Elmhurst receives confirmed Home Energy Model methodology for existing homes

Elmhurst Energy said on 12 November that it has received the confirmed Home Energy Model (HEM) methodology for existing dwellings, the calculation method being developed to replace the Reduced Data Standard Assessment Procedure (RdSAP) for Energy Performance Certificates (EPCs).

The accreditation body said the methodology, which it said the industry currently knows as HEMIE, sets out how the energy performance of existing homes will be calculated under the new EPC framework. Its technical teams are reviewing the document to understand the changes and what they mean for Domestic Energy Assessors (DEAs), it said.

Elmhurst did not say who issued the methodology or when. The Department for Energy Security and Net Zero (DESNZ) consulted on applying HEM to existing dwellings in England and Wales earlier this year; according to Elmhurst's January post, the deadline for responses was Wednesday 18 March 2026.

What changes for assessors

According to Elmhurst, HEMIE moves from monthly calculations to dynamic half-hourly modelling. Its explainer page says the Home Energy Model simulates homes at a 30-minute time resolution, and that a wrapper defines the standardised and user inputs required for a given use.

Elmhurst said the methodology has been designed to simplify data collection by using defaults, automated geometry generation and inferred values. Rather than relying mainly on age-band assumptions, building elements can be modelled from documented U-values, simplified construction descriptions or layer-by-layer build-ups.

Heating and hot water modelling covers a wider range of technologies, including heat batteries, heat networks, smart hot water tanks and advanced controls, Elmhurst said. Assessors will be able to record solar PV inverter details, and the model supports battery storage, PV diverters and smart hot water systems.

Measured Heat Transfer Coefficient (HTC) results from approved SMETER tests can be recorded alongside modelled values, according to Elmhurst. Assessors will record a limited set of appliances, namely a dishwasher, clothes drying appliance, separate fridge/freezer and oven/hob fuel type, while lighting energy use and heat gains are calculated automatically with no assessor input.

Despite the more detailed calculations, Elmhurst said the information collected for most existing homes under HEMIE is expected to remain largely the same as DEAs gather today. It told members to look out for further communications on the transition, including any actions required.

Background

The DESNZ consultation document described a modular approach in which the assessor enters as much data as they have access to in each dwelling, with defaults used for the rest.

In October 2025 Elmhurst reported that DESNZ had confirmed core HEM would be delivered through Energy Calculation as a Service (Ecaas), and that HEM is based on BS EN ISO 52016-1:2017, which differs from SAP. Jason Hewins, Elmhurst New Build Dwellings Manager, said at the time: "We are pleased to see the Home Energy Model consultation responses published by DESNZ."

The wider reform of the Energy Performance of Buildings regime proposes four headline metrics for domestic EPCs: fabric performance, heating system, smart readiness and energy cost. That consultation anticipated the new metrics would be introduced in the second half of 2026.

In a 10 March 2026 update to the HEM consultation page, however, the government said it had decided to move the launch of the reforms to the second half of 2027. It said it would work with industry and the devolved administrations to agree a new launch date and a shared implementation plan.

Elmhurst, based in Lutterworth, describes itself as the UK's largest independent provider of energy assessment, retrofit and property professional training and is a government and TrustMark approved accreditation body.