SAP (the Standard Assessment Procedure) is being replaced because it was designed in 1993 for pen-and-paper calculations, and its core methodology simply cannot model the modern low-carbon technologies that the UK's net zero targets depend on. Its monthly time resolution cannot capture how heat pumps, solar PV, battery storage, and smart controls actually perform. The Home Energy Model (HEM) replaces SAP with a half-hourly dynamic simulation that accurately reflects how 21st-century homes use energy.
A Brief History of SAP
SAP was first published in 1993 by the Building Research Establishment (BRE) on behalf of the UK Government. It was introduced as a standardised method for rating the energy performance of dwellings, supporting Part L of the Building Regulations (Conservation of Fuel and Power).
At the time, SAP was a sensible tool. Homes were heated almost exclusively by gas boilers, construction methods were straightforward, and the concept of on-site electricity generation was exotic. A monthly calculation — averaging heating demand, solar gains, and weather across each calendar month — was a reasonable simplification that could be performed by hand.
Over the following three decades, SAP was updated periodically:
- SAP 2005 — introduced the Environmental Impact Rating alongside the existing SAP energy cost rating
- SAP 2009 — used as the basis for Energy Performance Certificates when they became mandatory
- SAP 2012 — updated fuel prices and emission factors, improved treatment of some technologies
- SAP 10.2 — the current version, updating carbon emission factors and adding some technology improvements
Despite these updates, SAP's fundamental architecture remained unchanged. Each version refined the inputs and assumptions, but the underlying monthly calculation method — designed for a world of gas boilers and basic insulation — stayed the same.
The Limitations of SAP
By the 2020s, SAP's limitations had become acute. As the UK committed to net zero by 2050 and the government pushed for low-carbon heating in new homes, it became clear that SAP could not adequately support these policy objectives. The key problems were:
Monthly Resolution Is Too Crude
SAP averages everything over a month. It cannot capture the fact that a heat pump's efficiency varies hour by hour with outdoor temperature, or that solar PV generates electricity during the day but demand peaks in the evening. These dynamics are central to how modern homes actually perform, and averaging them away produces misleading results.
Poor Modelling of Modern Technologies
Several technologies critical to decarbonisation are either poorly modelled or entirely absent from SAP:
- Heat pumps are assessed using simplified fixed efficiency values that don't reflect real-world performance variations
- Battery storage cannot be modelled at all
- Solar PV self-consumption — the proportion of generated electricity used on-site rather than exported — cannot be calculated because SAP doesn't know when demand and generation coincide
- Smart controls and time-of-use tariffs cannot be assessed because their benefits depend on half-hourly patterns
Outdated Carbon Emission Factors
RdSAP 10 still uses carbon emission factors based on the 2012 electricity grid mix. Since then, the UK grid has decarbonised dramatically through the growth of wind, solar, and nuclear generation. Using historical factors significantly overestimates the carbon intensity of electrical heating, making heat pumps look worse than they actually are and undermining the policy case for electrification.
Software Inconsistencies
Under SAP, multiple third-party software providers each build their own implementation of the calculation. Different software can produce different results for the same dwelling, because each provider interprets the SAP specification independently. The October 2025 government consultation response confirmed that 49 of 84 respondents agreed these inconsistencies were a significant problem.
Insufficient for Net Zero
Perhaps most fundamentally, SAP cannot adequately support the UK's net zero policy framework. When the calculation methodology used for regulatory compliance cannot properly model the very technologies needed to decarbonise homes, it becomes a barrier rather than an enabler.
What the Climate Change Committee Recommended
The Climate Change Committee (CCC), the UK's independent statutory body advising on emissions targets, called for SAP's replacement in two key reports:
- In 2019, the CCC's report UK Housing: Fit for the Future? recommended a next-generation assessment methodology that could properly model dynamic technologies and support the net zero transition.
- In 2022, the CCC's progress report to Parliament reiterated that SAP was not fit for purpose and urged the government to accelerate its replacement.
These recommendations were influential. The government had already begun exploring alternatives, and the CCC's endorsement provided the policy backing needed to commit to a full replacement rather than another incremental SAP update.
How HEM Was Developed
In 2021, the government commissioned a consortium led by BRE (Building Research Establishment) to develop HEM. The project was overseen by DESNZ (Department for Energy Security and Net Zero), with a separate quality assurance consortium led by Etude, including Levitt Bernstein, UCL, and Julie Godefroy Sustainability.
