The current single A–G Energy Performance Certificate rating is being replaced by four separate metrics: Fabric Performance, Heating System, Smart Readiness, and Energy Cost. Each metric is calculated using the Home Energy Model (HEM) and gives a clearer, more honest picture of a specific aspect of a home's energy performance. Three metrics are banded A–G; Energy Cost is displayed as an estimated annual figure in pounds.
The Four Metrics at a Glance
| Metric | Measures | Scale | Key Implication |
|---|---|---|---|
| Fabric Performance | Building envelope heat retention | A–G | Shows building quality independent of heating |
| Heating System | Heating efficiency + carbon intensity | A–G | Fossil fuel heating could not achieve C (proposed) |
| Smart Readiness | Smart energy and grid interaction | A–G | Rewards battery storage, smart controls, flexibility |
| Energy Cost | Estimated annual energy running costs | £ per year | Directly comparable between properties |
1. Fabric Performance
The Fabric Performance metric measures how well the building's physical structure retains heat. It assesses the U-values of walls, roof, floor, and windows, as well as airtightness and thermal bridging. This metric is independent of the heating system, telling you about the quality of the building itself.
Under the current EPC system, a home with excellent insulation but an inefficient heating system might get a mediocre overall rating, masking the investment in fabric. The separate Fabric Performance metric corrects this by showing the building's intrinsic thermal quality on its own terms.
What Affects the Fabric Rating?
- Wall insulation: thickness, type, and continuity (cavity, external, or internal)
- Roof and loft insulation: depth and material
- Floor insulation, especially important in suspended timber floors
- Windows and doors: glazing type (single, double, triple), frame material, and U-value
- Airtightness: how well the building prevents uncontrolled air leakage
- Thermal bridges: heat loss at junctions between building elements
- Thermal mass, the building's capacity to absorb and store heat
Because HEM uses half-hourly dynamic simulation, the Fabric Performance calculation captures how the building actually behaves across different conditions, not just a single steady-state number. This means thermal mass and orientation are properly credited.
2. Heating System
The Heating System metric assesses the efficiency and carbon intensity of the home's heating and hot water system. This combines two factors: how much energy the system uses to produce useful heat, and how much carbon that energy produces.
| Heating Type | Proposed Position | Why |
|---|---|---|
| Heat pump (air or ground source) | Always C or above | High efficiency + low-carbon electricity |
| Low-carbon heat network | Always C or above | Low-carbon heat source |
| Gas, oil or LPG boiler (primary heating) | Cannot achieve C | Boundary set so fossil fuel heating does not reach C |
| Hybrid heat pump with gas boiler | D or below | Includes fossil fuel, but scores above the same boiler alone |
| Direct electric heating (no storage) | D or below | No storage or flexibility benefit |
| Electric heating with smart thermal storage | Typically C or above | Off-peak use lowers annual emissions versus direct electric |
These positions are proposals from the HEM: EPC consultation. Band boundaries have not yet been set, and the government notes it is not currently possible to estimate a given dwelling's ratings under the proposed system.
HEM calculates heat pump performance dynamically at each half-hourly timestep, using actual source temperatures and flow temperatures rather than SAP's fixed seasonal averages. This means efficient heat pump installations are more accurately rewarded. See SAP vs HEM for more on these differences.
3. Smart Readiness
Smart Readiness is an entirely new concept for UK EPCs. It measures a home's capacity to interact with smart energy systems and the wider electricity grid. This metric recognises that the value of a home's energy system increasingly depends on its ability to be flexible and responsive.
What Contributes to Smart Readiness?
- Smart heating controls: internet-connected thermostats, zoned heating, weather compensation
- Smart meter, enabling real-time energy monitoring and time-of-use tariffs
- Battery storage, storing solar PV generation or cheap off-peak electricity for later use
- EV charging: smart charger capable of vehicle-to-grid or managed charging
- Demand-side response: ability to shift energy use in response to grid signals
- Solar PV: on-site generation combined with smart export/self-consumption management
A home with a heat pump, solar PV, battery storage, and smart controls would score highly on Smart Readiness. A home with a basic timer-controlled gas boiler and no smart meter would score poorly. This metric supports the government's vision of homes as active participants in the energy system, not just passive consumers.
4. Energy Cost
The Energy Cost metric provides an estimated annual energy running cost in pounds. Unlike the current EPC, which uses an abstract “Energy Efficiency Rating” score, this metric gives buyers and tenants a directly comparable figure they can understand immediately.
The calculation uses standardised fuel prices (the same for every assessment at a given time, based on prices when the EPC is generated) and HEM's half-hourly simulation to estimate the overall annual energy bill. Solar PV self-consumption and battery storage are fully modelled, reducing the estimated cost for homes that generate and store their own electricity.
How HEM Improves EPC Accuracy
| Aspect | Current EPCs (SAP/RdSAP) | New EPCs (HEM) |
|---|---|---|
| Time resolution | Monthly averages | Half-hourly timesteps (17,520 per year) |
| Heat pump efficiency | Fixed seasonal COP | Dynamic COP at each timestep |
| Solar PV | Annual generation estimate | Half-hourly generation + self-consumption |
| Thermal mass | Simplified parameter | Dynamic modelling at each timestep |
| Carbon factors | Fixed annual (0.136) | Forward-looking (2025–2029 projected) |
| Battery storage | Self-use factor adjustment (RdSAP 10) | Charge/discharge modelled half-hourly |
| Smart controls | Limited credit | Dedicated Smart Readiness metric |
Frequently Asked Questions
Which EPC metric is most important?
No single metric is more important; they each measure a different aspect. Fabric Performance shows how well the building retains heat. Heating System shows heating efficiency and carbon intensity. Smart Readiness shows smart energy capacity. Energy Cost shows estimated running costs. The most relevant metric depends on your priorities.
How can I improve my Fabric Performance rating?
Fabric Performance is improved through physical building upgrades: wall insulation, loft insulation, floor insulation, higher-performance windows, draught-proofing, and addressing thermal bridges. These are typically permanent improvements that increase the property's intrinsic value. HEM's dynamic modelling captures these benefits more accurately than SAP/RdSAP.
What Heating System rating will a gas boiler get?
Under government proposals, a gas boiler would score D or below, regardless of its efficiency. The metric assesses both efficiency and carbon intensity, and fossil fuels produce far more carbon per unit of useful heat than heat pumps. Even a new A-rated condensing gas boiler could not achieve C or above. To score higher, you would need to switch to a low-carbon heating system.
What do I need for a good Smart Readiness rating?
Key factors include: smart heating controls (beyond a basic timer), smart meter, battery storage, EV charging capability, and demand-side response capability. The highest ratings require grid interaction and time-of-use tariff integration. A basic home with a timer-controlled gas boiler and no smart meter would score poorly.
How is the Energy Cost metric calculated?
Energy Cost is calculated using HEM's half-hourly simulation with standardised fuel prices. It estimates the overall annual energy bill for the property. Unlike SAP, HEM models solar PV self-consumption and battery storage, which directly reduce estimated costs. Displayed as a £ figure, not an A–G band.