HEM Preview · Live
Run today’s dwelling through the Home Energy Model.
HEM replaces SAP under the Future Homes Standard and is lodged through ECaaS. HEM Preview takes the extraction that already fills your SAP form, builds a schema-valid HEM core-input JSON with per-field provenance, and runs it through the open-source HEM engine and FHS wrapper for a results table — your dwelling against the FHS notional dwelling — and an unofficial compliance preview (DER/TER, DPER/TPER, DFEE/TFEE). A head start on HEM — not a lodgement.
HEM vs SAP — what actually changes
Not a rebrand of SAP. A different calculation, different inputs, and a different place to lodge them.
Monthly scoring becomes half-hourly simulation
SAP scores a dwelling against monthly reference tables. HEM simulates it half-hour by half-hour across a full year — heating demand comes out of the physics, not a lookup.
U-values become full constructions
SAP asks for a U-value per element. HEM asks for the construction itself — thermal resistance, areal heat capacity and how the mass sits within the build-up. The spec document matters more, not less.
The government calculates, you evidence
Under SAP, your software runs the calculation. Under HEM, the government's ECaaS service runs it. Your job shifts to assembling correct, evidenced inputs — which is exactly what Specvo extracts.
How the preview works
01
Same extraction as your SAP project
The drawings and spec you already uploaded feed the HEM export directly. No re-keying, no second intake.
02
Schema-valid core-input JSON
Built to the published HEM schema, with per-field provenance. Where your documents don't cover a field, a template default fills it — tagged as a default, never passed off as measured.
03
Open-source HEM engine runs it
The government's open-source HEM engine runs a full-year, half-hourly simulation with weather for your region.
04
Dwelling vs notional, plus an unofficial compliance preview
You get what HEM reports for your dwelling and for the FHS notional dwelling side by side: heating and cooling demand, delivered energy by system and fuel, mains import and peak load, system CoPs and the fabric figures (HTC, HLP). Above it, the government's open-source FHS wrapper gives DER vs TER, DPER vs TPER and DFEE vs TFEE with pass/fail and an EER band — marked UNOFFICIAL, because only the ECaaS result counts at lodgement. The FHS input JSON itself is downloadable in the shape ECaaS accepts.
Honest about what it knows
Every export opens with its own provenance summary:
“N fields from your documents / M template defaults — labelled, never hidden.”
Each field in the JSON carries a provenance tag: extracted from your drawings and spec, or filled by a template default. You see the split before you rely on the number — and you know exactly which fields to review when HEM goes live.