HEM Calculations: A Guide to the Home Energy Model

What is the Home Energy Model?

HEM is the government's new calculation methodology for assessing the energy performance of homes, developed to replace the Standard Assessment Procedure (SAP). Its headline change is time resolution: HEM simulates a home in half-hourly steps, compared with SAP's monthly approach.

This allows a more detailed representation of how energy demand and supply change through the day. It is particularly relevant to heat pumps, solar generation and energy storage, where the timing of energy use matters. The government has also published reference code to make the calculation more transparent. DESNZ's introduction explains these design principles; its original launch forecasts have since been superseded by the status update above.

HEM is a methodology. An assessor uses software that applies it to a home. A Building Regulations standard sets the requirements that the home must meet, while an EPC communicates assessed performance. Keeping these roles separate helps make sense of the changes.

HEM and the Future Homes Standard: what is confirmed?

The government published the 2026 edition of Approved Document L for England in March 2026, with corrections published in September. Earlier editions continue to apply to buildings subject to previous regulatory standards.

The Future Homes Hub identifies 2027 as the implementation year for the new standards, with transitional provisions. The relevant application dates, commencement criteria and building type all need checking. Its delivery programme brings together the documents and industry implementation work.

The standard and the calculation method have separate timetables. Government has confirmed SAP 10.3 as the initial FHS calculation route while HEM is delayed. Do not assume that publication of a HEM technical paper means the method is approved for your submission.

We have not identified a confirmed HEM launch date in the official status material reviewed for this guide. Nor should an older consultation's proposed start date be treated as a current deadline. Ask your assessor to confirm the applicable edition and methodology with the project team and building control.

What is the difference between HEM and SAP?

For a closer look at what these differences mean for a design, read SAP vs HEM: three changes to prepare for.

The government's response to the HEM consultation explains the development decisions. HEM is a substantial methodological change, so an existing SAP result should not be treated as a HEM result or converted using a simple adjustment. For an explanation of today's assessment process, see our SAP calculations guide.

How does a HEM calculation work?

The core calculation models heat transfer through the building, demand for heating, cooling and hot water, and the energy used by systems to meet that demand. It also accounts for electricity generation, self-use, storage and grid imports and exports. The government's core calculation summary (PDF) describes the sequence.

A wrapper supplies the assumptions and processing needed for a particular use. It can standardise inputs such as occupancy before the simulation and process the results afterwards, for example by applying emissions factors. A core model's capabilities do not mean every input is freely adjustable in a regulatory assessment. The wrapper determines how the model is used. Read the official explanation of wrappers (PDF).

HEM:FHS means the model applied to Future Homes Standard assessment. HEM:EPC refers to its application to Energy Performance Certificates. The distinction matters when comparing outputs: first establish what the assessment is for and which assumptions it uses.

Why do heat pumps, solar panels and storage matter?

The benefit of a technology can depend on when it operates. Solar panels generate during daylight hours; household demand can peak later. Storage can move energy between those periods. A heat pump may serve both space heating and hot water, with those demands sharing its available capacity.

HEM's core structure allows interacting components and systems to be modelled together. That is more useful than considering a technology only as an isolated annual saving. It does not mean that adding a battery, a larger heat pump or more panels will automatically produce a compliant design.

For the technical detail, DESNZ publishes separate papers on fabric heat loss, ventilation, thermal mass, shading, hot water, heat pumps, controls and PV generation. These are the appropriate starting point for understanding how individual features are treated.

Our practical advice is to coordinate the envelope and services early. Ask the heating designer, architect and assessor to work from the same drawings and product information, then assess alternatives before procurement fixes the specification.

How does HEM relate to EPC reform?

EPC reform is a connected programme with its own decisions and delivery timetable. The January 2026 HEM:EPC consultation covers England and Wales and proposes how HEM would assess existing dwellings and produce new certificate metrics.

The consultation considers fabric performance, heating systems, smart readiness and energy costs, alongside the use of detailed evidence and defaults for existing homes. These proposals should not be confused with the approved requirements for a new home's Building Regulations submission. Read the HEM:EPC consultation.

A standardised energy assessment is also different from forecasting a particular household's bills. Occupancy, temperatures, weather and tariffs affect actual consumption. A more detailed model still needs its assumptions explained.

What does HEM mean for your project?

New builds

Start by establishing the regulations and transition provisions that apply to each plot. Agree the assessment route before comparing specifications. Our new build SAP service supports design assessments, practical compliance recommendations and completion documentation.

