The Home Energy Model (HEM) is set to replace Standard Assessment Procedure (SAP) in the next few years as the methodology for calculating the energy performance of homes in the UK.
SAP, of course, is the methodology that currently sits behind Energy Performance Certificates (EPCs) to give an Energy Efficiency Rating (EER) to existing homes.
EPCs are a key piece of information for the property sector, used in a whole host of different ways.
They’re also vital for understanding the carbon emissions that UK housing is responsible for, as we aim towards net zero housing. The UK government currently has a target to bring as many homes as possible to an EPC C rating by 2035.
So, what is the new Home Energy Model methodology? How does it differ from SAP? Will it change how EPCs work?
That’s what this guide to the Home Energy Model (HEM) covers.
Last updated: 15 July 2024
What is the Home Energy Model?
The Home Energy Model (HEM) is a new methodology for modelling domestic energy use – vital for understanding the energy efficiency of UK homes and the carbon emissions housing is responsible for.
The Home Energy Model is currently under development by the UK government and the Building Research Establishment (BRE).
The eventual plan is that the Home Energy Model will replace SAP for all use cases.
A key feature of Home Energy Model, therefore, is that is has been designed in two parts to enable its use across several different policy needs:
- Part 1: A core calculation engine which is used to model the energy demand in a home
- Part 2: ‘Wrapper’ components that add standardised assumptions, primary energy factors, and emission calculations – all of which could differ for different policy needs.
Why is the Home Energy Model being introduced?
The Home Energy Model is being introduced to replace SAP with a more accurate methodology for modelling domestic energy use in the UK.
The Standard Assessment Procedure (SAP) has many issues.
SAP was first introduced in 1993 and last updated in 2012, so it’s now a very outdated methodology for modelling domestic energy use – especially as a methodology that now needs to drive the property sector towards decarbonisation.
Several climate experts and property industry experts have recommended to the government that SAP is no longer fit for purpose as the primary methodology behind understanding and improving the energy performance and efficiency of UK homes.
Notably, this has included recommendations by the Climate Change Committee (CCC) in their ‘UK Housing: Fit for the Future’ paper (2019) and their Independent Assessment of the UK’s Heat and Building’s Strategy (2022).
This has led to the government undertaking a project to overhaul SAP and develop a new and improved methodology for modelling domestic energy use, which has been named the Home Energy Model.
“Our key aim is to make a new methodology which is fit for a net zero future.”
UK government, Home Energy Model consultation
When will the Home Energy Model be implemented?
The Home Energy Model is currently under development by the UK government and BRE, with the aim of beginning to roll out use of the model in 2025.
The current timeline is as follows:
- On 27 March 2024 the Home Energy Model: Future Homes Standard Assessment consultation closed – this was an open consultation by the UK government asking industry experts and parties who currently use SAP for new build homes to input on the proposed methodology.
- From April 2024 onwards the government and BRE will now review the responses to the consultation and further develop the Home Energy Model.
- Later in 2024 the government aims to begin consultation on the use of the Home Energy Model for existing homes and particularly the impact on EPCs.
- In 2025 rollout of the Future Homes Standard, which will include the use of the Home Energy Model for new buildings only, will commence.
As you can tell from this, the timeline on the Home Energy Model for EPCs is currently unknown, but there will definitely be no changes to the current methodology before 2025.
How does the Home Energy Model relate to the Future Homes Standard (FHS)?
The current consultation on the Homes Energy Model is focused on its use to ensure new buildings are compliant with the Future Homes Standard once in place from 2025.
So what is the Future Homes Standard?
The Future Homes and Buildings Standard (FHS) aims to ensure that all new buildings (including homes) are ‘zero carbon ready’ so that we can meet net zero targets and mitigate the impacts of climate change.
In essence, FHS is largely a new version of Part L of the Building Regulations, which currently covers the conservation of fuel and power in new buildings i.e. energy efficiency – though it may also mean changes to other elements of the Building Regulations too.
FHS includes several changes to existing building regulations, but the two most significant proposals are:
- Phasing out fossil fuel heating and hot water systems in new buildings. The government has recognised that gas boilers (including hybrid heat pumps and hydrogen-ready boilers) and biofuel systems will not meet the required standards of being commercially available and zero carbon ready, and so the FHS aims to phase these heating systems out of use.
- Replacing SAP with HEM. To ensure new homes are zero carbon ready we first need to be able to accurately understand their energy use and efficiency, for which the current SAP methodology is not fit for purpose. The government has therefore proposed the introduction of a more sophisticated methodology and tool, the Home Energy Model.
