Mirror of eval_api_sap_accuracy.py that decomposes each cert's SAP error
into per-component energy/cost deltas WITHOUT generating an Elmhurst
worksheet. Calibrates the consumer price from the certs we already get
right (gas £0.0809/kWh n=291, elec £0.2839/kWh n=326 over |SAP err|<0.4),
then for every cert compares our_component_kWh × price to the lodged
heating_cost_current / hot_water_cost_current / lighting_cost_current and
back-calculates a numeric energy target (lodged_cost / price).
Clusters errors by (component × direction). On the 905-cert sample this
reveals heat:high (we over-state heating energy → under-rate SAP) as the
dominant broken cluster: 332 certs, only 36.7% within 0.5. Output CSV at
<cache>/_cost_decomposition.csv.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
