The Home Energy Model (HEM) is the government’s confirmed replacement for the Standard Assessment Procedure (SAP) for rating the energy performance of dwellings in the UK. It is not yet live. SAP 10.3 remains the sole approved methodology when the Future Homes Standard (FHS) launches in Q1 2026. HEM will follow the FHS by a minimum of three months, with a cloud-based tool delivered via the Energy Calculation as a Service (ECaaS) platform by autumn 2026, according to GOV.UK.
Table of Contents
- Why is the Home Energy Model replacing SAP?
- How does the Home Energy Model actually work?
- How does HEM differ from SAP in its calculations?
- When will the Home Energy Model launch in the UK?
- What does the Future Homes Standard mean for HEM compliance?
- How will HEM change Energy Performance Certificates?
- How can CCA Environmental help with HEM and SAP compliance?
- Key Takeaways
- References
Why is the Home Energy Model replacing SAP?
SAP was first introduced in 1993 and last updated in 2012, so it no longer reflects modern construction methods, heating technologies or the way homes consume energy across a day, according to Saint-Gobain. The existing model is built on the Building Research Establishment Domestic Energy Model (BREDEM), as noted by Elmhurst Energy.
SAP calculates energy performance at a monthly time step. This means it cannot capture dynamic effects such as how a heat pump’s efficiency varies with outside temperature, or when solar gains actually occur during the day. The Home Energy Model was announced by government in December 2023 as part of three consultations, two of which dealt specifically with HEM, according to Elmhurst Energy.
The main consultation, titled “Home Energy Model: replacement for the Standard Assessment Procedure (SAP)”, ran from 10am on 13 December 2023 to 11:59pm on 27 March 2024. The closing date was extended from 6 March 2024, according to GOV.UK. The government received 120 responses from organisations, trade bodies, industry professionals, academics and members of the public.
The stated aim is a more accurate and transparent methodology. The government intends to publish the HEM codebase so all changes are visible before formal implementation, according to Elmhurst Energy.
How does the Home Energy Model actually work?
The Home Energy Model is divided into two main parts: the Core Calculation Engine, which performs the energy calculations, and Wrappers, which prepare inputs and process outputs for different policy uses, according to Energy Trust.
The core engine models heat transfer within a home, heating and cooling demand, hot water demand, and the energy required to meet those demands. The calculation engine works through five main calculation stages during each time step, as described by Energy Trust.
The engine first calculates hot water needs, the energy required to provide hot water, and the fuel or electricity consumed by the water heating system. It then calculates electricity generated by technologies such as solar PV and any heat gains associated with that generation.
The reference code is constructed on a modular framework, according to Elmhurst Energy. This modular structure is what allows the government to publish the codebase openly, so assessors and software developers can see how each component behaves before formal implementation.
How does HEM differ from SAP in its calculations?
The Home Energy Model simulates homes at a 30-minute resolution, which equates to 17,520 timesteps per year, replacing SAP’s monthly averages with dynamic thermal modelling based on BS EN ISO 52016, according to Home Energy Model. SAP 10.2 uses 12 monthly periods, while HEM uses half-hourly periods.
SAP 10.2 applies two fixed thermal zones for a dwelling. HEM uses user-defined zones, with a single zone specified for the Future Homes Standard assessment, according to Home Energy Model. This gives more flexibility in how different parts of a home are modelled.
Heat pump modelling also changes significantly. SAP 10.2 applies fixed coefficient of performance (COP) values for heat pumps, whereas HEM uses dynamic COP curves that respond to changing conditions, according to Home Energy Model. HEM also integrates localised weather data, and solar gains are calculated hourly based on the actual position of the sun combined with building orientation and shading objects, according to Saint-Gobain.
The practical consequence is a more responsive model of how a home performs across a real year, rather than a smoothed monthly average. This matters most for technologies whose performance varies with weather and time of day.
When will the Home Energy Model launch in the UK?
The launch of the Home Energy Model has been delayed to ensure robustness, and it will no longer launch at the same time as the Future Homes Standard, according to GOV.UK. The wider FHS package remains on track for Q1 2026, so SAP 10.3 will initially be the sole approved methodology.
