Include the C6 half heat-network standing charge on top of a water-only dwelling's fuel standing 🟥

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2021,6 +2021,17 @@ def _heat_network_standing_charge_gbp(
     return None
 
 
+def _standing_charges_total_gbp(
+    epc: EpcPropertyData,
+    main: Optional[MainHeatingDetail],
+    *,
+    main_fuel_code: Optional[int],
+    water_heating_fuel_code: Optional[int],
+    tariff: Tariff,
+) -> float:
+    raise NotImplementedError
+
+
 def _main_heating_efficiency(epc: EpcPropertyData) -> float:
     """SAP 10.2 (206) main system 1 efficiency as a 0..1 fraction.
 

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -62,6 +62,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _heat_network_dlf,  # pyright: ignore[reportPrivateUsage]
     _heat_network_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _heat_network_standing_charge_gbp,  # pyright: ignore[reportPrivateUsage]
+    _standing_charges_total_gbp,  # pyright: ignore[reportPrivateUsage]
     _main_fuel_code,  # pyright: ignore[reportPrivateUsage]
     _mev_decentralised_kwh_per_yr_from_cert,  # pyright: ignore[reportPrivateUsage]
     _mev_default_data_iuf,  # pyright: ignore[reportPrivateUsage]
@@ -9104,6 +9105,61 @@ def test_non_heat_network_main_returns_none_so_caller_uses_fuel_standing() -> No
     assert _heat_network_standing_charge_gbp(epc, main) is None
 
 
+def test_water_only_heat_network_half_charge_adds_to_the_dwellings_fuel_standing() -> None:
+    # Arrange — SAP 10.2 §C6 (PDF p.54): "Include one-half of the normal heat
+    # network standing charge in the calculation of fuel costs unless the space
+    # heating is also a heat network." WHC 950 is a DHW-only heat network, and
+    # this dwelling's space heating is electric storage (SAP 401) — not a
+    # network — so the "unless" does not fire and the half charge applies.
+    #
+    # C6 says "INCLUDE [...] IN the calculation of fuel costs", in deliberate
+    # contrast to the full case's "the TOTAL standing charge IS the normal heat
+    # network standing charge" (next paragraph, p.55). Only the full case is
+    # phrased as a replacement, so the half is ADDITIVE to whatever Table 12
+    # note (a) already charges the dwelling for its own fuel. Note (a): "The
+    # standing charge for off-peak electricity is added to space and water
+    # heating costs where either main heating or hot water uses off-peak
+    # electricity" — a Dual meter + SAP 401 is §12 Rule 2 → 7-hour → Table 32
+    # code 32 = £24/yr.
+    #
+    # Half of the RdSAP10 Table 32 code-51 heat-network row (£120/yr) is £60.
+    # Total £84. Cert 22032926 (22 Gwydyr Mansions) is the corpus instance.
+    from dataclasses import replace
+
+    from domain.sap10_calculator.tables.table_12a import Tariff
+
+    base = _typical_semi_detached_epc()
+    storage_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=0,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,  # electric storage heaters
+        sap_main_heating_code=401,
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),  # Dual
+        sap_heating=make_sap_heating(
+            main_heating_details=[storage_main],
+            water_heating_code=950,  # DHW-only heat network (boilers)
+        ),
+    )
+
+    # Act
+    total = _standing_charges_total_gbp(
+        epc,
+        storage_main,
+        main_fuel_code=29,
+        water_heating_fuel_code=51,  # Table 32 heat-network row
+        tariff=Tariff.SEVEN_HOUR,
+    )
+
+    # Assert — £24 off-peak electric (note (a)) + £60 half network (C6).
+    assert abs(total - 84.0) <= 1e-9
+
+
 def test_mev_default_data_iuf_is_table_329_system_type_10_value() -> None:
     # Arrange / Act — SAP 10.2 Table 4g note 3 (PDF p.176) directs the
     # default SFP to "the appropriate in-use factor for default data from