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HEM-TP-02: General summary of wrappers — extracted text
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General summary of how wrappers work with the Home Energy Model
A technical explanation of the methodology
October 2025
OFFICIAL
Acknowledgements
This methodology has been developed for the Department for Energy Security & Net Zero by a number of organisations and individuals, including Sustenic, Quidos, Scene Connect, City Science, Hoare Lea, Oxford Brookes University, University of Bath, 10-x, Building Research Establishment (BRE), AECOM, Kiwa Ltd., Loughborough University Enterprises Limited, Chris Martin and John Tebbit.
Quality assurance has been undertaken by a consortium led by Etude, including Levitt Bernstein, People Powered Retrofit, University of Strathclyde’s Energy Systems Research Unit, Julie Godefroy Sustainability, and UCL.
Document reference: HEM-TP-02
Document version: v2.0
Issue date: October 2025
Home Energy Model version: HEM 1.0
© Crown copyright 2025
This publication is licensed under the terms of the Open Government Licence v3.0 except where otherwise stated. To view this licence, visit nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: psi@nationalarchives.gsi.gov.uk.
Where we have identified any third-party copyright information you will need to obtain permission from the copyright holders concerned.
Any enquiries regarding this publication should be sent to us at: homeenergymodel@energysecurity.gov.uk
Contents
Background to the Home Energy Model ___________________________________________ 4
What is the Home Energy Model? ______________________________________________ 4
Where can I find more information? _____________________________________________ 4
Related content _______________________________________________________________ 5
Methodology _________________________________________________________________ 6
- Overall structure and separation of core and wrappers____________________________ 6
Future development ___________________________________________________________ 7
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HEM-TP-02 General summary of wrappers
Background to the Home Energy Model
What is the Home Energy Model?
The Home Energy Model (HEM) is a calculation methodology designed to assess the energy performance of homes, which will replace the government’s Standard Assessment Procedure (SAP).
Where can I find more information?
This document is part of a wider package of material relating to the Home Energy Model:
Home Energy Model technical documentation (e.g. this document)
What: This document is one of a suite of technical documents, which go into further detail on the methodology and the validation exercises that have been carried out. We intend to update and produce further technical documentation throughout the model development process.
Audience: The technical documentation will be of interest to those who want to understand the detail of how the Home Energy Model works and how different technologies are treated.
The Home Energy Model consultation
What: The Home Energy Model consultation introduces the overhaul to the SAP methodology and sought views on the approach taken by the new Home Energy Model. The Home Energy Model consultation summarises the feedback to the consultation and the actions taken subsequently in development, ahead of the initial release of HEM.
Audience: The Home Energy Model consultation will be of interest to those who want to understand the proposed changes to the SAP methodology and wider SAP landscape.
The Home Energy Model reference code
What: The full Python source code for the Home Energy Model and the Home Energy Model: FHS assessment has been published as a Git repository. This code is identical to that sitting behind the consultation tool. We are currently considering whether the open- source code could serve as the approved methodology for regulatory uses of the Home Energy Model.
Audience: The reference code will be of interest to those who want to understand how the model has been implemented in code, and those wishing to fully clarify their
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| Col1 | Col2 | Col3 |
|---|---|---|
| Home Energy Model technical documentation (e.g. this document) | ||
| **What:**This document is one of a suite oftechnical documents, which go into further detail | ||
| on the methodology and the validation exercises that have been carried out. We intend to | ||
| update and produce further technical documentation throughout the model development | ||
| process. | ||
| Audience: The technical documentation will be of interest to those who want to | ||
| understand the detail of how the Home Energy Model works and how different | ||
| technologies are treated. | ||
| Col1 | Col2 | Col3 |
|---|---|---|
| The Home Energy Model reference code | ||
| What: The full Python source code for the Home Energy Model and the Home Energy | ||
| Model: FHS assessment has been published as a Git repository. This code is identical to | ||
| that sitting behind the consultation tool. We are currently considering whether the open- | ||
| source code could serve as the approved methodology for regulatory uses of the Home | ||
| Energy Model. | ||
| Audience: The reference code will be of interest to those who want to understand how | ||
| the model has been implemented in code, and those wishing to fully clarify their |
HEM-TP-02 General summary of wrappers
understanding of the new methodology. It will also be of interest to any potential contributors to the Home Energy Model.
Related content
This document provides a general summary of wrappers and how they relate to the core calculation. For a general summary of the core calculation, see HEM-TP-01 General summary of core calculation. For a summary of the FHS assessment wrapper, see HEMFHS-TP-06 Summary of FHS assessment wrapper.
To understand how this methodology has been implemented in computer code, please see:
Src/hem_core/hem.py
5
| Col1 | understanding of the new methodology. It will also be of interest to any potential | Col3 |
|---|---|---|
| contributors to the Home Energy Model. | ||
HEM-TP-02 General summary of wrappers
Methodology
1. Overall structure and separation of core and wrappers
The Home Energy Model software is structured to facilitate the use of different sets of assumptions for different policy requirements. Fundamentally, this means that the software is divided into:
• A core calculation component which models heat transfer in the building, demand on
heating, cooling and hot water systems and the resulting energy demand for electricity, mains gas etc.
• “Wrapper” components which add pre-processing (e.g., standardisation of occupancy
assumptions) and post-processing (e.g., application of emissions and primary energy factors) steps to the core calculation which are suitable for different policies.
In summary, at the highest level the calculation can be thought of as having three steps where steps 1 and 3 are optional:
-
Wrapper (if applicable): Apply pre-processing of inputs.
-
Core: Run building simulation.
-
Wrapper (if applicable): Apply post-processing of outputs.
The core calculation can be run on its own, but this requires a large amount of input data for a realistic calculation (e.g., hot water draw-off event times and durations for the entire calculation period). Therefore, it is expected that the core calculation would usually be used with some kind of pre-processing step, whether defined in the software itself or externally (e.g., for unofficial purposes).
The application of pre-processing steps by the wrapper means that the input data set required when using the wrapper is different to the input data set required by the core calculation, and the purpose of the wrapper is to transform its own input data set to that required by the core calculation. Usually, it would be expected that this would mean expanding a simpler set of inputs to a more detailed set of inputs and making certain assumptions in order to do so, but it is not a technical requirement that the wrapper input data set is simpler than the core input data set. In transforming the input data set, the wrapper may add, remove or overwrite input data items with which it is provided and will do so according to the policy requirements it is intended to fulfil.
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HEM-TP-02 General summary of wrappers
Future development
In future, additional wrappers may be defined to suit new purposes, and existing wrappers may be refined.
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This publication is available from: https://www.gov.uk/government/publications/home-energy- model-technical-documentation