About a year ago, when I was working for Greengage Environmental, I wrote this article explaning Operational Energy Modelling. I thought it was worth sharing it here as well.
Or you can just type “Operational Energy Modelling” on Google. It used to be at the top 😉
Although what is known as operational energy modelling has become increasingly common for both new and existing buildings in recent years, it might still be unfamiliar to those not dealing with energy models on a regular basis. In this article, we’ll answer the main questions in a straightforward way, shedding light on the latest addition to the ever-evolving discipline of energy efficiency in buildings.
- What is operational energy modelling exactly?
- Why do we need it?
- How does it work?
But before we answer these questions, it’s important to put the topic into context.
Where are we coming from?
In 2002, the European Energy Performance of Buildings Directive (EPBD) introduced the requirement for Member States to develop a framework for calculating the integrated energy performance of buildings, to be used for both creating Energy Performance Certificates (EPCs) and setting minimum energy performance requirements for new buildings and major renovations. The adoption of this directive in the UK led to the development in subsequent years of the SAP and SBEM methodologies for domestic and non-domestic energy performance assessments respectively.
The whole-building energy models created using SAP or SBEM methodology provide an estimation of the annual energy consumption of the existing or proposed building based on its main specifications and a series of standardised use profiles for its building type and activities, which is then compared to the energy consumption and calculated carbon emissions of a notional building that is created using the same geometry and profiles and a standard set of specifications. This comparison is the basis of regulatory compliance (Part L) and EPC ratings, the latter having the objective of promoting the improvement of energy performance through market dynamics, by allowing buyers and tenants to include energy performance in their decision making.
However, as the push for achieving net zero ambitions increases and energy price rises loom, there has been a growing debate and concern about the suitability of that methodology for achieving actual energy and carbon savings in real life once buildings are in operation. The ‘performance gap’ between the energy that buildings actually consume and what was calculated by the Part L/EPC models is well known and documented 1 2 3 and there is a general lack of confidence about their usefulness and the quality and accuracy of the results obtained by this method. Discussing the multiple reasons why this happens would take another full article, but there is a fundamental reason why Part L/EPC calculations cannot predict the operational energy consumption of a building with accuracy: it is not their purpose.
As outlined before, the main purpose of these methodologies is evaluating the energy efficiency features of a building as an asset for a typical occupancy, to verify that it complies with minimum requirements (Building Regulations Part L) or to be compared with other similar buildings by a prospective buyer/tenant (EPCs). Their purpose was never estimating the actual energy consumption of a building for a specific use and specific building users. In fact, they only cover the energy uses regulated by Part L: heating, cooling, ventilation, hot water and lighting, and do not include (in SAP) or estimate very roughly (in SBEM) the energy consumption of any other equipment in the building. Therefore, although this was a common misunderstanding before the popularisation of operational energy modelling, they are not the best tool for making accurate predictions of future energy consumption and specific methods have now been developed for this purpose.
Now that we have the context and before we get into further details about the different methodologies and standards, we can give very simple answers to those fundamental questions about operational energy modelling:
- What is operational energy modelling exactly?
- Predicting the future energy consumption of the building as accurately as possible.
- Why do we need it?
- Because energy performance should be evaluated in real life operation.
- How can we do it?
- By putting as much information as possible about the future occupancy, equipment and operation of the building in an appropriate model.
The answers are in essence that simple. It is all about creating a good model with enough information about the building specifications, the expected occupancy and use of equipment that it can predict, with a reasonable level of confidence, the energy consumption that will be obtained from the meter readings once in operation. It is not so much about the accuracy of the modelling methods as it is about the information that we can gather for feeding it, although certain energy uses such as HVAC systems can be quite complex to model accurately in certain buildings.
It is also essential that these models are used to inform design decisions that will drive energy efficiency in real-life operation. This is particularly important where targets are set for operational energy performance in use (e.g. NABERS ratings and operational net zero carbon commitments).
Now, let us answer again the main questions in further detail to get a better understanding of what the current practice is.
What is currently known as operational energy modelling?
Apart from the above-mentioned differences with compliance/EPC models, another fundamental difference in operational energy assessments is the metrics used for obtaining the final results and comparing them with benchmarks. The main result obtained by operational energy modelling is the Energy Use Intensity (EUI) of the building, which is the total energy consumption in all uses divided by the total area of the building (kWh/m²). This result can then be compared directly to actual EUI figures based on metered data, which is its ultimate purpose.
Therefore, operational energy modelling (also known as energy performance modelling) is basically the process by which we obtain a predictive EUI for a new build or retrofit project based on the proposed specifications and as much information as we can gather about the expected use and management of the building once in operation.
This also means that operational energy benchmarking is not based on the comparison with a notional building created with the same geometry, orientation and distribution of spaces as the proposed design, which may not be the most appropriate for the site. Benchmarking comparisons are based just on the total energy use per floor area, and they become absolute targets rather than relative to a specific design.
Why is it becoming the new standard?
For some years now, the industry has acknowledged that business-as-usual Part L/EPC-based energy assessments, even if these have become more demanding, are not delivering the real energy and carbon emission savings that we need urgently. Therefore, other sustainability standards and certifications have been promoting the change of approach to operational energy predictions.
