The Home Energy Model (HEM) – A major improvement over SAP
The first version of the Home Energy Model (HEM), the one dedicated to the Future Homes Standard (HEM:FHS), is released in a few months. Like other software updates it will take time to get used to, but represents a step change and will enable you to do many things better than SAP.
What is HEM?
The Home Energy Model (HEM) is a new calculation methodology for estimating the energy consumption of housing. HEM will replace SAP as the software used to demonstrate compliance with Approved Document Part L of the Building Regulations, generate EPCs, and show that applications comply with planning policy sustainability requirements, such as the London Plan.
HEM is built around a “core engine” and “wrappers” that define some of the input assumptions and generate results. It can calculate the predicted energy consumption of a house or flat.
What is a HEM wrapper?
A wrapper in the Home Energy Model provides some fixed assumptions and validates a calculation against a particular standard, then gives a specific set of output metrics.
The first two wrappers developed are policy-specific and use the HEM core engine to demonstrate compliance with the Future Homes Standard (HEM:FHS) and to produce EPCs (HEM:EPC). The core engine and its wrappers are built on robust and up-to-date foundations, including international standards, particularly BS EN ISO 52016-1:2017.
HEM is open source, written in Python, and you can develop additional wrappers. For example, a ‘Home Retrofit’ wrapper could be developed to enable homeowners to plan their retrofit, or a stock model could be created to optimise a housing portfolio against key performance indicators.
What will HEM look like?
HEM is the calculation code, different software providers will develop software interfaces to make it easy to use and for specific purposes. Vulcan and Elmhurst are both developing interfaces for HEM.
The software will communicate with the central Energy Calculation as a Service (ECaaS) platform, which hosts the approved version of the HEM calculation engine. Rather than running calculations locally, software will send the building data to ECaaS, which performs the calculation and returns the results. This approach ensures that all users are running the same verified version of the methodology and allows updates to the calculation engine to be managed centrally.
Why is the Home Energy Model so much better than SAP?
SAP was introduced in 1993 and has played a major role in helping to design and assess more energy-efficient homes over the last 30 years. However, it was never designed as a predictive energy modelling tool, and it now struggles to represent modern building technologies, low-carbon heating, smart systems, and the requirements of a net zero energy system.
HEM is written from the ground up, drawing on the European Energy Performance of Buildings (EPB) standards framework (EN ISO 52000 series and related standards), and reconsidering what a modern building simulation package should be capable of. It is a dynamic half hourly model with a powerful core engine.
Etude contributed to developing HEM software, leading the Quality Assurance and Validation team. This gives us a unique insight into HEM’s functionality, potential uses and limitations. We have run the HEM core engine and FHS wrapper through many sensitivity tests, comparisons with other reputable energy modelling tools such as PHPP and ESP-r. From this we believe there are many reasons why HEM is much better than SAP, but here are our top five:
- It is much more accurate than SAP at predicting energy demand, energy use, and renewable energy generation, and therefore can inform better decisions.
- It can model the behaviour of heat pumps and the dynamic electrical demand of housing very effectively, which is crucial for the future of energy networks.
- It is much more accurate at modelling air infiltration and ventilation systems, rewarding efforts to make dwellings more airtight and improving ventilation systems.
- It operates at a half-hourly timestep rather than SAP’s monthly approach. This higher resolution enables better modelling of renewable energy self-consumption and export, energy storage, smart technologies, and time-of-use tariffs.
Who has been involved in the development of HEM?
The Home Energy Model programme has been led by the UK Government Department for Energy Security and Net Zero (DESNZ). After an initial research period (see SAP 11), many organisations and collaborators were involved in its development, with the key organisations being BRE, Sustenic and City Science.
Etude provided Quality Assurance and Validation services, leading a multi-disciplinary team also including University College London (UCL), People Powered Retrofit, Levitt Bernstein, the University of Strathclyde, and Julie Godefroy Sustainability.
The Ministry of Housing, Communities and Local Government (MHCLG) is also involved through the ‘sister programme’: Energy Calculation as a Service (ECaaS), a cloud-based API platform that performs calculations using the Home Energy Model (HEM). Under ECaaS, an authoritative calculation engine is hosted by MHCLG.
Is the Home Energy Model going to be that good from day one?
From the quality assurance and validation work we have undertaken, we can already see that HEM:FHS is much better than SAP in a number of key areas (see above). Will it be perfect and bug-free? Probably not. None of the software we use is. The building industry has a key role to play in helping find bugs we have not yet identified so that they can be fixed by DESNZ. HEM has a great future, but we may have to be a little patient and forgiving in its first three months of life.
There are many reasons to be excited about HEM. We would like to applaud the Department for Energy Security and Net Zero for its ambition to develop the best Home Energy Modelling tool ever.
For more information please get in touch with Thomas Lefevre (thomas.lefevre@etude.co.uk)