The new Home Energy Model is the methodology that will be used to assess the energy efficiency of new homes and ensure they comply with FHS.
FHS is the first scenario or ‘wrapper’ for which the Home Energy Model will be used, but it will go on to be used for other purposes and policies, including EPCs.
FHS (and the use of HEM within it) is currently under development in 2024, with the government aiming to roll out the legislation in 2025.
HEM vs SAP: what are the key differences?
So, what are the key differences between SAP and the new Home Energy Model?
In brief:
- A centralised, open source methodology
- Wrappers to distinguish different use cases
- New technologies are recognised
- Half hourly simulations
- Updated standardised assumptions
Let’s take a closer look at each of these.
It’s also worth bearing in mind that the Home Energy Model is still under development – these changes are based on the latest information from the Department for Energy Security and Net Zero (DESNZ) Home Energy Model: Future Homes Standard assessment consultation but the final methodology could be different.
We’ll keep this article updated as the development progresses, and you can sign up to our green newsletter to be notified of the latest updates.
A centralised, open source methodology
SAP is currently owned by the government and made available to those parties that need to use the methodology via a PDF which includes a worksheet for making the calculations. Energy assessors typically use this as the basis for their own SAP calculations, or a tool created by a third party using the methodology e.g. Elmhurst Energy’s SAP calculations tool.
This decentralised approach, with many individuals and organisations taking the methodology and interpreting and using it in their own ways, has introduced differing interpretations in how the SAP methodology and calculations are used. It also means that when updates are made to the SAP methodology there’s no guarantee of these updates being reflected in third party software. This has resulted in inconsistencies.
To rectify this, the government is introducing improved transparency into the Home Energy Model:
- Open source code: the final HEM methodology will be published as open source code via a Github repository to improve transparency and understanding of how exactly the methodology works.
- Centralised software: alongside the methodology, the government plans to provide a centralised, cloud-based version of the Home Energy Model calculation engine which software providers can take and build a user interface around – meaning that all HEM calculation tools will use the same core software and that updates to the calculations will be reflected across-the-board, reducing inconsistencies.
Wrappers to distinguish different use cases
The SAP methodology has been in use since 1993 and has adopted several different purposes in that time.
The main two use cases are for regulatory compliance:
- To demonstrate compliance with Part L of the Buildings Regulation for new build homes
- To produce Energy Performance Certificates (EPCs) which are a requirement when a home is sold or let.
On top of this, SAP is used to determine eligibility for government retrofit schemes such as the Boiler Upgrade Scheme. And SAP is also used more generally as a method for estimating the energy consumption of UK buildings by a range of different organisations e.g. research, policymaking, architects, construction.
SAP has become a ‘one-size-fits-all’ solution, using the same standardised assumptions for all of the above use cases – which leads to inaccuracies.
The Home Energy Model will use ‘wrappers’ to distinguish between different use cases and improve accuracy.
At the heart of the Home Energy Model is the core calculation engine which is used to model the energy demand in a home. This will remain the same across all use cases. Layered on top of this will be ‘wrapper’ components that add standardised assumptions, primary energy factors, and emission calculations – all of which could differ for different policy needs.
New technologies are recognised
A key issue with the SAP calculations is that there have been substantial technological advances since the methodology was last updated in 2012.
This has resulted in new technologies which are not included in the calculations, or existing technologies whose calculations are not up-to-date – leaving blind spots and inaccuracies in our understanding of the energy efficiency of UK homes.
Information on new technologies is currently gathered in Appendix Q of SAP, but updates to the methodology to include Appendix Q items happen very infrequently.
The aim with the Home Energy Model is that:
- Pre-release: a large-scale consultation of new and existing technologies will take place throughout the development of the Home Energy Model, and these will be factored into the calculation engine before it goes live.
- Once live: information on new technologies will still be gathered in Appendix Q in the same way, but these will be incorporated into the Home Energy Model’s core calculation engine on a much more frequent basis – and because of the centralised software this will be instantly reflected in new assessments.
However, because EPCs last for 10 years once created we could still have the scenario that an EPC is created and then 6 months later the calculations are updated to reflect the latest information on technologies – the methodology may now be up-to-date, but the EPC will remain inaccurate for 9.5 more years. It will be interesting to see how this factors into the Home Energy Model EPC consultation process, which is due to begin in 2024.
Half-hourly time resolution
The SAP methodology currently has a time resolution of one month i.e. predicted energy use in a home is modelled on a month-by-month basis.
With the Home Energy Model this will change to a half-hourly time resolution – improving the accuracy of energy consumption modelling.
