As the industry adjusts to the Future Homes Standard, Jon Ponting of AES Sustainability Consultants examines why the long-delayed Home Energy Model will be a transformative break from the long-established SAP method.
For almost two decades, the Standard Assessment Procedure (SAP) has served as the foundation for measuring the energy performance of new homes in the UK. Introduced in the 1990s and made mandatory from 2006, SAP has been refreshed several times as each new Part L update has arrived. Yet despite these periodic revisions, its core structure remains rooted in assumptions and methodologies that are now significantly outdated. As energy efficiency has grown from a peripheral consideration into a central component of modern construction, SAP’s limitations have become increasingly apparent, prompting the need for a more sophisticated and transparent system – cue the Home Energy Model (HEM).
HEM is not simply another iteration of SAP, nor a rebranding exercise. It represents a complete redesign, one that fundamentally rethinks how we model, track and interpret the energy performance of homes. The Government has consciously avoided the label ‘new SAP’ because the underlying differences are so substantial. While both SAP and HEM produce results related to energy use and efficiency, almost everything about how HEM calculates those results is new. Its level of detail, level of accuracy and capacity to incorporate modern and emerging technologies mark a dramatic advance beyond anything SAP can currently offer.
Perhaps the most transformative change is the shift from monthly modelling to half-hourly modelling. SAP analyses homes’ performance using 12 month averages, meaning it can only approximate conditions such as how solar panels generate across a typical month or how energy demand shifts throughout the day. HEM, by contrast, operates using 30-minute intervals, resulting in roughly 17,520 data points over the course of a year. This dramatic increase in resolution enables HEM to account for daily variation in sunlight, cloud cover, peak-hour electricity use, overnight demand, battery storage behaviours and how energy systems interact with one another in real time.
Equally important is HEM’s transparency. Many of SAP’s assumptions and equations date back decades, often without clear documentation on their original rationale. Over time, this has led to a model that behaves in ways that can feel opaque, even to experienced assessors. HEM, however, is built on a fully traceable, research linked methodology. Every calculation and variable maps back to defined evidence or modelling work, enabling users to understand precisely why results appear as they do. This clarity not only improves confidence but also makes it possible to interrogate results in ways SAP cannot support. It also positions HEM to incorporate new technologies more effectively. For example, thermal batteries, which are now increasingly common, are not supported by SAP at all, leaving assessors with no meaningful way to model them. HEM, built with modern energy systems in mind, can integrate these technologies and evolve with future innovations far more readily.
The timeline for HEM’s introduction has shifted several times. The original intention was for HEM to launch at the same time as the new Future Homes Standard and updated Part L Building Regulations. However, the Government ultimately chose to delay HEM to avoid the risks associated with a simultaneous, sector-wide changeover. The current expectation is that the first version will be released in summer 2026, with the new Part L taking effect in 2027. From that point, there will be a two-year period of dual running during which housebuilders and developers can choose whether to use SAP or HEM for new projects. After this period, Government is expected to announce a six-month notice window ahead of SAP’s withdrawal for new sites, likely placing full adoption of HEM around 2029 or 2030.
It is also important to recognise that HEM will not be fully complete at launch. The first release will not include EPC calculations or running cost assessments. These will be introduced in HEM Version Two, anticipated for late 2026 or early 2027.
This staged rollout reflects the model’s scale and complexity, as well as the need to build the training, accreditation and auditing frameworks that will surround it. Although the HEM technical engine is largely ready, it cannot be deployed until these industry-wide processes are established. Assessors will require training, conventions must be developed, and accreditation pathways need to be put in place. Unlike previous SAP updates, where most assessors could adapt with minimal effort, HEM involves a fundamentally different way
of working and will therefore require dedicated upskilling.
Another factor accelerating the relevance of HEM is the upcoming reform of Energy Performance Certificates (EPCs) planned for 2030. Under new rules, all privately rented and social housing will be required to meet revised EPC standards. Crucially, SAP will not be capable of producing EPCs that comply with the 2030 framework. Any new EPCs which are generated after the reforms come into force will require HEM.
Although existing EPCs produced under SAP will remain valid, landlords and housing providers undertaking new assessments will need to ensure they are using HEM. This long-term shift makes early familiarisation with HEM particularly valuable for organisations with large or evolving portfolios.
Overall, the Home Energy Model represents a long-awaited modernisation of how the UK models energy use in new homes. By offering far greater accuracy, full transparency and the flexibility to accommodate new and emerging technologies, it brings energy assessment into line with contemporary building practices and net zero ambitions.
While the transition will require adjustment, including training, dual running and new accreditation processes, the benefits are substantial. With HEM expected to be fully integrated into regulation by the end of the decade, now is the moment for housebuilders, developers, assessors and landlords to begin preparing for a system which is designed for the future, not the past.
Jon Ponting is Future Homes Standard policy lead at AES Sustainability Consultants