BREEAM UK introduced in its 2018 version credits for undertaking predicted operational energy modelling and analysis at both design and post-construction stages. In 2020 NABERS UK was launched, which is a rating scheme purely based on operational performance imported from Australia, where it has demonstrated its success in driving down energy and emissions for two decades. The Greater London Authority introduced in the 2021 London Plan a requirement for operational energy to be estimated in planning applications, and then monitored and reported in actual operation for at least five years (the Be Seen policy).
The most ambitious voluntary decarbonisation initiatives, such as RIBA’s 2030 Climate Challenge 4 and LETI’s Climate Emergency Design Guide 5, also base their operational energy targets on the actual building EUI. And so do the imminent UK Net Zero Carbon Buildings Standard 6 being developed by the industry’s leading organisations, which is expected to become the de facto net zero standard. They have confirmed the use of the EUI metric for setting the operational targets and their approach will be that to be called Net Zero Carbon, the energy of the building will need to be verified with at least 1 year of metered energy use.
The new RICS Whole Life Carbon Assessment (WLCA) guidance, in effect from July 2024, also requires operational energy modelling to be undertaken to estimate operational carbon emissions when completing WLCAs.
But further to the requirements of these certifications and standards, there are also multiple reasons why building owners are becoming more and more interested in getting a better understanding of how their new or existing buildings will use energy, such as reducing operational costs, planning improvements with more confidence, future-proofing their assets, or making them more attractive to tenants. Although the EUI is usually the main output, operational energy models can also provide detailed outputs of the different energy uses and insights into the different elements affecting building performance. In their most advanced version, they can even be converted into calibrated ‘digital twins’ that mirror the behaviour and dynamics of the real-world buildings to untap further opportunities for energy efficiency and decarbonisation.
How do we carry out these assessments?
In order to facilitate the change from the business-as-usual compliance approach to the new operational energy prediction one, some guidance has been developed defining certain methods and workflows that can help modellers and other project team members in gathering the required information and developing a reliable set of predictive energy results. The main reference document, which has become the de facto standard for this, is CIBSE’s TM54 – Evaluating operational energy use at the design stage 7.
This guidance is organised as a series of steps covering the different elements that a good operational energy prediction requires, putting a lot of emphasis on the importance of considering the uncertainty in many of the variables that determine energy consumption, such as the weather, the occupancy patterns, the use of equipment, or how the systems will be operated and managed. To acknowledge this uncertainty, it recommends providing EUI results as a likely performance range instead of a single figure.
Additional guidance, targeted to a broader range of audiences, can be found in LETI’s Operational Modelling Guide 8, where they incorporate and expand on TM54 recommendations. Similarly, the NABERS UK scheme has also created a guide 9 and a framework, the Design for Performance (DfP) process, specifically for office projects aimed at achieving high operational performance ratings. This process involves committing to certain performance through an agreement with the scheme, going through an independent design review process of the proposed design, including detailed operational energy modelling, and eventually obtaining a NABERS UK energy rating once the building is in operation for a minimum of 12 months and at least 70% occupied.
However, although this guidance is important for providing common ground to operational energy modelling, it is actually just the starting point for creating an accurate and useful model. As discussed previously, gathering specific information about the building is key, but it is also essential that the modellers have the necessary skills and a deep understanding of all the different elements that influence energy performance in buildings since, depending on the building type, the most important factors can vary. It is also fundamental that the importance and the requirements of these models are well understood by the different members of the project team, and that modellers can work in close collaboration with key members, such as the architects, building services engineers and client representatives, to ensure operational energy modelling is used as a design tool to drive high energy performance in operation.
Key takeaways
- The purpose of Part L/EPC energy calculations is not predicting the performance of real buildings accurately.
- New energy efficiency and net zero carbon standards are based on real metered performance rather than energy models.
- We need to be able to predict real performance more accurately so we can design and operate buildings to be as energy efficient as possible in real life.
- Putting as much information as possible about the real building specifications and about the future occupancy and operation of the building is key for obtaining accurate results.
- Operational energy modelling guides, such as CIBSE TM54 or NABERS Design for Performance, are good aids in setting the methodology to follow, but do not provide all the necessary information for making accurate predictions in each building.
- Modellers need to have a deep understanding of energy performance in buildings. Certain energy uses, such as HVAC systems, can be quite complex to model accurately in certain buildings and require a good level of expertise and the appropriate tools.
- Collaboration between modellers, design team and client is key to ensure that the modelling process facilitates the delivery of a high performance building in operation.
- van Dronkelaar C, Dowson M, Burman E, Spataru C and Mumovic D (2016) – A Review of the Energy Performance Gap and Its Underlying Causes in Non-Domestic Buildings. ↩︎
- Innovate UK (2016) – Building Performance Evaluation programme. ↩︎
- Zero Carbon Hub (2014) – Closing the gap between design and as-built performance. ↩︎
- London Energy Transformation Initiative (2019) – Climate Emergency Design Guide. ↩︎
- Royal Institute of British Architects (2021) – RIBA 2030 Climate Challenge. ↩︎
- UK Net Zero Carbon Buildings Standard (2023) – Technical Update & Consultation ↩︎
- The Chartered Institution of Building Services Engineers (2022) – Technical Memorandum 54: Evaluating operational energy use at the design stage. ↩︎
- London Energy Transformation Initiative (2023) – Operational Modelling Guide. ↩︎
- NABERS UK (2023) – Guide to Design for Performance. ↩︎