slice S-B7: per-end-use fuel cost — HW uses water-fuel, lighting always electric

SAP 10.3 §12 charges fuel costs by end-use, not by main heating fuel.
For a gas-heated dwelling with an electric immersion hot-water cylinder,
HW bills at the electric rate (13.19 p/kWh) not the gas main-heating
rate (3.48 p/kWh) — a 3.8× cost difference for HW that propagates
straight to ECF. Lighting, central-heating pumps, and fans always
electric regardless of main fuel.

Discovered by hand-tracing cert 8035-9023 (Detached bungalow, actual
SAP 43, predicted 63). Trace showed our hot-water + lighting + pumps
lines were charging mains-gas rates throughout, under-counting cost by
~£290/yr.

100-cert parity probe (biggest single Session-B slice so far):
  MAE 5.70 → 4.90   (-0.80, -14%)
  RMSE 7.48 → 6.68  (-11%)
  within ±1:  20% → 24%
  within ±3:  37% → 46%
  within ±5:  54% → 67%
  bias +1.50 → -1.44 (over-corrected by ~3 SAP points)

The over-correction (bias now slightly negative) means we're now
under-predicting on average. Next slice tackles where we're charging
too much electricity — probably HW on dwellings with combi boilers (no
immersion, water still on main fuel) and the water_heating_code 901
("from main system") inheritance path.

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

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

@@ -160,6 +160,12 @@ _DEFAULT_THERMAL_MASS_PARAMETER_KJ_PER_M2_K: Final[float] = 250.0
 _DEFAULT_PUMPS_FANS_KWH_PER_YR: Final[float] = 130.0
 
 
+# SAP 10.3 §12: lighting + central-heating pumps + fans always bill at
+# the standard-electricity rate regardless of the main heating fuel —
+# Table 32 code 30 (standard electricity), 13.19 p/kWh.
+_STANDARD_ELECTRICITY_P_PER_KWH: Final[float] = 13.19
+
+
 # SAP 10.3 Table 9 main_heating_control codes → control type (1/2/3).
 # Type 1: no time + temp control, or one but not both.
 # Type 2: programmer + room thermostat (+/− TRVs).
@@ -374,6 +380,25 @@ def _space_heating_fuel_cost_gbp_per_kwh(main: Optional[MainHeatingDetail]) -> f
     return _fuel_cost_gbp_per_kwh(main)
 
 
+def _hot_water_fuel_cost_gbp_per_kwh(
+    main: Optional[MainHeatingDetail], water_heating_fuel: Optional[int]
+) -> float:
+    """Hot water bills at the *water-heating* fuel's rate — distinct from
+    the main heating fuel for gas-heated dwellings whose DHW runs off an
+    electric immersion. Falls back to the main fuel when the cert
+    doesn't lodge a separate water fuel."""
+    if water_heating_fuel is not None:
+        return fuel_unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP
+    return _fuel_cost_gbp_per_kwh(main)
+
+
+def _other_fuel_cost_gbp_per_kwh() -> float:
+    """Pumps, fans, and lighting always bill at the standard-electricity
+    rate (SAP 10.3 §12; Table 32 code 30) regardless of the main heating
+    fuel — these end uses are electric in every UK dwelling."""
+    return _STANDARD_ELECTRICITY_P_PER_KWH * _PENCE_TO_GBP
+
+
 
 
 def _co2_factor_kg_per_kwh(main: Optional[MainHeatingDetail]) -> float:
@@ -495,7 +520,9 @@ def cert_to_inputs(epc: EpcPropertyData) -> CalculatorInputs:
         pumps_fans_kwh_per_yr=_DEFAULT_PUMPS_FANS_KWH_PER_YR,
         lighting_kwh_per_yr=lighting_kwh,
         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),
+        hot_water_fuel_cost_gbp_per_kwh=_hot_water_fuel_cost_gbp_per_kwh(
+            main, epc.sap_heating.water_heating_fuel
+        ),
+        other_fuel_cost_gbp_per_kwh=_other_fuel_cost_gbp_per_kwh(),
         co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
     )

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

@@ -203,19 +203,27 @@ def test_main_heating_efficiency_reads_sap_main_heating_code() -> None:
     assert inputs_lo.main_heating_efficiency == 0.70
 
 
-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). For mains-
-    # gas dwellings all three end-use fuel costs collapse to the same value.
-    epc = _typical_semi_detached_epc()
+def test_gas_heating_with_electric_immersion_charges_hw_at_electricity_rate() -> None:
+    # Arrange — Default test fixture: mains-gas main heating but the
+    # SapHeating fixture uses water_heating_fuel=26 (also mains gas) so
+    # all three lines collapse to gas. Override water_heating_fuel to 29
+    # (electricity) to verify the mapper picks the water fuel rate.
+    gas_only = _typical_semi_detached_epc()
+    electric_hw = _typical_semi_detached_epc()
+    electric_hw.sap_heating.water_heating_fuel = 29  # electricity
 
     # Act
-    inputs = cert_to_inputs(epc)
+    inputs_gas = cert_to_inputs(gas_only)
+    inputs_hw = cert_to_inputs(electric_hw)
 
-    # Assert
-    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
+    # Assert — gas main → space heating at gas rate; HW switches to electric
+    # rate when water_heating_fuel is electric; lighting/pumps always electric.
+    assert inputs_gas.space_heating_fuel_cost_gbp_per_kwh == 0.0348
+    assert inputs_gas.hot_water_fuel_cost_gbp_per_kwh == 0.0348
+    assert inputs_gas.other_fuel_cost_gbp_per_kwh == 0.1319
+    assert inputs_hw.space_heating_fuel_cost_gbp_per_kwh == 0.0348
+    assert inputs_hw.hot_water_fuel_cost_gbp_per_kwh == 0.1319
+    assert inputs_hw.other_fuel_cost_gbp_per_kwh == 0.1319
 
 
 def test_main_heating_control_code_maps_to_sap_control_type() -> None:
@@ -309,6 +317,7 @@ def test_electric_storage_heater_space_heating_at_off_peak_rate() -> None:
             ),
         ],
         sap_heating=make_sap_heating(
+            water_heating_fuel=29,  # all-electric house: water is also electric
             main_heating_details=[
                 MainHeatingDetail(
                     has_fghrs=False,