:
install heat pumps
add PV
buy renewable electricity
is not enough.
The first step should be understanding and reducing the building's actual likely energy requirement.
This is where TM54-style analysis becomes useful.
TM54 and NABERS UK
TM54 and NABERS are not the same thing, but they sit within the same broader shift towards operational performance.
TM54 helps designers evaluate likely operational energy.
NABERS UK provides a performance-based framework for UK offices, including Design for Performance and measured Energy for Offices ratings.
Both challenge the idea that minimum compliance automatically guarantees a low-energy building.
It does not.
TM54 and the UK Net Zero Carbon Buildings Standard
The same logic is now visible at industry level.
Version 1 of the UK Net Zero Carbon Buildings Standard was published in March 2026 and provides a unified framework for verifying buildings as Net Zero Carbon Aligned.
That strengthens the case for treating operational energy as a genuine project outcome rather than a theoretical design statistic.
If a project eventually needs to demonstrate real performance, understanding likely operational consumption during design becomes increasingly valuable.
When should a TM54 assessment happen?
Ideally, operational energy should evolve alongside the design.
Stage 1
Establish targets, benchmarks and initial assumptions.
Stage 2
Model the concept and compare major design options.
Stage 3
Update the model as MEP, façade and occupancy information improves.
Stage 4
Test detailed systems, controls and specifications.
Stage 5
Track major substitutions that could influence energy performance.
Stage 6
Compare commissioned systems with the assumptions used in the model.
Stage 7
Compare predicted consumption with measured energy data.
The last step is particularly valuable.
It creates a feedback loop.
Why post-occupancy measurement matters
No matter how sophisticated the model becomes, it is still a model.
Real data tells you what actually happened.
After occupation, compare:
predicted electricity
measured electricity
predicted heating demand
measured heating
expected renewable generation
actual output
design operating hours
real schedules
When they differ, investigate why.
The answer might be occupant behaviour.
Or commissioning.
Or controls.
Or an incorrect design assumption.
That information can improve the current building.
It can also improve the next project.
Part L compliance is the beginning, not the end
There is nothing wrong with compliance calculations.
They serve an essential regulatory purpose.
The mistake is expecting them to answer every energy-performance question.
Think of it this way:
Part L asks whether the building meets the rules.
TM54 asks how the building is likely to behave.
Post-occupancy monitoring tells you how it actually behaved.
A climate-intelligent project should care about all three.
How Climery can help
Climery's building-performance and MEP teams work together so operational-energy modelling can directly influence the building's engineering strategy.
Support can include:
CIBSE TM54 operational-energy assessments
Dynamic simulation modelling
Energy-use-intensity target setting
MEP options modelling
Thermal comfort analysis
HVAC optimisation
Controls strategy
Metering strategy
NABERS-aligned analysis
Net-zero operational-energy strategy
Post-occupancy evaluation
Model-to-meter performance review
The aim is not to make the model look good.
The aim is to help the building perform well.
Want to understand how much energy your project is actually likely to use before the design is locked? Speak to Climery about operational-energy modelling during the early design stages.
FAQs
What is CIBSE TM54?
TM54 is CIBSE guidance for evaluating the likely operational energy performance of buildings at design stage.
What is the difference between TM54 and Part L?
Part L modelling is primarily intended to demonstrate Building Regulations compliance using defined methodologies. TM54 focuses on evaluating likely operational energy using assumptions and scenarios relevant to the real building.
Is TM54 the same as SBEM?
No. SBEM is commonly used within non-domestic regulatory compliance calculations. TM54 is focused on evaluating operational energy performance.
When should TM54 modelling be undertaken?
It is most valuable from early design onwards because the results can influence building form, façade, MEP strategy and controls before those elements are fixed.
Can TM54 guarantee actual energy consumption?
No energy model can guarantee exactly how a completed building will be operated. TM54 improves the evaluation by explicitly examining realistic assumptions, scenarios and sensitivities.
