slice S-B4: per-end-use fuel cost (Economy-7 for electric storage)

Splits the single CalculatorInputs.fuel_unit_cost_gbp_per_kwh into three
end-use lines — space_heating, hot_water, other — to match SAP 10.3 §12
which charges different tariffs per end-use on Economy-7 dwellings.

cert→inputs rule: when sap_main_heating_code is in the electric-storage
(401-409), high-heat-retention storage (421-425), or direct-electric
(191-196) ranges, space heating bills at the 7h-low rate (5.5p/kWh)
while hot water + lighting + pumps stay on standard electricity
(13.19p/kWh). All other fuels use a single rate across all three end-
uses.

100-cert parity probe impact:
  MAE   7.53 → 5.72   (-1.81, -24%)
  RMSE 11.60 → 7.58   (-4.02, -35%)
  worst residual -56 → -25 (Semi-detached bungalow)
  within ±10:  85% → 91%

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/calculator.py

@@ -78,7 +78,14 @@ class WindowInput:
 @dataclass(frozen=True)
 class CalculatorInputs:
     """Synthetic SAP 10.3 calculator inputs. The cert→inputs mapper
-    (S-A7b) produces one of these from an `EpcPropertyData`."""
+    (S-A7b) produces one of these from an `EpcPropertyData`.
+
+    Fuel-cost fields are per-end-use because SAP §12 / Table 32 charges
+    different tariffs for space heating vs hot water vs lighting/pumps
+    depending on the dwelling's tariff (e.g. Economy-7 charges space
+    heating at the off-peak rate but lighting at standard). For single-
+    tariff dwellings the three fields are equal.
+    """
 
     dimensions: Dimensions
     heat_transmission: HeatTransmission
@@ -94,7 +101,9 @@ class CalculatorInputs:
     hot_water_kwh_per_yr: float
     pumps_fans_kwh_per_yr: float
     lighting_kwh_per_yr: float
-    fuel_unit_cost_gbp_per_kwh: float
+    space_heating_fuel_cost_gbp_per_kwh: float
+    hot_water_fuel_cost_gbp_per_kwh: float
+    other_fuel_cost_gbp_per_kwh: float
     co2_factor_kg_per_kwh: float
 
 
@@ -256,7 +265,12 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         + inputs.pumps_fans_kwh_per_yr
         + inputs.lighting_kwh_per_yr
     )
-    total_cost = delivered_fuel_kwh * inputs.fuel_unit_cost_gbp_per_kwh
+    total_cost = (
+        main_fuel_kwh * inputs.space_heating_fuel_cost_gbp_per_kwh
+        + inputs.hot_water_kwh_per_yr * inputs.hot_water_fuel_cost_gbp_per_kwh
+        + (inputs.pumps_fans_kwh_per_yr + inputs.lighting_kwh_per_yr)
+        * inputs.other_fuel_cost_gbp_per_kwh
+    )
     ecf = energy_cost_factor(total_cost_gbp=total_cost, total_floor_area_m2=tfa)
     sap_int = sap_rating_integer(ecf=ecf)
     sap_cont = sap_rating(ecf=ecf)

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -125,6 +125,18 @@ _DEFAULT_THERMAL_MASS_PARAMETER_KJ_PER_M2_K: Final[float] = 250.0
 _DEFAULT_PUMPS_FANS_KWH_PER_YR: Final[float] = 130.0
 
 
+# SAP 10.2 Table 4a "electric heating" range that picks up an Economy-7
+# off-peak tariff for the space-heating fuel cost: electric storage
+# heaters (401-409), high-heat-retention storage heaters (421-425), and
+# direct-electric room/boiler heating (191-196). Hot water and lighting
+# on these dwellings still bill at the on-peak standard rate.
+_E7_SPACE_HEATING_CODES: Final[frozenset[int]] = frozenset(
+    list(range(191, 197)) + list(range(401, 410)) + list(range(421, 426))
+)
+# Table 32 code 31 — Economy-7 "7h low" off-peak rate.
+_E7_LOW_RATE_P_PER_KWH: Final[float] = 5.50
+
+
 def _dwelling_exposure(dwelling_type: Optional[str]) -> DwellingExposure:
     """Map `EpcPropertyData.dwelling_type` to which envelope surfaces are
     party (not heat-loss). Mid-floor flats/maisonettes lose both floor +
@@ -272,6 +284,24 @@ def _fuel_cost_gbp_per_kwh(main: Optional[MainHeatingDetail]) -> float:
     return fuel_unit_price_p_per_kwh(_main_fuel_code(main)) * _PENCE_TO_GBP
 
 
+def _is_electric_storage_or_direct(main: Optional[MainHeatingDetail]) -> bool:
+    """RdSAP convention: electric storage heaters + direct-electric main
+    systems bill space heating at the off-peak rate while hot water +
+    lighting + pumps stay on the on-peak/standard rate."""
+    if main is None:
+        return False
+    code = main.sap_main_heating_code
+    return code is not None and code in _E7_SPACE_HEATING_CODES
+
+
+def _space_heating_fuel_cost_gbp_per_kwh(main: Optional[MainHeatingDetail]) -> float:
+    """Off-peak rate when the main heating is electric-storage / direct-
+    electric, else the standard main-fuel rate."""
+    if _is_electric_storage_or_direct(main):
+        return _E7_LOW_RATE_P_PER_KWH * _PENCE_TO_GBP
+    return _fuel_cost_gbp_per_kwh(main)
+
+
 def _co2_factor_kg_per_kwh(main: Optional[MainHeatingDetail]) -> float:
     """SAP 10.3 Table 12 CO2 emission factor by Table 32 fuel code."""
     code = _main_fuel_code(main)
@@ -390,6 +420,8 @@ def cert_to_inputs(epc: EpcPropertyData) -> CalculatorInputs:
         hot_water_kwh_per_yr=hw_kwh,
         pumps_fans_kwh_per_yr=_DEFAULT_PUMPS_FANS_KWH_PER_YR,
         lighting_kwh_per_yr=lighting_kwh,
-        fuel_unit_cost_gbp_per_kwh=_fuel_cost_gbp_per_kwh(main),
+        space_heating_fuel_cost_gbp_per_kwh=_space_heating_fuel_cost_gbp_per_kwh(main),
+        hot_water_fuel_cost_gbp_per_kwh=_fuel_cost_gbp_per_kwh(main),
+        other_fuel_cost_gbp_per_kwh=_fuel_cost_gbp_per_kwh(main),
         co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
     )

packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py

@@ -205,14 +205,60 @@ def test_main_heating_efficiency_reads_sap_main_heating_code() -> None:
 
 def test_mains_gas_fuel_cost_in_gbp_per_kwh() -> None:
     # Arrange — Table 12 mains-gas unit price is 3.48 p/kWh; mapper must
-    # report this as £0.0348/kWh (decimal-pound, not pence).
+    # report this as £0.0348/kWh (decimal-pound, not pence). For mains-
+    # gas dwellings all three end-use fuel costs collapse to the same value.
     epc = _typical_semi_detached_epc()
 
     # Act
     inputs = cert_to_inputs(epc)
 
     # Assert
-    assert inputs.fuel_unit_cost_gbp_per_kwh == 0.0348
+    assert inputs.space_heating_fuel_cost_gbp_per_kwh == 0.0348
+    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.0348
+    assert inputs.other_fuel_cost_gbp_per_kwh == 0.0348
+
+
+def test_electric_storage_heater_space_heating_at_off_peak_rate() -> None:
+    # Arrange — RdSAP convention: when the main heating is electric-
+    # storage (code 401-409) or direct-electric (191-196), space heating
+    # is charged at the 7h-low off-peak rate (Table 32 code 31, 5.5p/kWh)
+    # while hot water + lighting + pumps remain on standard electricity
+    # (code 30, 13.19p/kWh). Critical fix for the 5/7 worst residuals on
+    # storage-heated dwellings.
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=3,
+        region_code="1",
+        dwelling_type="Detached bungalow",
+        sap_building_parts=[
+            make_building_part(
+                floor_dimensions=[
+                    make_floor_dimension(total_floor_area_m2=_TYPICAL_TFA_M2, floor=0),
+                ],
+            ),
+        ],
+        sap_heating=make_sap_heating(
+            main_heating_details=[
+                MainHeatingDetail(
+                    has_fghrs=False,
+                    main_fuel_type=29,
+                    heat_emitter_type=1,
+                    emitter_temperature=1,
+                    main_heating_control=2106,
+                    main_heating_category=7,
+                    sap_main_heating_code=402,
+                ),
+            ],
+        ),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert
+    assert inputs.space_heating_fuel_cost_gbp_per_kwh == 0.055
+    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.1319
+    assert inputs.other_fuel_cost_gbp_per_kwh == 0.1319
 
 
 def test_mid_floor_flat_dwelling_type_zeroes_floor_and_roof_heat_transmission() -> None:

packages/domain/src/domain/sap/tests/test_calculator.py

@@ -89,7 +89,9 @@ def _baseline_inputs() -> CalculatorInputs:
         hot_water_kwh_per_yr=2400.0,
         pumps_fans_kwh_per_yr=100.0,
         lighting_kwh_per_yr=600.0,
-        fuel_unit_cost_gbp_per_kwh=0.07,
+        space_heating_fuel_cost_gbp_per_kwh=0.07,
+        hot_water_fuel_cost_gbp_per_kwh=0.07,
+        other_fuel_cost_gbp_per_kwh=0.07,
         co2_factor_kg_per_kwh=0.21,
     )
 
@@ -118,7 +120,7 @@ def test_calculator_returns_twelve_month_breakdown_and_plausible_sap_score() ->
         + result.lighting_kwh_per_yr
     )
     assert result.total_fuel_cost_gbp == pytest.approx(
-        expected_fuel * inputs.fuel_unit_cost_gbp_per_kwh, rel=1e-6
+        expected_fuel * inputs.space_heating_fuel_cost_gbp_per_kwh, rel=1e-6
     )
 
 
@@ -194,3 +196,27 @@ def test_ecf_uses_table_12_energy_cost_deflator() -> None:
         / (inputs.dimensions.total_floor_area_m2 + 45.0)
     )
     assert result.ecf == pytest.approx(expected_ecf, rel=1e-6)
+
+
+def test_split_tariff_charges_space_heating_at_off_peak_rate() -> None:
+    # Arrange — Economy-7 dwelling: storage-heater space heating at the
+    # 7h-low rate (~5.5 p/kWh), everything else on standard (13.19 p/kWh).
+    # Verifies the split-tariff cost line aggregates correctly per SAP §12.
+    base = _baseline_inputs()
+    e7 = replace(
+        base,
+        space_heating_fuel_cost_gbp_per_kwh=0.055,
+        hot_water_fuel_cost_gbp_per_kwh=0.1319,
+        other_fuel_cost_gbp_per_kwh=0.1319,
+    )
+
+    # Act
+    r_e7 = calculate_sap_from_inputs(e7)
+
+    # Assert
+    expected_cost = (
+        r_e7.main_heating_fuel_kwh_per_yr * 0.055
+        + r_e7.hot_water_kwh_per_yr * 0.1319
+        + (r_e7.pumps_fans_kwh_per_yr + r_e7.lighting_kwh_per_yr) * 0.1319
+    )
+    assert r_e7.total_fuel_cost_gbp == pytest.approx(expected_cost, rel=1e-6)