Energy Performance Certificates (EPCs) are being fundamentally reformed. The current single A–G rating is being replaced by a four-metric framework — Fabric Performance, Heating System, Smart Readiness, and Energy Cost — calculated using the Home Energy Model (HEM) instead of SAP and RdSAP. New HEM-based EPCs are targeted for launch from October 2026, running alongside the existing system until October 2029.
Why Are EPCs Changing?
The current EPC system has been widely criticised for being too simplistic and, in many cases, misleading. The single A–G rating based on energy cost conflates multiple factors — a home with cheap gas heating can score better than one with an efficient heat pump, because the rating is based on fuel costs rather than energy efficiency or carbon emissions.
The underlying calculation methodology also needs updating. EPCs for existing homes are currently produced using RdSAP, a simplified version of SAP that uses monthly calculations and age-based defaults. This approach cannot accurately model modern technologies like heat pumps, solar PV with battery storage, or smart heating controls. The Home Energy Model replaces this with a dynamic half-hourly simulation that captures real-world performance far more accurately.
What Is Changing?
Four New Headline Metrics
The single A–G energy efficiency rating is being replaced by four separate metrics, each providing a clearer picture of a different aspect of a home's performance:
| Metric | What It Measures | Banding |
|---|---|---|
| Fabric Performance | How well the building envelope retains heat — walls, roof, floor, windows, airtightness | A–G |
| Heating System | Efficiency and carbon intensity of the heating system (fossil fuel heating cannot achieve C or above) | A–G |
| Smart Readiness | Capacity for smart controls, demand flexibility, and interaction with the electricity grid | A–G |
| Energy Cost | Estimated annual energy running costs in pounds sterling | £ value |
This separation means buyers and landlords can see exactly where a property performs well and where it falls short, rather than relying on a single number that hides important detail. For a full breakdown of each metric, see our New EPC Metrics page.
HEM-Based Calculation
All new EPCs will be calculated using HEM instead of SAP or RdSAP. This means:
- Half-hourly simulation instead of monthly averages — capturing how a home actually performs throughout the day and across seasons
- Forward-looking carbon factors based on the projected 2025–2029 grid mix, rather than SAP's historical 2012 figures
- Accurate heat pump modelling — HEM calculates COP dynamically at each timestep based on actual conditions, rather than using fixed seasonal averages
- Solar PV self-consumption modelled at each half-hourly interval, properly crediting homes that use their own generated electricity
- Centralised calculation via ECaaS — every assessment uses the identical engine, eliminating inconsistencies between different software providers
Modular Input Approach
For existing dwellings, the current RdSAP model uses a rigid data collection framework with age-based defaults for unknown elements. The new HEM-based approach proposes a modular input framework where assessors collect what they can directly observe, and HEM applies calibrated defaults for elements that cannot be determined on site. This is more flexible and should produce more accurate results, particularly for homes that have been improved since construction.
Current EPCs vs New HEM-Based EPCs
| Feature | Current EPCs (SAP/RdSAP) | New EPCs (HEM) |
|---|---|---|
| Headline rating | Single A–G based on energy cost | Four separate metrics (Fabric, Heating, Smart, Cost) |
| Calculation method | Monthly steady-state (SAP/RdSAP) | Half-hourly dynamic simulation (HEM) |
| Carbon factors | Historical (2012 grid mix) | Forward-looking (2025–2029 projected) |
| Heat pump modelling | Fixed seasonal efficiency | Dynamic COP at each timestep |
| Solar PV | Annual generation estimate | Half-hourly generation + self-consumption |
| Software | Multiple third-party engines | Centralised ECaaS API |
| For existing homes | RdSAP (rigid defaults) | HEM reduced data (modular inputs) |
| Smart technology | Limited or no credit | Smart Readiness metric + dynamic modelling |
EPC Reform Timeline
| Milestone | Date | Status |
|---|---|---|
| EPC reform consultation published | December 2024 | Complete |
| EPC reform consultation closed | February 2025 | Complete |
| HEM: EPC Assessment consultation published | 21 January 2026 | Complete |
| HEM: EPC consultation closes | 18 March 2026 | Upcoming |
| Government response to EPC consultations | 2026 (expected) | Awaited |
| New HEM-based EPCs launch | October 2026 (target) | Expected |
| Existing EER continues alongside new metrics | October 2026 – October 2029 | Expected |
| Existing EER discontinued — HEM EPCs only | October 2029 (target) | Expected |
Who Is Affected?
Homeowners & Landlords
Existing EPCs remain valid for 10 years from issue. When you next need an EPC (for selling or letting), it will be produced under the new system if the launch has occurred. The four-metric format gives you clearer information about where your property performs well and where improvements would have the most impact. Properties with fossil fuel heating will score poorly on the Heating System metric, which may influence property values over time.
Energy Assessors
The transition from RdSAP to HEM-based assessments represents a significant change in working practice. Data collection will be more detailed, all calculations will run through ECaaS, and assessors will need to understand the new four-metric framework. Training and accreditation updates are expected ahead of launch. See our SAP Assessor Guide for preparation advice.
Property Industry
Estate agents, mortgage lenders, and lettings agents will need to understand and communicate the new four-metric format. The transition period (2026–2029) means both old and new format EPCs will be in circulation simultaneously, requiring systems that can handle both.
Explore EPC Guides
How EPCs Are Changing
Detailed guide to the new EPC assessment process, what changes for homeowners, and the transition timetable.
The EPC Wrapper
How HEM's EPC wrapper works, standardised assumptions, and the modular architecture.
New EPC Metrics Explained
Full breakdown of the four new headline metrics: Fabric, Heating, Smart Readiness, and Energy Cost.
RdSAP to HEM Transition
How RdSAP is being replaced by HEM for existing dwelling assessments, and what assessors need to know.
Frequently Asked Questions
When will the new HEM-based EPCs launch?
The government is targeting October 2026 for the launch of new HEM-based EPCs, running alongside the existing Energy Efficiency Rating. The HEM: EPC consultation (closing 18 March 2026) sets out the proposed framework. The existing EER continues until October 2029, after which all EPCs will use HEM-based metrics only.
What are the four new EPC metrics?
The four proposed metrics are: Fabric Performance (how well the building retains heat), Heating System (efficiency and carbon intensity), Smart Readiness (capacity for smart controls and flexibility), and Energy Cost (estimated annual running costs). Each except Energy Cost will be banded A–G. See our New EPC Metrics page for full details.
Will my existing EPC still be valid?
Yes. Existing EPCs remain valid for 10 years from issue, regardless of when the new system launches. New HEM-based EPCs will be produced for new assessments from October 2026 onwards. The existing A–G rating runs alongside the new metrics until October 2029, after which all new EPCs use the four-metric framework only.
What replaces RdSAP for existing homes?
RdSAP will be replaced by a HEM-based reduced data methodology. Instead of RdSAP's rigid model, HEM uses a modular input framework where assessors collect what they can observe and HEM applies defaults for unknowns. See our RdSAP Transition page for details.
How will the new EPCs affect energy assessors?
Assessors will need to learn the new HEM-based process, which requires more detailed data collection than RdSAP. All calculations run through the centralised ECaaS platform. Software providers will build interfaces on top of the ECaaS API. Training and accreditation updates are expected ahead of October 2026. See our assessor guide for preparation advice.
Related Pages
What is the Home Energy Model?
Comprehensive introduction to HEM — what it is and how it works.
SAP vs HEM
Side-by-side comparison of SAP and HEM methodologies.
Future Homes Standard
Full guide to the FHS and what it means for new homes.
Timeline & Status
Live tracker of HEM, FHS, and EPC reform milestones.