Rather than building from scratch, the development team based HEM on established international standards — principally BS EN ISO 52016-1:2017 for dynamic thermal modelling and BS EN 16798-7:2017 for ventilation calculations. This grounded the methodology in peer-reviewed physics rather than the bespoke simplifications that characterised SAP.
A key decision was to make HEM open source, published under the MIT Licence with Crown Copyright. Two implementations were developed:
- A Python reference implementation by BRE (on Azure DevOps), serving as the definitive methodology reference
- A Rust performance implementation by MHCLG (on GitHub), optimised for speed and used by the ECaaS platform
The Consultation Process
The government published the HEM consultation in December 2023, alongside the Future Homes Standard 2023 consultation. The consultation period was extended to March 2024 following a software issue, and the industry test environment was made available from mid-2025.
The government response, published in October 2025, confirmed the key decisions:
- The name “Home Energy Model” was supported by a majority of the 83 respondents who commented, praised as more descriptive than “SAP”
- The half-hourly methodology received strong support, with industry accepting the increased data requirements as a necessary trade-off for accuracy
- The centralised ECaaS delivery model was endorsed, with 49 of 84 respondents confirming that SAP's multi-provider inconsistencies were a real problem
- 68 of 80 respondents expressed eagerness to collaborate on the open source codebase
Concerns were also raised — including the treatment of window shading, heat transfer between zones, traditional buildings, and the balance between complexity and accuracy. These are being addressed in ongoing development.
SAP vs HEM at a Glance
The table below summarises how HEM addresses each of SAP's key limitations. For a full detailed comparison, see our SAP vs HEM page.
| SAP Limitation | How HEM Addresses It |
|---|---|
| Monthly calculations — averages demand over 30 days | Half-hourly dynamic simulation — 17,520 timesteps per year |
| Ignores orientation — south-facing treated same as north | Models solar gains by orientation, pitch, and shading for every timestep |
| No thermal mass modelling — lightweight and heavyweight homes score the same | Full dynamic thermal modelling — concrete, masonry, and heavy floors rewarded for thermal storage |
| Simplified heat pump COP — uses fixed seasonal values | Dynamic COP modelling — varies with outdoor temperature, flow temperature, and part-load conditions each half-hour |
| Basic ventilation — fixed infiltration assumptions | Detailed ventilation modelling — MVHR efficiency, duct losses, and demand-controlled ventilation all modelled |
| Single national climate — one weather dataset for all of the UK | Regional weather data — 14 climate zones with hourly temperature, wind, and solar radiation |
| No overheating assessment — Part O handled separately | Integrated overheating risk — assessed within the same model using the same building data |
| Cannot model battery storage at all | Full battery modelling — charge/discharge cycles, self-consumption optimisation, and grid interaction |
| Solar PV assessed as annual total only | Half-hourly PV generation matched against half-hourly demand — captures self-consumption accurately |
Benefits of the Replacement
Replacing SAP with HEM delivers several fundamental improvements:
- Accurate technology modelling — heat pumps, solar PV, batteries, MVHR, and smart controls are all modelled dynamically, rewarding good design and proper installation
- Forward-looking carbon factors — reflecting the actual decarbonisation of the electricity grid rather than using decade-old data
- Consistency — the centralised ECaaS platform eliminates discrepancies between different software providers
- Transparency — the open source codebase enables industry scrutiny, collaboration, and trust
- Policy alignment — HEM can properly support the Future Homes Standard, EPC reform, and the UK's path to net zero
- Future-proofing — the modular architecture means the core physics engine can evolve independently of policy wrappers, adapting to new technologies without requiring a fundamental rebuild
In short, HEM is the assessment methodology the UK needs for the homes it is now building. SAP served its purpose for 30 years, but the world it was designed for no longer exists.
What This Means for You
SAP's replacement is not just a technical update — it changes how homes are rated, designed, built, and sold. Here is what the transition means for each audience.
For Homeowners
Your Energy Performance Certificate (EPC) may change under HEM, even if your home has not changed at all. Properties with good orientation, thermal mass, or heat pumps may see improved ratings because HEM can recognise benefits that SAP averaged away. Conversely, properties that benefited from SAP's simplified assumptions may see lower scores.
The new EPCs will also show different metrics — including a fabric performance rating and heating demand figure — giving you a clearer picture of your home's actual energy performance. If you are considering improvements like solar PV, insulation, or a heat pump, HEM will give you a more honest assessment of the benefit.
