The SAP/EI rating is computed on UK-average weather (Appendix U Tables
U1-U3 region 0) so ratings are nationally comparable, but Appendix U
paragraph 1 (PDF p.124) requires that "other calculations (such as for
energy use and costs on EPCs) are done using local weather. Weather data
for each postcode district are taken from the PCDB". `Sap10Calculator.
calculate` ran ONE cascade (UK-average) and fed it to SAP, CO2 AND primary
energy, so every cert's EPC-displayed CO2/PE were computed on the wrong
climate. Because most of England is warmer than the UK-average, this
systematically OVER-counted heating demand on the emissions/PE outputs.
The two cascades (`cert_to_inputs` rating, `cert_to_demand_inputs`
postcode) already existed; this wires the demand cascade into the
production entry point and grafts its CO2/PE onto the rating result (SAP
unchanged). The corpus gauge's longstanding +5% CO2/PE over-estimate was
mostly this climate bug, NOT (as previously diagnosed) per-cert mapper
fidelity:
CO2 MAE 0.26 -> 0.12 t/yr (bias +0.18 -> +0.04)
PE MAE 13.6 -> 3.8 kWh/m2 (bias +9.0 -> +0.24)
SAP within-0.5 = 69.7% (rating cascade, unchanged)
Worksheet-validated to 1e-4 on simulated case 45 (heat-pump ground-floor
flat, postcode W6): the P960 prints the current dwelling twice — Block 1
on UK-average weather (SAP 60.5318, CO2 692.13) and Block 2 on postcode
weather (CO2 626.78, PE 6581.59). Both reproduce exactly. Added a tracked
case-45 Summary fixture + two-cascade cascade pin as a permanent guard,
and ratcheted the corpus CO2/PE ceilings to 0.13 / 4.2. The e2e Elmhurst
suite (Block-1 line refs) now pins the rating cascade directly; the two
Vaillant overlay snapshots refreshed to demand-cascade CO2/PE.
pyright not installed in this codespace (strict gate not run locally);
change is type-trivial (dataclasses.replace over SapResult).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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