17,520 moments
Instead of twelve monthly blocks, HEM calculates every half hour across a year.
Home Energy Model
A better way to understand how a home uses energy.
The government is developing HEM as the intended successor methodology to SAP. It looks at a home half hour by half hour, making weather, heating, ventilation, solar and storage easier to represent together.
EPCs remain in force. HEM implementation details and the certificate wrapper are still developing and must be checked before a formal assessment.
What changes
From a broad monthly picture to the rhythm of a real year.
The building itself
Geometry, materials, windows, ventilation and thermal mass shape how heat moves.
Systems in context
Heat pumps, hot water, solar and storage respond to weather and time of use.
A standard household
Published occupancy assumptions keep official comparisons consistent between homes.
Two truths, kept separate
What a standard home should do. What your home actually does.
HEM uses a standardised household so homes can be compared fairly. Real households are different. Some ration heat, some choose more comfort after improvements, and daily moisture and hot-water patterns vary.
The assessment track
A versioned calculation using published assumptions. Useful for comparison and compliance, but still a forecast.
The operating track
With permission, the GridSync record can hold real temperatures, humidity, generation and energy use. Gateway Pro is designed to coordinate compatible systems without asking the resident to become an energy manager.
One survey, one shared record
Capture, model, specify, verify.
Capture
One trained surveyor records geometry, openings, services, roof and practical constraints.
Model
The versioned twin records assumptions and identifies where evidence is missing.
Specify
Each competent specialist uses the same record, while retaining their professional responsibility.
Verify
The as-built condition, commissioning and measured operation return to the same record.
What one capture does not replace
Intrusive investigations, structural and electrical tests, asbestos surveys, statutory inspections and specialist judgement remain separate whenever the property or work requires them.
Why it matters
Better evidence before investment, then a fair test after installation.
For households, that means a plan designed around the whole building. For portfolio owners and funders, it means common data, clearer assumptions and release decisions based on accepted evidence. Breathe’s HEM alignment is a capability under development, not regulatory approval.
Source status, published and developing. Department for Energy Security and Net Zero, Home Energy Model replacement for SAP and Future Homes Standard technical documentation. Policy position reviewed 6 October 2026. Timings, wrappers and final metrics must be reconfirmed before a board or regulatory decision.