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Pre-S0380.65 the cascade applied the annual-flat SAP 10.2 Table 12
code-30 CO2 factor (0.136 kg/kWh) to all electric main heating fuel,
ignoring both the Table 12d monthly variation AND the Table 12a
Grid 1 (SH) high-rate fraction. For winter-peaked heat-pump loads on
an off-peak tariff this hid ~600 kg/yr of CO2 — cert 000565 worksheet
line 261:
Main 1 (high-rate cost) 20826.4764 kWh × 0.1581 = 3293.6388 kg
Main 1 (low-rate cost) 13884.3176 kWh × 0.1460 = 2027.2261 kg
───────────── ────────────
blended 34710.7941 × 0.1533 = 5320.87 kg
Cascade impact on cert 000565:
- co2_kg_per_yr 5823.16 → 6427.86 (Δ −624 → Δ −20)
- main_heating_co2_factor 0.1360 → 0.1533 (matches spec line 261)
- sap_score 29 EXACT (unchanged — CO2 doesn't enter
the energy-rating cost factor)
Spec basis:
- SAP 10.2 Table 12a Grid 1 (p.191): SH high-rate fraction by
`Table12aSystem` × tariff. ASHP_OTHER + TEN_HOUR = 0.6 high
(Slice S0380.61 already exercises this on the cost side).
- SAP 10.2 Table 12d (p.194): monthly CO2 factors by electricity
fuel code. 7-hour split = codes 32 (high) / 31 (low); 10-hour
split = codes 34 (high) / 33 (low). 18-hour (38/40) and 24-hour
(35) fall through to code-30 monthly factors in the table —
no dual-rate split applies for those tariffs.
- RdSAP 10 §12 page 62 (Slice S0380.60): meter_type=Dual + HP
without PCDB record → Rule 1 → TEN_HOUR tariff.
Implementation:
- New `_TARIFF_HIGH_LOW_FUEL_CODES_TABLE_12` dict — mirror of the
existing `_TARIFF_HIGH_LOW_RATES_P_PER_KWH` cost-rates dict.
Only SEVEN_HOUR + TEN_HOUR have a Table 12d split entry.
- New `_main_heating_co2_factor_kg_per_kwh(main, tariff, monthly_kwh)`
helper — mirror of `_space_heating_fuel_cost_gbp_per_kwh` on the
CO2 side. Falls back to the annual `_co2_factor_kg_per_kwh` for
non-electric mains, STANDARD tariff, mains without a Table 12a
Grid 1 row yet (storage / direct-acting electric — TODO matches
cost-helper coverage gap), tariffs without a Table 12d split,
and zero-fuel degenerate cases.
- Wire helper into `CalculatorInputs.main_heating_co2_factor_kg_per_kwh`
using the `energy_requirements_result.main_1_fuel_monthly_kwh`
profile already precomputed in `cert_to_inputs`.
Tests:
- `test_dual_meter_ashp_main_heating_co2_factor_applies_table_12a
_grid_1_split` — minimal ASHP code 224 + meter_type=1 cert
asserts the effective factor exceeds the pre-S0380.65 annual
flat (0.136 + 0.005) per spec.
- `test_standard_meter_ashp_main_heating_co2_factor_falls_back_
to_annual_table_12` — meter_type=2 (Standard) pass-through pin
at 0.136. Locks in non-regression for non-dual-meter certs.
Test suite: 480 pass + 9 expected 000565 cascade-gap fails (was
478/9 pre-S0380.65). Pyright net-zero on both touched files
(cert_to_inputs.py 34/34; test_cert_to_inputs.py 11/11).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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| .. | ||
| addresses | ||
| data_transformation | ||
| epc | ||
| sap10_calculator | ||
| sap10_ml | ||
| tasks | ||
| postcode.py | ||