fix(cost): HP-DHW from PCDB heat pump bills Table 12a ASHP_APP_N WH split

When DHW is heated by the main heat pump (WHC 901/902/914 = "from main
system") and the main carries a PCDB Table 362 record,
`_hot_water_fuel_cost_gbp_per_kwh` billed the electric HW at 100% off-peak
low rate (its long-standing TODO). SAP 10.2 Table 12a Grid 1 WH column
(PDF p.191) puts HP-DHW on the ASHP/GSHP-from-database row: 0.70
high-rate fraction at 7-hour and 10-hour → 0.70×14.68 + 0.30×7.50 =
12.526 p/kWh (10-hour), not 7.50 p. The low-rate collapse over-credited
the cat-4 HP-DHW cluster.

Fix: pass the cert WHC into the helper and, for HP-DHW (WHC ∈ {901,902,
914} + PCDB-HP main), bill at the ASHP_APP_N WH blended rate. Electric
IMMERSION (WHC 903) is a different Table 12a row (off-peak immersion 0.17
/ Table 13) and stays on the 100%-low-rate fallback until that slice
lands.

cat-4 cluster (20 certs): mean|err| 2.43→2.11, mean signed +0.06→-0.52
(now per-cert scatter, no systematic bias); cert 9472 +6.4→+3.2, 2789
+6.8→+4.0, 4135 +2.7→within 0.5. Headline mean|err| 1.727→1.720.
Regression green (2447 pass incl. golden 6035 + ASHP cohort at 1e-4);
pyright net-zero.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -103,6 +103,7 @@ from domain.sap10_calculator.tables.table_12a import (
     rdsap_tariff_for_cert,
     space_heating_high_rate_fraction,
     tariff_from_meter_type,
+    water_heating_high_rate_fraction,
 )
 from domain.sap10_calculator.tables.table_32 import (
     additional_standing_charges_gbp,
@@ -2231,6 +2232,7 @@ def _hot_water_fuel_cost_gbp_per_kwh(
     tariff: Tariff,
     prices: PriceTable,
     *,
+    water_heating_code: Optional[int] = None,
     inherit_main_for_community_heating: bool = False,
 ) -> float:
     """Hot water bills at the *water-heating* fuel's rate. When the
@@ -2239,8 +2241,16 @@ def _hot_water_fuel_cost_gbp_per_kwh(
     water fuel is a non-electric fuel (gas / oil / LPG), tariff is
     not consulted — those fuels are single-rate per Table 32. For
     cert 000565 HW routes to gas combi via WHC 914 → tariff branch
-    not taken. TODO: Table 12a Grid 1 WH high-rate-fraction split for
-    electric WH on off-peak (currently uses 100% low rate).
+    not taken.
+
+    HP-DHW exception: when DHW is heated by the main system (WHC
+    ∈ {901, 902, 914}) and that main is a PCDB Table 362 heat pump, the
+    HW bills per SAP 10.2 Table 12a Grid 1 WH column (PDF p.191) — the
+    ASHP/GSHP-from-database row carries a 0.70 high-rate fraction at
+    7-hour and 10-hour, NOT 100% off-peak low rate. Electric IMMERSION
+    (WHC 903) is a different Table 12a row (off-peak immersion 0.17 /
+    Table 13) and stays on the 100%-low-rate fallback until that slice
+    lands.
 
     `inherit_main_for_community_heating`: per S0380.173, when WHC
     ∈ {901, 902, 914} AND main is a heat network, ignore the cert-
@@ -2253,6 +2263,18 @@ def _hot_water_fuel_cost_gbp_per_kwh(
         return _fuel_cost_gbp_per_kwh(main, prices)
     water_electric = _is_electric_water(water_heating_fuel)
     if water_electric and tariff is not Tariff.STANDARD:
+        if (
+            water_heating_code in _WATER_INHERIT_FROM_MAIN_CODES
+            and main is not None
+            and main.main_heating_index_number is not None
+            and heat_pump_record(main.main_heating_index_number) is not None
+        ):
+            high_rate, low_rate = _tariff_high_low_rates_p_per_kwh(tariff)
+            high_frac = water_heating_high_rate_fraction(
+                Table12aSystem.ASHP_APP_N, tariff
+            )
+            blended = high_frac * high_rate + (1.0 - high_frac) * low_rate
+            return blended * _PENCE_TO_GBP
         return _off_peak_low_rate_gbp_per_kwh(tariff)
     if water_heating_fuel is not None:
         return prices.unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP
@@ -6963,6 +6985,7 @@ def cert_to_inputs(
             _water_heating_main(epc),
             _rdsap_tariff(epc),
             prices,
+            water_heating_code=epc.sap_heating.water_heating_code,
             inherit_main_for_community_heating=_community_hw_inherit,
         )
         hw_co2_factor = _hot_water_co2_factor_kg_per_kwh(

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -3212,6 +3212,48 @@ def test_space_heating_electric_room_heater_off_peak_bills_at_direct_acting_high
     assert abs(gas_rate - 0.0550) > 1e-6
 
 
+def test_hot_water_from_pcdb_heat_pump_bills_at_app_n_wh_high_rate() -> None:
+    # Arrange — when DHW is heated by the main heat pump (WHC 901/902/914
+    # "from main system") and that main carries a PCDB Table 362 record,
+    # SAP 10.2 Table 12a Grid 1 WH column (PDF p.191) bills it on the
+    # ASHP/GSHP-from-database row: 0.70 high-rate fraction at 7-hour and
+    # 10-hour. `_hot_water_fuel_cost_gbp_per_kwh` previously billed any
+    # electric off-peak HW at 100% low rate (its TODO), over-crediting the
+    # HP-DHW cat-4 cluster. Electric IMMERSION (WHC 903) is a different
+    # Table 12a row (off-peak immersion 0.17 / Table 13) and must stay on
+    # the 100%-low-rate fallback here.
+    from domain.sap10_calculator.tables.table_12a import Tariff
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _hot_water_fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    )
+    pcdb_heat_pump_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity (heat pump), API enum
+        heat_emitter_type=1,
+        emitter_temperature=0,
+        main_heating_control=2210,
+        main_heating_category=4,
+        sap_main_heating_code=None,
+        main_heating_index_number=104351,  # PCDB Table 362 heat pump
+    )
+
+    # Act — DHW from the main HP (WHC 901) vs a separate electric
+    # immersion (WHC 903), both on a 10-hour off-peak tariff.
+    rate_from_hp = _hot_water_fuel_cost_gbp_per_kwh(
+        29, pcdb_heat_pump_main, Tariff.TEN_HOUR, SAP_10_2_SPEC_PRICES,
+        water_heating_code=901,
+    )
+    rate_immersion = _hot_water_fuel_cost_gbp_per_kwh(
+        29, pcdb_heat_pump_main, Tariff.TEN_HOUR, SAP_10_2_SPEC_PRICES,
+        water_heating_code=903,
+    )
+
+    # Assert — HP-DHW: 0.70 × 14.68 p + 0.30 × 7.50 p = 12.526 p; immersion
+    # stays at the 10-hour low rate 7.50 p (£0.0750).
+    assert abs(rate_from_hp - 0.12526) <= 1e-6
+    assert abs(rate_immersion - 0.0750) <= 1e-6
+
+
 def test_space_heating_pcdb_heat_pump_without_sap_code_bills_at_app_n_high_rate() -> None:
     # Arrange — an API-path heat pump resolves via its PCDB Table 362
     # index alone (data_source=1, no Table-4a SAP code lodged), so