Slice 45c: PV demand cascade uses postcode-specific climate (PCDB Table 172) per Appendix U

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

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

@@ -211,18 +211,18 @@ _PV_PITCH_DEG_DEFAULT: Final[float] = 30.0  # RdSAP10 §11.1 default
 _HOURS_PER_DAY_OVER_1000: Final[float] = 0.024
 _DAYS_PER_MONTH: Final[tuple[int, ...]] = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
 
-# SAP 10.2 Appendix U Table U4 row 0 = UK-average climate region. Rating
-# cascade uses UK average per Appendix U; demand cascade will switch to
-# postcode-specific climate in a follow-on slice.
-_PV_RATING_REGION: Final[int] = 0
-
-
-def _pv_annual_s_kwh_per_m2(orientation_code: int, pitch_code: int) -> float:
+def _pv_annual_s_kwh_per_m2(
+    orientation_code: int,
+    pitch_code: int,
+    climate: "int | PostcodeClimate",
+) -> float:
     """SAP 10.2 Appendix U3.3 equation (U4): annual solar radiation
-    (kWh/m²/yr) on a surface of given orientation and tilt under UK-
-    average climate. Sums the monthly Appendix U3.2 surface flux over
-    the year. Returns 0.0 for unrecognised orientation codes (cert
-    octants outside 1..8) — these PV arrays contribute nothing."""
+    (kWh/m²/yr) on a surface of given orientation and tilt. Sums the
+    monthly Appendix U3.2 surface flux over the year. `climate` selects
+    Table U3/U4 region (UK average = 0 for the rating cascade) or a
+    `PostcodeClimate` from PCDB Table 172 for the demand cascade.
+    Returns 0.0 for unrecognised orientation codes (cert octants outside
+    1..8) — these PV arrays contribute nothing."""
     orientation = ORIENTATION_BY_SAP10_CODE.get(orientation_code)
     if orientation is None:
         return 0.0
@@ -232,7 +232,7 @@ def _pv_annual_s_kwh_per_m2(orientation_code: int, pitch_code: int) -> float:
         s_m = surface_solar_flux_w_per_m2(
             orientation=orientation,
             pitch_deg=pitch_deg,
-            region=_PV_RATING_REGION,
+            region=climate,
             month=month_idx + 1,
         )
         total += days * s_m
@@ -746,27 +746,36 @@ def _secondary_fuel_cost_gbp_per_kwh(
     return prices.unit_price_p_per_kwh(sec_fuel) * _PENCE_TO_GBP
 
 
-def _pv_array_generation_kwh_per_yr(array: PhotovoltaicArray) -> float:
+def _pv_array_generation_kwh_per_yr(
+    array: PhotovoltaicArray,
+    climate: "int | PostcodeClimate",
+) -> float:
     """SAP 10.2 Appendix M (M1) for a single array: EPV = 0.8 × kWp × S × ZPV.
     S is the Appendix U3.3 annual solar radiation for the array's
-    orientation and tilt under UK-average climate; ZPV is the Table M1
+    orientation and tilt under `climate` (UK average region 0 for ratings,
+    PCDB Table 172 PostcodeClimate for demand); ZPV is the Table M1
     overshading factor. Arrays with missing peak power contribute zero."""
     if array.peak_power is None:
         return 0.0
-    s = _pv_annual_s_kwh_per_m2(array.orientation, array.pitch)
+    s = _pv_annual_s_kwh_per_m2(array.orientation, array.pitch, climate)
     z = _PV_OVERSHADING_FACTOR.get(array.overshading, 1.0)
     return _PV_MODULE_EFFICIENCY_FACTOR * array.peak_power * s * z
 
 
-def _pv_generation_kwh_per_yr(epc: EpcPropertyData) -> float:
+def _pv_generation_kwh_per_yr(
+    epc: EpcPropertyData,
+    climate: "int | PostcodeClimate",
+) -> float:
     """Annual PV generation (kWh/yr) summed per-array. Per SAP 10.2
     Appendix M §M1: "If there is more than one PV array … apply this
     process to each and sum the monthly electricity generation figures."
+    `climate` selects UK-average (region 0) for the rating cascade or
+    postcode-specific (PCDB Table 172) for the demand cascade.
     """
     arrays = epc.sap_energy_source.photovoltaic_arrays
     if not arrays:
         return 0.0
-    return sum(_pv_array_generation_kwh_per_yr(a) for a in arrays)
+    return sum(_pv_array_generation_kwh_per_yr(a, climate) for a in arrays)
 
