What is the Home Energy Model (HEM)?

Last updated 14 August 2026·Applies to England & Wales

The Home Energy Model is the calculation engine intended to replace SAP and RdSAP — the methodology that works out what a home's energy assessment says. It is the machinery beneath the reformed EPC, and it is still under development.

Status: HEM is confirmed as the direction for the reformed domestic EPC regime, which the government plans to launch in the second half of 2027. The methodology and implementation detail remain under development. Nothing on this page is current law, and no certificate issued today uses HEM. Treat it as planning context, not as a rule to comply with.

Methodology, not a certificate

It helps to separate three things that get conflated:

The methodology — the model that takes a property's characteristics and computes its energy performance. Today that is SAP for new builds and RdSAP for existing homes. HEM is the intended replacement.
The metrics — what gets reported. The reformed regime is planned to report four: fabric performance, heating system, smart readiness and energy cost.
The certificate — the document a buyer or tenant sees.

"A HEM EPC" is shorthand for a certificate produced by the new methodology and reported against the new metrics. Our guide to the planned EPC reform covers the metrics and the timeline; this page is about the model itself.

Why replace SAP and RdSAP

Today's rating is driven largely by estimated energy cost, and that basis has a known distortion: measures that cut bills can flatter the score without improving how well the building retains heat. A property can gain points from bolt-on technology while its walls and roof perform exactly as badly as before.

The reform is explicitly fabric-first — fabric performance is the headline metric — and it needs a model capable of representing the building physics that supports that. HEM is designed with a modular structure intended to handle newer technologies and more granular, time-of-use energy modelling than the existing methodology was built for.

What it could mean for a property's rating

A different model reporting different metrics will not simply reproduce today's letter against a new scale. Directionally, and subject to the methodology being finalised:

Well-insulated, airtight homes should read better under a fabric-first measure than under a cost-driven one.
Homes scoring well largely through generation or low-cost tariffs may find the fabric metric tells a less flattering story than today's single letter.
The single headline letter becomes less informative once four metrics are reported, because a property can be strong on one and weak on another.

The practical implication for spending. Because the announced 2030 rental standard is expressed against fabric performance plus one of smart readiness or heating system, insulation and airtightness are the measures that improve today's score and the metric the reformed system is built around. That makes fabric work the lower-risk spend while the detail is unsettled — the same conclusion our D-to-C upgrade guide reaches from the cost side.

What to do now

Do not wait for HEM. Certificates issued today use the current methodology and remain valid for ten years from lodgement — see how long an EPC is valid for.
Favour fabric measures over bolt-on technology where you have a choice.
Landlords: plan against announced policy, not a guarantee. The transition, including grandfathering before October 2029, is announced policy subject to Parliamentary approval rather than current law.
Keep invoices. Qualifying spend from 1 October 2025 is intended to count towards the announced £10,000 MEES cost cap.

Frequently asked questions

What does HEM stand for?

Home Energy Model. It is the calculation methodology intended to replace SAP and RdSAP as the basis for domestic energy assessments.

When do HEM-based EPCs start?

The government plans to launch the reformed domestic EPC regime in the second half of 2027. The methodology and implementation detail are still under development, so treat the timing as planned rather than fixed.

Will my current EPC still be valid?

A certificate issued under the current methodology remains valid for ten years from lodgement, and the announced transition intends current-style and reformed certificates to run in parallel for a period.

Is HEM the same as the four EPC metrics?

No. HEM is the calculation model; the four metrics — fabric performance, heating system, smart readiness and energy cost — are what the reformed certificate is planned to report.