Read the full homeowner guide →
For Energy Assessors
The transition from SAP to HEM is not just a software update — the methodology is fundamentally different. You will need new training, new qualifications, and new workflows. The centralised ECaaS platform replaces third-party SAP software, and the data input requirements are more detailed (HEM needs orientation, shading, thermal mass, and ventilation data that SAP did not require).
The good news: assessors who invest in understanding HEM early will have a significant competitive advantage during the transition period. The fundamentals of building physics still apply — HEM just models them more accurately.
Read the full assessor guide →
For Architects
Under SAP, design decisions around orientation, thermal mass, and window placement had limited measurable impact because the methodology could not model their benefits. Under HEM, these decisions have direct, quantifiable effects on the energy rating. A south-facing living room with appropriately sized glazing and a concrete floor slab will score measurably better than an identical room facing north with a lightweight timber floor.
This means architects can use energy modelling as a genuine design tool, not just a compliance checkbox. Early-stage HEM modelling will become part of the design process, informing decisions about orientation, form factor, glazing ratios, and construction type.
Read the full architect guide →
For Builders
HEM can model the performance gap between design intent and as-built reality more accurately than SAP ever could. Airtightness, MVHR commissioning, heat pump installation quality, and thermal bridging at junctions all have measurable impacts in HEM. On-site quality matters more, because the model can distinguish between a well-built home and one with poor detailing.
The practical implication: construction teams need to understand why certain details matter, not just follow drawings. An unsealed service penetration or a crushed MVHR duct is no longer a hidden compromise — it affects the modelled performance.
Frequently Asked Questions
Why is SAP being replaced?
SAP is being replaced because its 1993-era monthly calculation method cannot accurately model modern low-carbon technologies like heat pumps, solar PV, battery storage, and smart controls. The Climate Change Committee recommended replacement in 2019 and 2022, recognising that SAP cannot support the UK's net zero objectives.
When was SAP first introduced?
SAP was first published in 1993 by BRE on behalf of the UK Government. It has been updated periodically (SAP 2005, 2009, 2012, and 10.2), but its core monthly calculation structure has remained fundamentally unchanged for over 30 years.
What are the main limitations of SAP?
SAP's key limitations are: monthly time resolution that cannot model dynamic technologies; simplified heat pump modelling; inability to model battery storage or solar self-consumption; outdated carbon factors based on the 2012 grid; inconsistencies between software providers; and insufficient support for net zero policy. For a full comparison, see SAP vs HEM.
Who recommended replacing SAP?
The Climate Change Committee (CCC) recommended replacing SAP in its 2019 report UK Housing: Fit for the Future? and again in its 2022 progress report to Parliament. The government had already begun exploring alternatives, and the CCC's endorsement provided the policy mandate for a full replacement.
Who developed the Home Energy Model?
HEM was developed by a consortium led by BRE, commissioned by DESNZ. Quality assurance was led by Etude, including Levitt Bernstein, UCL, and Julie Godefroy Sustainability. The project was commissioned in 2021, with the methodology based on international standards BS EN ISO 52016-1:2017 and BS EN 16798-7:2017.
Was SAP giving wrong results?
SAP was not “wrong” within its own framework — it calculated what it was designed to calculate. The problem was that its framework was too simple for modern homes. By averaging over monthly periods, SAP could not distinguish between a well-oriented home with thermal mass and a poorly-oriented lightweight one. The results were internally consistent but increasingly disconnected from real-world performance, particularly for homes with low-carbon technologies.
Will my EPC rating change when SAP is replaced by HEM?
Possibly. When HEM is used for existing dwelling EPCs (expected from 2026–2027 onwards), some properties will see changes — even without physical alterations. Properties with good orientation, thermal mass, or heat pumps may see improved ratings. Properties that benefited from SAP's simplifications may score lower. The new EPCs will also show additional metrics beyond the familiar A–G rating.
Can I still use SAP after HEM is introduced?
SAP will remain during a transitional period. For new builds, SAP 10.2 can still be used for projects started before the FHS comes into force, with the transitional period ending around late 2027. For existing dwellings, RdSAP continues for EPCs until HEM-based assessment is formally approved and assessors are retrained. The exact switchover date has not yet been confirmed.
Related Pages
What is the Home Energy Model?
Comprehensive introduction to HEM — what it is, how it works, and who it affects.
SAP vs HEM — What’s Changed?
Detailed side-by-side comparison of the two methodologies.
Timeline & Status
Live tracker of HEM, FHS, and EPC reform milestones.
Future Homes Standard
The regulatory framework that HEM underpins for new-build compliance.