Accurately predicting building energy use with TM54 assessments

Have you ever wondered how to predict a building's operational energy? If you are involved in the design process for the built environment and you haven’t, now is the time to change that.

Accurately predicting and reducing a building’s operational energy is increasingly important for developers aiming to meet tightening sustainability goals while reducing whole-life carbon and project running costs.

A CIBSE TM54 Operational Energy assessment evaluates a building’s operational energy use by modelling how it will perform once occupied. Unlike compliance‑based calculations, TM54 considers occupancy and use patterns to provide a more realistic prediction of Heating Ventilation and Air Conditioning (HVAC) loads, lighting, and small power demands, thus helping clients understand true energy performance and reduce performance gaps.

We have long relied on Excel workbooks in conjunction with robust modelling software to deliver high-quality TM54 assessments. Historically, these assessments were carried out using IES software. As the industry evolved, we have transitioned to DesignBuilder, unlocking fresh opportunities but also introducing some technical challenges. In particular, around the variety of HVAC modelling options available within the platform.

Rather than allowing these challenges to slow our progress, our team used them as an opportunity to innovate. We’ve worked collaboratively, both internally and with DesignBuilder, to refine and enhance our internal modelling processes, ensuring our TM54 assessments remain accurate, efficient, and aligned with best practice.

Over recent months, our team has:

• Developed bespoke scripts to help model complex mechanical designs across multiple HVAC modules. These include modelling peak lopping cooling in Mechanical Ventilation Heat Recovery (MVHR) units and modelling hybrid ventilation units, commonly used in schools.
• Built reusable templates within the detailed HVAC environment to speed up modelling common HVAC configurations.
• Partnered with the DesignBuilder’s team to produce scripts that support post-processing and improve data handling, allowing us to extract desired variables and format them into the Excel calculation sheet much faster.
• Streamlined our operational energy Excel calculation sheet for compatibility with all DesignBuilder HVAC modules, aligning outputs clearly and reducing manual intervention.

In our transitional process, we have consistently validated DesignBuilder and the new modelling methodologies against previous project work to ensure accuracy is maintained.  

DesignBuilder vs Excel calculation tool

The main HVAC heating and cooling demand is always modelled within building modelling software. However, within our TM54 calculation sheet are multiple other calculations which provide an option to use information from DesignBuilder or perform a faster calculation in Excel. One example of this is the process for calculating internal lighting energy.

Despite the Excel calculation sheet not being as detailed as using DesignBuilder, the Excel calculation has been developed to consider key factors, including lighting controls, daylight dimming, emergency lighting and parasitic power. It uses approved British Standards (BS EN 15193), Lighting Energy Numeric Indicator (LENI) principles, NCM Modelling, and CIBSE TM54 methodology to estimate these.  

A comparison of lighting energy results between the Excel calculation tool and when modelled in DesignBuilder demonstrated only a 2.99% difference in lighting energy. Demonstrating the validity of both calculation methods for predicting in-use lighting energy.

DesignBuilder Simple vs Detailed HVAC modelling methodology

DesignBuilder offers two different HVAC modelling approaches: Simple and Detailed HVAC. Detailed HVAC allows consideration of part load efficiencies for the fans, heating, and cooling systems, as well as greater control over the fresh air supply temperature.  

We have been able to assess the difference in energy consumption when using these options, and the following differences in EUI for the building were seen. The assessment showed minimal difference in heating and fan energy in comparison to the simple HVAC methodology. Although the cooling energy did increase, when considering the other end uses, it was found that the whole building’s EUI was only 0.43kWh/m2 GIA/yr greater, representing only a 0.57% increase in EUI. 

Considering these case studies, we can confidently demonstrate our ability to adjust and adapt the required level of detail based on specific project needs whilst maintaining accuracy using this method. This approach is particularly beneficial at the early stages of a project, where we’re able to provide fast and accurate results that can be used to test multiple design options and guide the design towards the most efficient solution for our clients. We can then refine the modelling at the later design stages, once a single concept is being developed, to maximise detail and accuracy, to provide the best prediction of operational energy.

By investing in smarter workflows and deeper technical capability, we’re proud to continue delivering TM54 assessments that provide clarity, confidence, and meaningful insight into real world building performance. If you're looking to gain a more accurate understanding of your building’s operational energy, our team is ready to help.