The half-hourly time resolution will also make it possible to accurately assess the benefits of installing renewable energy generation technologies or battery storage in a home and harnessing these to adjust energy supply or demand in the home i.e. energy flexibility.
With energy flexibility key to decarbonising the UK electricity grid, this is a huge step forward.
Updated standardised assumptions
Any energy modelling methodology relies on standardised assumptions to an extent – factors like the typical occupancy behaviour or the carbon intensity of a fuel are required for accurate calculations.
The standardised assumptions used within the SAP methodology are often outdated e.g. carbon intensity factors were last updated in 2022, and many of the assumptions have simply been shown to be inaccurate over the lifetime of the methodology.
The Home Energy Model, therefore, is an opportunity to update those assumptions for improved accuracy.
The planned updates on standardised assumptions are too extensive to cover in full here, but we’ll highlight a few of the key updates below. For a full overview, go to the Home Energy Model Future Homes Standard Assessment consultation document.
- Occupancy. SAP assumes a number of occupants based on total floor area. HEM will include the number of bedrooms as an additional dependency.
- Local weather. The SAP methodology uses UK average weather data as inputs in calculating both the EER (EPC) and EIR ratings. In reality, the weather varies significantly across locations. The Home Energy Model will allow for localised weather as an input. The data used for weather data will be updated hourly based on ten locations across the UK. HEM also incorporates UK climate projection scenarios from the Met Office to give a forward-looking view of weather, as climate change is already changing weather patterns significantly.
- Appliances. The assumed energy demand of all household appliances e.g. TVs have been reduced by around 25% in HEM to reflect improved energy efficiency.
- Cooking appliances. SAP currently assumes that half of cooking in every home is done with gas as the fuel and half electricity. The actual fuel type used e.g. gas oven, electric hob will be included in HEM calculations.
- Emission factors. Static, outdated emissions factors have been a big issue with SAP and RdSAP. The emission factors used to calculate the emissions from different fuel types were last updated for the latest version of SAP (SAP 10) to reflect the government’s 2022 greenhouse gas conversion factors. Before that, they had not been updated since 2012 – and the current RdSAP methodology (used for EPCs for existing homes) still uses those 2012 emission factors, reflecting a very out of date baseline. HEM will shift from using static, historical emissions factors to future emissions factors, projecting the average emission factors from 2025 to 2029. This makes the calculations more accurate, as they reflect the reality that the grid is becoming lower carbon over time.
How will the Home Energy Model change EPCs?
As we’ve mentioned, so far the development of and consultations on the Home Energy Model have centred around its use in ensuring new build homes are compliant with the Future Homes and Buildings Standard.
The Home Energy Model will definitely replace SAP as the methodology for measuring the energy efficiency of existing homes and generating an EPC – which will include the development of a new ‘wrapper’ for this purpose.
However, when that will happen is currently unknown.
So far, all we know from the Home Energy Model consultation is that work on the EPC wrapper is planned to be the next phase of development, and that the government intends to begin consultations on the EPC wrapper in 2024.
“EPC reform is not covered in any of these publications. We are developing proposals to improve EPCs and intend to consult on these in the coming months.”
Home Energy Model consultation
BRE have also published their recommendations of key changes that should be made when SAP is replaced with the Home Energy Model for EPCs, which includes:
- One headline energy efficiency metric instead of the two metrics currently gained from SAP: the Energy Efficiency Rating (EER) which is a measure of energy costs and the Energy Intensity Rating (EIR) which is a measure of carbon intensity.
- Factoring energy flexibility into the energy efficiency calculations
- Improved reach and validity of EPCs i.e. ensuring that more homes have an in-date EPC (our analysis found that only 49% of homes have an EPC at all, and of those only 26% are dated within the last five years).
Some industry experts have expressed frustration that the EPC use case has not been included in the initial phase of the Home Energy Model, such as Stuart Fairlie, the Managing Director of Elmhurst Energy.
Indeed, because there is little information on how HEM will impact the creation and use of EPCs, there are many questions left unanswered. What will it mean for properties that have already been assessed via SAP for their EPC? Will all homes need a new EPC under the HEM methodology? If not, and given the issues highlighted with SAP, how do we get accurate data for the 29 million existing homes in the UK? Will we end up with different methodologies for different UK homes if new builds move to HEM and existing homes stay on SAP? How will the two methodologies be integrated?
Much remains to be seen.
📣 Stay up-to-date with the latest news on the Home Energy Model
As the Home Energy Model continues to be developed and rolled out (including for EPCs) we’ll keep this article updated with news and changes – so make sure you bookmark it to come back to later. You can also sign up to our green newsletter to be notified of updates.
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