The Home Energy Model is the government's replacement for the Standard Assessment Procedure as the engine that scores home energy performance HEM consultation. Where SAP works on monthly averages, HEM simulates a home in half-hourly steps, which lets it see things SAP structurally cannot: time-of-use tariffs, battery storage cycling, heat pump behaviour across a cold snap, solar self-consumption.
How HEM differs from SAP
- Half-hourly calculation. Energy use is modelled through the day rather than smeared across a month HEM consultation, so flexibility and storage finally score.
- Wrapper architecture. A core physics engine with policy "wrappers" on top: one wrapper produces EPC outputs, others serve building regulations and future policies, all from one model HEM consultation.
- Built for the four metrics. The reformed certificate's energy cost, fabric, heating system and smart readiness readings are natural HEM outputs.
The dates that matter
Two anchor points are on the record. HEM was consulted on as SAP's replacement, with a further consultation on HEM-based EPCs published alongside the March 2026 partial response HEM consultation. And the confirmed PRS standard switches its measurement basis to HEM-derived metrics from 1 October 2029 Gov response Jan 2026, one year before the band C deadline. For anything more precise, watch the reform tracker; intermediate milestones are still consultation material, and we label them that way.
What changes for you
Nothing about your current certificate: it remains valid to its printed date under the methodology that produced it GOV.UK SAP. The practical effect arrives with your next assessment after the switchover, when a home's score starts rewarding flexibility and storage as well as insulation. Landlords sequencing 2030 works should read EPC C by 2030 alongside this page; EPC vs SAP explains the system being retired, and RdSAP 10 is the bridge you will be assessed under in the meantime.