The UK’s energy landscape is set to change with the introduction of the Home Energy Model (HEM), which is due to replace the Standard Assessment Procedure (SAP) in 2025. SAP has been the primary framework for assessing home energy performance since 1993. As technology and energy usage patterns evolve, however, its reliance on outdated methods has become a limitation. HEM promises to address these shortcomings, using a more accurate, sub-hourly, dynamic modelling approach.
Despite numerous refinements, SAP’s core methodology has remained unchanged, employing a ‘steady state’ method to average energy usage over a month. This limits precision, particularly in accounting for modern dynamic technologies such as energy storage, advanced control systems, and dynamic energy tariffs, which operate on a much faster timescale.
Procedural limitations have also arisen. For instance, SAP enforces fixed assumptions about occupancy, used across all applications to facilitate direct comparisons. As SAP’s role expanded to support various policies, however, this one-size-fits-all approach was found to cause differences in predicted and actual energy consumption.
Recognising these challenges, the UK government initiated the development of a new model – HEM. This will use a sub-hourly modelling approach to capture granular variation in energy usage, significantly improving its ability to model the interplay of various building technologies and user behaviours.
HEM promises a framework that can adapt to innovation in energy technology, anticipating advancements in residential spaces. Aligned with the Future Homes Standard, it will ensure energy assessments are accurate and reflect current and future energy dynamics.
The architecture behind HEM
HEM is built on the EN 52016 suite of standards, incorporating both new and existing methodologies. The core EN 52016 model conducts heat-balance analyses that compute the movement of heat within and between the building’s structural and internal components. This includes calculating temperature changes and heat flows for each component, such as walls and air spaces, for each timestep. The design allows it to perform simulations using any desired timestep, although it has been configured and tested using a half-hourly step to match the UK’s energy metering systems. Developed in Python, its architecture is open-source and inherently modular, encouraging ongoing enhancements and collaborative development. A user-friendly codebase makes it accessible to a broad audience.
A significant advancement in HEM is the introduction of ‘wrappers’ – layers of code that integrate specific policy or use-case assumptions into the core model without altering its underlying structure. This addresses SAP’s rigidity by allowing dynamic input of assumptions regarding occupancy, climate and appliance, enabling more accurate energy profiles that reflect real-world usage. This makes HEM adaptable for various applications, from occupancy-standardised asset ratings to modelling the energy use of dwellings of specific occupants.
Validation
HEM has undergone extensive inter-model comparisons to validate its building physics algorithms against established modelling tools, such as the Passivhaus Planning Package and Environmental Systems Performance – Research (ESP-r). These comparisons align input values across models, ensuring a fair assessment of each model’s ability to simulate energy dynamics within dwellings accurately.
Validation phases confirm HEM’s potential while highlighting areas for refinement, ensuring that the model evolves in response to emerging technologies and standards (See Figure 1).
While HEM is a significant advancement, it has limitations that require ongoing refinement. One challenge is its simplified approach to handling ventilation-related losses and gains, which can affect accuracy, particularly in assessing cooling energy requirements and the potential for overheating. Currently, HEM is configured as a two-zone model, which may not fully capture the complexity of overheating in different rooms. Planned enhancements will improve granularity and accuracy.
Another challenge is operational speed. Unlike SAP, which provides near-instantaneous results, HEM simulations take approximately one minute. While a step forward in computational detail and accuracy, it poses challenges for applications such as stock modelling, where numerous simulations must be run quickly. As the model is optimised, processing times are expected to decrease.
HEM also requires detailed inputs from users, which enables accurate modelling but increases complexity. Guidance and tools are being developed to help users navigate this.
Despite these challenges, HEM’s open-source, modular nature allows for continuous improvements by a broad community, facilitating immediate enhancements.
Looking forward
HEM’s potential extends beyond individual homes. Its flexible, open-source architecture could enhance neighbourhood energy models and financial tools, leveraging detailed, real-time data to aid smart, sustainable energy use. By engaging with HEM, manufacturers and stakeholders can refine their products and drive innovation in energy solutions.
Easy access
SAP’s rigid software specification often limited the scope of participation and innovation. In contrast, HEM’s open, modular structure allows anyone with basic software skills to download, use and modify the model. Individual components can be updated or replaced independently, allowing continual refinement without overhauling the entire system. This accessibility encourages a dynamic process of improvement and adaptation, where users can identify bugs, suggest changes, or even propose code enhancements that can be integrated swiftly into the main codebase after appropriate reviews.
Its success relies on diverse insights and adaptability to feedback. The UK government is promoting engagement through public consultations and transparent processes for proposing and integrating code changes, and must continue to facilitate widespread input into the model’s development.
User and developer forums will provide a structured environment where improvements can be discussed and integrated, while clear guidelines for codebase modifications will help focus efforts on changes that offer significant benefits.
We are on the threshold of a new era in home energy assessment. The open nature of HEM’s codebase has the potential to revolutionise how energy modelling is conducted, encouraging collaborative development and potentially spurring the creation of spin-off software products and systems.
HEM is poised to adapt continually to the evolving landscape of building technology, driving forward the UK’s energy efficiency and decarbonisation goals. l
l John Henderson is a principal consultant and Jose Ortiz a director at Sustenic
