From SAP to the Home Energy Model: The difference, and why it matters

At Wincer Kievenaar, we integrate energy assessment directly into our design process through our own in-house SAP assessor. This upcoming transition brings us both an inspiring challenge and an opportunity, and it will reshape how we design and communicate energy performance in our residential projects. But it’s important to note: SAP isn’t being phased out just yet.

In this article, we’re going to discuss the transition and what it means for our work as architects who put sustainability at the forefront.

SAP, SAP 10.3, and the Future Homes Standard

SAP, or the Standard Assessment Procedure, is the UK government’s system for a building’s energy performance assessment. It’s been in place since 1993 and is used to determine a home’s SAP rating, demonstrate compliance, and generate the legally required Energy Performance Certificate (EPC). However, this model is limited and outdated; it is simply no longer useful in providing accurate details for modern, energy-efficient homes.

The Home Energy Model has been designed to solve SAP’s problems, opening the doors to more flexible and highly accurate energy performance assessments. For architects, this means more informed design decisions at early stages.

While the Home Energy Model is still being developed to eventually replace SAP, the upcoming SAP 10.3 update will act as a temporary bridge between the current model and the HEM. This allows us to comply preemptively with the Future Homes Standard, before it is officially released. SAP, for now, will remain a core part of how we manage compliance within our residential design work.

Once the government finalises and releases the Future Homes Standard, we are expecting them to publish a timeline for phasing out SAP. This period of transition will be key for us to prepare our team and systems for the exciting next steps.

Why is SAP being replaced?

SAP has served the industry well since 1993, but its limitations have become increasingly apparent in today’s design and construction landscape.

• Monthly time-step calculations don’t capture real-world energy use. Our home energy consumption fluctuates day to day, and even hour to hour, which is missed by SAP’s outdated monthly calculations, limiting accuracy.
• SAP makes simplified assumptions about occupancy and heating systems. The resulting generic patterns do not accurately reflect how people actually live, which can affect the reliability of each home’s predicted energy performance.
• The modelling capabilities for low-carbon systems and smart technologies are limited. There are many emerging energy solutions, such as heat pumps, smart controls, solar, and battery storage, which are difficult to accurately represent within SAP. This makes it far less useful when it comes to low-carbon home design.

As our homes become more complex, energy-conscious and performance-driven, a more responsive and accurate model is essential. This is especially important for a practice like ours that prioritises sustainable, future-ready residential design.

What is the Home Energy Model (HEM)?

The Home Energy Model is the UK Government’s upcoming calculation tool for assessing the energy performance of homes. This next-generation model will replace SAP with a more detailed approach, monitoring performance every half hour and taking into account thermal zones and local conditions, providing a far more realistic picture of day-to-day home energy usage.

Some of the key features expected are:

• Half-hourly interval calculations for more realistic energy profiles. It will better capture how each home’s energy demand fluctuates throughout the day. Thanks to this, the results will be much closer to real-world behaviour, offering improved accuracy for design.
• Modular architecture for flexible updates and future-proofing. Different components of the new model, such as heating systems, renewable generation, and ventilation, can independently evolve within the model as technology does.
• Open-source Python codebase for transparency and collaboration. Unlike the closed structure of SAP, the new model will be open-source and written in the Python programming language. This means anyone can access the code, which offers greater transparency in how results are generated for architects and energy assessors.
• Detailed input requirements for building fabric, systems, and appliances. More granular data on a building’s construction, equipment, and systems will be required, including controls, appliances, window and wall specs, and even the behavioural patterns of occupants. This will all contribute to creating a complete picture of how a home actually performs.

Our experience with HEM test software

As part of our commitment to staying ahead of industry changes, our team has taken part in early testing of the Home Energy Model software. We’ve engaged with and explored the new system and contributed to the build process by sharing detailed feedback with the development team.

This hands-on involvement has given us valuable insight and a clear understanding of how the model works, what challenges to expect, and how to prepare our design and assessment workflows and teams for the transition. It also means we’re ready to support our clients and collaborators in making the most of HEM as it becomes the industry standard.

What this means for Wincer Kievenaar

The new Home Energy Model marks an exciting step for us at Wincer Kievenaar. Representing a major shift in how homes are assessed for energy performance, it aligns perfectly with our commitment to sustainability and design. Here’s how we’re approaching the shift:

Design integration

HEM’s more detailed and granular modelling means more alignment between our teams for architectural design and energy strategy. Early-stage decisions around home orientation, zoning, and fabric performance will have a bigger impact on compliance outcomes.

Workflow evolution

Our in-house SAP assessor will make the switch to HEM’s new tools and methodologies. They’ll use cloud-based modelling platforms and software, manage detailed data, and collaborate even more closely with our architects and designers to keep results accurate from the very start.

Team upskilling and development

We’re already preparing our team for the shift by engaging in HEM training and pilot projects, reviewing our current workflows to identify any areas that may need adaptation, and exploring HEM-compatible software and data integration strategies we can implement.

Client communication

The Home Energy Model will make more accurate and transparent reporting possible, which will enhance our ability to better inform clients on energy performance, meet new upcoming regulations, and showcase our ongoing commitment to sustainable design

Looking ahead and next steps

The shift from SAP to HEM is far more than a technical update. This transition is an exciting opportunity for Wincer Kievenaar Architects to deliver clients more accurate and future-ready designs and enhance client value through technically sound and easier-to-understand modelling. It will also strengthen internal collaboration between our designers and energy assessors, allowing for more aligned designs for energy-efficient structures. This change can help us produce even better work as leaders in sustainable architecture.

We’re committed to staying ahead of the curve. The clients, consultants, and developers working with us can rest assured that our team is actively preparing for the transition; we’re ready to guide your project through the new energy modelling landscape with ease.