Conversions and changes of use

Tell us what the building is now and what it will become. A conversion needs its own compliance review; choosing a new calculation method does not establish the applicable requirements. Our conversion SAP service explains how we approach these projects.

Extensions

If your concern is extensive glazing, start with the extension assessment route. Our extension service covers area weighted U-value checks and whole house SAP calculations where appropriate. Send the existing and proposed plans so we can identify the suitable assessment.

For current work, we will confirm the SAP scope and required documents in your quote. HEM preparation should help organise a project; it should not hold up an assessment that can proceed under its applicable requirements.

What information should you prepare?

The following is our practical preparation list, based on the information we already request for detailed SAP work. It is not a published final HEM submission checklist:

• Dimensioned floor plans, elevations and sections, with orientation and clear plot references.
• Wall, floor and roof build-ups, insulation and U-values.
• Window and door schedules, glazing performance and shading details.
• Thermal junction details, airtightness targets and ventilation specifications.
• Selected heating and hot water products, emitters, controls and cylinder information.
• PV layout and product details, plus any proposed energy storage and controls.
• At completion, final specifications, relevant test and commissioning records, and construction evidence requested by your assessor.

Keep revisions together and identify assumptions that still need decisions. Ask the assessor which product data and evidence their approved assessment route accepts before commissioning additional surveys or tests.

What are industry experts saying?

CIBSE: better modelling needs reliable evidence

Writing in May 2024, CIBSE's head of net zero, Julie Godefroy, welcomed the separation of the model from policy assumptions while highlighting additional complexity and inputs. That is a useful explanation of HEM's potential, rather than a statement of today's launch status. Read CIBSE's consultation commentary.

In March 2026, Godefroy supported the direction of EPC reform but called for stronger links to measured performance, clarity about default and detailed inputs, and transparent cost assumptions. The practical lesson we take from this is to examine the evidence behind a result as carefully as the headline rating. Read CIBSE's EPC analysis.

Elmhurst: distinguish decisions from speculation

Elmhurst's January 2026 skills commentary separates confirmed developments from speculation about assessor upskilling and the informal term “RdHEM”. Its discussion is dated: it should not be used as proof of current accreditation requirements. For a commission, ask which qualifications and software approvals apply at that time. Read Elmhurst's perspective.

Future Homes Hub: delivery involves the whole project team

The Hub's implementation work covers fabric quality, heating, ventilation, grid connections and assessor capability. Our takeaway is to treat readiness as a coordination task across design and construction. A calculation is most useful when the specified systems can be installed, commissioned and operated as intended. Explore the Hub's delivery programme.

Frequently asked questions about HEM calculations

Is HEM the same as the Future Homes Standard?

No. HEM is the calculation methodology; the Future Homes Standard sets requirements. The initial FHS assessment route is SAP 10.3 while HEM is delayed.

Should I replace my existing SAP assessment?

Ask your assessor to review the regulations, project dates and any design changes first. A new methodology announcement alone is not a reason to commission a replacement assessment.

Can I use HEM reference code for a compliance submission?

Published code supports transparency and technical investigation. Before relying on any calculation for compliance, confirm that the methodology, wrapper and software are approved for that purpose.

How much will a HEM calculation cost?

We have not published a HEM fee or delivery promise. We will confirm scope, deliverables and price when we offer that assessment. For current SAP work, email [email protected] with your drawings.

Will HEM guarantee lower bills?

No. Modelling supports design comparisons and assessment. Actual bills depend on the home, its occupants and operating conditions; a rating is not a personalised bill guarantee.

Does this guide apply throughout the UK?

We work across the UK. The Building Regulations discussion in this guide focuses on England. The HEM:EPC consultation cited above applies to England and Wales. Check the relevant national requirements for projects elsewhere.

Sources and further reading

Use the latest official publication for a project decision. Consultation documents explain proposals and development history; industry commentary adds interpretation.

• GOV.UK: HEM:FHS assessment and launch status — approval and timing.
• GOV.UK: Approved Document L, 2026 edition — England's published guidance.
• DESNZ: HEM consultation response — development decisions.
• DESNZ: HEM technical documentation — detailed calculation papers.
• DESNZ: HEM:EPC consultation — proposals for certificates and existing homes.

This review records the sources checked on 17 October 2026. The next review should check HEM approval and availability, assessment conventions, assessor requirements and the final EPC decisions.