Pin end-to-end off-peak day/night bill regression 🟩

An Economy-7 storage dwelling now prices heating at the 0.20-day/0.80-
night blend through cert -> calculator -> bill, instead of raising
UnpricedFuel and aborting the modelling_e2e batch.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/domain/billing/test_off_peak_bill_integration.py

@@ -0,0 +1,77 @@
+"""End-to-end regression for the Off-Peak Meter day/night bill split (ADR-0014
+2026-06-24 amendment).
+
+Drives a real off-peak (Economy-7) storage-heating cert through the whole chain
+— `cert_to_inputs` → `calculate_sap_from_inputs` → `EnergyBreakdown.from_sap_result`
+→ `BillDerivation.derive` against the committed Fuel Rates snapshot. Before the
+amendment this raised `UnpricedFuel` ("no rate for fuel ELECTRICITY_OFF_PEAK"),
+which aborted the `modelling_e2e` batch (e.g. property 717572). It must now price
+the dwelling day/night instead of crashing.
+"""
+
+from __future__ import annotations
+
+from dataclasses import replace
+
+from domain.sap10_ml.tests._fixtures import (
+    make_building_part,
+    make_minimal_sap10_epc,
+    make_sap_heating,
+)
+from datatypes.epc.domain.epc_property_data import EpcPropertyData, MainHeatingDetail
+from domain.sap10_calculator.calculator import calculate_sap_from_inputs
+from domain.sap10_calculator.rdsap.cert_to_inputs import cert_to_inputs
+from domain.fuel_rates.fuel import Fuel
+from domain.billing.bill import BillSection, EnergyBreakdown
+from domain.billing.bill_derivation import BillDerivation
+from repositories.fuel_rates.fuel_rates_static_file_repository import (
+    FuelRatesStaticFileRepository,
+)
+
+_TYPICAL_TFA_M2 = 60.0
+
+
+def _off_peak_storage_epc() -> EpcPropertyData:
+    # Integrated storage heaters (SAP code 408, Table 12a Grid 1 high-rate
+    # fraction 0.20) on a Dual / Economy-7 (7-hour) meter.
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,  # electric storage heaters
+        sap_main_heating_code=408,
+    )
+    base = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part(construction_age_band="E")],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+    return replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),
+    )
+
+
+def test_off_peak_storage_dwelling_bills_heating_day_night_not_crashes() -> None:
+    # Arrange — score the off-peak cert and price it at the committed snapshot.
+    result = calculate_sap_from_inputs(cert_to_inputs(_off_peak_storage_epc()))
+    rates = FuelRatesStaticFileRepository().get_current()
+
+    # Act — the previously-crashing path: off-peak electricity reaches the bill.
+    breakdown = EnergyBreakdown.from_sap_result(result)
+    bill = BillDerivation(rates).derive(breakdown)
+
+    # Assert — heating bills on the off-peak carrier at the 0.20-day/0.80-night
+    # blend (0.20×29.73 + 0.80×13.89 = 17.058 p/kWh), not a crash and not the
+    # flat standard rate (24.67 p) or pure night rate (13.89 p).
+    heating = bill.sections[BillSection.HEATING]
+    assert heating.kwh > 0.0
+    implied_rate_p = heating.cost_gbp / heating.kwh * 100.0
+    assert abs(implied_rate_p - 17.058) <= 1e-6
+    # The off-peak meter contributes one standing charge (56.99 p/day).
+    assert any(line.fuel is Fuel.ELECTRICITY_OFF_PEAK for line in breakdown.lines)
+    assert bill.total_gbp > 0.0