HEM is expected to follow after at least a three-month delay. The HEM engine will continue to be developed via the Energy Calculation as a Service (ECaaS) platform, delivering a cloud-based tool available by autumn 2026, according to GOV.UK.
Once both methodologies are available, they will run in parallel for a limited period. The dual running period is expected to last at least 24 months to give industry time to adapt, according to Home Energy Model. During 2026, the government will also explore integrating HEM into Energy Performance Certificates (EPCs), according to GOV.UK.
What does the Future Homes Standard mean for HEM compliance?
The Future Homes Standard requires at least 75% lower carbon emissions in new homes, according to Home Energy Model. This is the policy driver behind the methodology change, but the transition is staged rather than immediate.
As of May 2025, the Department for Energy Security and Net Zero (DESNZ) announced that both SAP 10.3 and the Home Energy Model could be used in England to assess compliance with the upcoming Future Homes Standard, according to Elmhurst Energy. In practice, the FHS will launch with SAP 10.3 as the sole compliance route, with HEM following later.
SAP has two main uses: demonstrating compliance of new homes with Part L of the Building Regulations, and generating Energy Performance Certificates for all homes, according to Elmhurst Energy. HEM must eventually serve both functions.
Assessors and developers therefore need to plan for a period where SAP 10.3 is the only route, followed by a dual running phase. The exact administration of that dual running period has not yet been published in full, according to GOV.UK.
How will HEM change Energy Performance Certificates?
Energy Performance Certificates are one of SAP’s two core uses, so any replacement methodology flows directly into how certificates are produced, according to Elmhurst Energy. The government will explore integrating HEM into EPCs during 2026, but the reformed EPC design and any new metrics have not yet been confirmed, according to GOV.UK.
HEM’s half-hourly simulation is expected to affect the time and cost of producing an EPC, though the scale of that impact is not yet published. The more detailed calculation engine requires more processing and potentially more input data, which has practical implications for assessors and software providers.
Changes to the Product Characteristics Database (PCDB) and any new product classes are also still to be confirmed. Until the government response to the 120 consultation responses is published in full, these remain open questions for assessors and software providers, according to GOV.UK.
How can CCA Environmental help with HEM and SAP compliance?
CCA Environmental works with clients on SAP and energy assessment compliance as the methodology transitions toward the Home Energy Model. The practical priority for 2026 is that SAP 10.3 remains the sole approved methodology at Future Homes Standard launch, so current compliance routes still apply, according to GOV.UK.
Planning should account for the minimum three-month gap after the FHS, the dual running period of at least 24 months, and the ECaaS cloud tool arriving by autumn 2026. CCA Environmental can advise on which methodology applies to a given project and when the switch becomes relevant.
The transition from SAP to HEM is a staged process rather than a single cutover. Understanding the timing and the differences between the two methodologies is the first step in preparing for compliance under the new regime.
Key Takeaways
- The Home Energy Model will replace SAP for the energy rating of dwellings, but it is not live yet.
- SAP 10.3 will initially be the sole approved methodology when the Future Homes Standard package lands in Q1 2026.
- HEM is expected to follow the FHS by a minimum of three months, with the ECaaS cloud tool available by autumn 2026.
- HEM simulates at 30-minute resolution across 17,520 timesteps per year, replacing SAP’s 12 monthly periods.
- HEM uses dynamic COP curves for heat pumps and user-defined thermal zones, where SAP 10.2 uses fixed COP values and two fixed zones.
- Once both are available, HEM and SAP will run in parallel for at least 24 months.
- The government will explore integrating HEM into EPCs during 2026, but the reformed EPC metrics are not yet confirmed.
References
- Home Energy Model: replacement for the Standard Assessment Procedure (SAP) – GOV.UK — published 13 December 2023, last updated 2 October 2025
- The Home Energy Model – Elmhurst Energy — date unknown
- Home Energy Model: UK’s Complete HEM Guide — date unknown
- Understanding The Home Energy Model: Core Calculation Engine – Energy Trust — published 11 June 2026
- What is the Home Energy Model (HEM) and does it replace SAP? – Saint-Gobain — date unknown