 
 def _pv_export_credit_gbp_per_kwh() -> float:
@@ -1828,6 +1837,7 @@ def _fuel_cost(
     pumps_fans_kwh: float,
     lighting_kwh: float,
     cooling_kwh: float,
+    climate: "int | PostcodeClimate",
     electric_shower_kwh: float = 0.0,
 ) -> FuelCostResult:
     """SAP10.2 §10a fuel-cost precompute — produce a `FuelCostResult` from
@@ -1913,7 +1923,7 @@ def _fuel_cost(
         other_uses_gbp_per_kwh=other_uses_gbp_per_kwh,
         instant_shower_kwh_per_yr=electric_shower_kwh,
         instant_shower_gbp_per_kwh=other_uses_gbp_per_kwh,
-        pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc),
+        pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc, climate),
         pv_export_credit_gbp_per_kwh=pv_export_credit_gbp_per_kwh,
         additional_standing_charges_gbp=standing,
         appendix_q_saved_gbp=0.0,
@@ -2252,7 +2262,7 @@ def cert_to_inputs(
             wh_result.electric_shower_monthly_kwh if wh_result is not None else (0.0,) * 12,
             _STANDARD_ELECTRICITY_FUEL_CODE,
         ),
-        pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc),
+        pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc, climate),
         pv_export_credit_gbp_per_kwh=_pv_export_credit_gbp_per_kwh(),
         secondary_heating_fraction=secondary_fraction_value,
         secondary_heating_efficiency=secondary_efficiency_value,
@@ -2296,6 +2306,7 @@ def cert_to_inputs(
             pumps_fans_kwh=pumps_fans_kwh,
             lighting_kwh=lighting_kwh,
             cooling_kwh=energy_requirements_result.cooling_fuel_kwh_per_yr,
+            climate=climate,
         ),
     )
 

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

@@ -28,7 +28,11 @@ from domain.ml.tests._fixtures import (
     make_window,
 )
 from domain.sap.calculator import Sap10Calculator, SapResult
-from domain.sap.rdsap.cert_to_inputs import pcdb_combi_loss_override, cert_to_inputs
+from domain.sap.rdsap.cert_to_inputs import (
+    cert_to_demand_inputs,
+    cert_to_inputs,
+    pcdb_combi_loss_override,
+)
 from domain.sap.tables.pcdb import GasOilBoilerRecord, gas_oil_boiler_record
 from domain.sap.worksheet.tests import _elmhurst_worksheet_000477 as _w000477
 from domain.sap.worksheet.water_heating import (
@@ -462,6 +466,27 @@ def test_pv_generation_differentiates_arrays_by_pitch() -> None:
     assert tilted_kwh > vertical_kwh
 
 
+def test_pv_generation_uses_postcode_climate_in_demand_cascade() -> None:
+    # Arrange — SAP 10.2 Appendix U: rating cascade uses UK-average
+    # climate (region 0); demand cascade uses postcode-specific climate
+    # from PCDB Table 172 ("Other calculations… are done using local
+    # weather"). PV generation depends on monthly Sh via Appendix U3.3,
+    # so a cert whose postcode resolves to a non-UK-avg region must
+    # produce different annual PV generation under the two cascades.
+    epc = _typical_semi_detached_epc()
+    epc.postcode = "DE22"  # PCDB Table 172 — Midlands, region 6
+    epc.sap_energy_source.photovoltaic_arrays = [
+        PhotovoltaicArray(peak_power=2.0, pitch=2, orientation=5, overshading=1),
+    ]
+
+    # Act
+    rating_gen = cert_to_inputs(epc).pv_generation_kwh_per_yr
+    demand_gen = cert_to_demand_inputs(epc).pv_generation_kwh_per_yr
+
+    # Assert
+    assert rating_gen != demand_gen
+
+
 def test_open_chimneys_raise_infiltration_ach() -> None:
     # Arrange — Direction check: chimneys add Table 2.1 volume to the
     # infiltration calc, so an otherwise identical dwelling with 2 open

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

@@ -139,18 +139,20 @@ _EXPECTATIONS: tuple[_GoldenExpectation, ...] = (
         cert_number="2130-1033-4050-5007-8395",
         actual_sap=82,
         expected_sap_resid=+3,
-        expected_pe_resid_kwh_per_m2=-51.0719,
+        expected_pe_resid_kwh_per_m2=-51.9024,
         expected_co2_resid_tonnes_per_yr=+0.1422,
         notes=(
             "End-terrace + 1 extension, TFA 64, gas combi PCDB index 17505, "
             "postcode DE22 (PCDB Table 172 match), PV: 2× 2.04 kWp arrays "
-            "(SE + NW, overshading 1 + 2). Slice 45a/b applied SAP10.2 "
-            "Appendix M per-array yield with the real Appendix U3.3 "
-            "S(orient, p) integral + Table M1 ZPV — pulled SAP residual "
-            "+9 → +3 and PE residual −69.57 → −51.07 vs the prior lump-"
-            "sum 850 × total_kWp. Remaining drift: demand cascade still "
-            "uses UK-average climate for S; switching to postcode-specific "
-            "climate (PCDB Table 172) lands in a follow-on slice."
+            "(SE + NW, overshading 1 + 2). Slices 45a/b/c implement SAP10.2 "
+            "Appendix M per-array yield with the real Appendix U3.3 S(orient, "
+            "p) integral + Table M1 ZPV, and split rating (UK-avg climate) "
+            "from demand (DE22 PCDB Table 172 climate). Net effect: SAP "
+            "residual +9 → +3, PE residual −69.57 → −51.90 vs the prior "
+            "lump-sum 850 × total_kWp. The remaining −51.90 PE drift sits "
+            "outside the PV cascade — candidates include the dwelling-use "
+            "vs export β-factor split (Appendix M §3) and the secondary "
+            "heating credit, both untouched so far."
         ),
     ),
     _GoldenExpectation(