Slice 35: Plumb postcode climate through cert_to_inputs (demand cascade)

Adds an optional `postcode_climate: Optional[PostcodeClimate]` parameter
to every cert→inputs section helper that touches climate:
- `cert_to_inputs(epc, postcode_climate=...)`
- `ventilation_from_cert` (overrides UK-avg wind tuple)
- `mean_internal_temperature_section_from_cert`
- `space_heating_section_from_cert`
- `space_cooling_section_from_cert`
- `solar_gains_section_from_cert`
- `energy_requirements_section_from_cert`
- `fuel_cost_section_from_cert`
- `environmental_section_from_cert`

`_climate_source(postcode_climate)` returns `int | PostcodeClimate`
(region 0 = UK-avg fallback). The four Appendix U lookup functions
(`external_temperature_c`, `wind_speed_m_per_s`, `horizontal_solar_
irradiance_w_per_m2`, `_latitude_deg`) now accept the union and
dispatch on isinstance — region path is unchanged, postcode path reads
directly from `PostcodeClimate`.

CalculatorInputs gains `monthly_external_temp_c_override` so the
calculator's per-month solve uses the postcode tuple computed in
cert_to_inputs instead of looking up `external_temperature_c(region, m)`
(which would always be UK-avg).

Adds two public helpers:
- `local_climate_for_cert(epc)` — postcode lookup with None fallback
- `cert_to_demand_inputs(epc)` — convenience: cert_to_inputs with
  postcode climate from the cert's postcode field

Verification (000474 with postcode "bd3 8aq" injected — fixtures
currently lodge placeholder "A1 1AA"; real postcodes land in slice 36):
  Rating  main_1_fuel = 11964.8924  (PDF Block 1: 11964.8924 ✓)
  Demand  main_1_fuel = 12288.0014  (PDF Block 2: 12288.0014 ✓ EXACT)
  Rating  ext_temp Jan = 4.3°C (UK-avg)
  Demand  ext_temp Jan = 4.2°C (BD3)

840/840 existing pins still pass — refactor is backward-compatible.

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

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

@@ -172,6 +172,11 @@ class CalculatorInputs:
     hot_water_fuel_cost_gbp_per_kwh: float
     other_fuel_cost_gbp_per_kwh: float
     co2_factor_kg_per_kwh: float
+    # Pre-computed monthly external temperature (°C). When provided, the
+    # calculator's per-month solve uses this directly instead of looking up
+    # `external_temperature_c(region, month)`. Set by cert_to_inputs from
+    # either UK-average (rating cascade) or PCDB postcode (demand cascade).
+    monthly_external_temp_c_override: Optional[tuple[float, ...]] = None
     # Per-end-use effective CO2 factors. For electricity end-uses with
     # known monthly kWh distribution, cert_to_inputs computes the days-
     # weighted average Table 12d factor: Σ(kWh_m × CO2_m) / Σ(kWh_m). Gas
@@ -305,7 +310,11 @@ def _solve_month(
     time_constant_h: float,
     heat_loss_parameter: float,
 ) -> MonthlyEntry:
-    t_ext = external_temperature_c(inputs.region, month)
+    t_ext = (
+        inputs.monthly_external_temp_c_override[month - 1]
+        if inputs.monthly_external_temp_c_override is not None
+        else external_temperature_c(inputs.region, month)
+    )
     g_int = inputs.internal_gains_monthly_w[month - 1]
     g_sol = inputs.solar_gains_monthly_w[month - 1]
 

packages/domain/src/domain/sap/climate/appendix_u.py

@@ -20,6 +20,8 @@ from __future__ import annotations
 
 from typing import Final
 
+from domain.sap.tables.pcdb.postcode_weather import PostcodeClimate
+
 
 # Table U1 — Mean external temperature (°C), 22 regions × 12 months.
 # Row order: region 0 (UK average) first, then regions 1-21 in spec order.
@@ -94,16 +96,29 @@ def _validate(region: int, month: int) -> None:
     _validate_month(month)
 
 
-def external_temperature_c(region: int, month: int) -> float:
+def external_temperature_c(
-    """Mean external temperature (°C) for a SAP climate region in a month."""
+    region_or_climate: "int | PostcodeClimate", month: int
-    _validate(region, month)
+) -> float:
-    return _TABLE_U1[region][month - 1]
+    """Mean external temperature (°C) per month. Accepts either a SAP region
+    index (0..21) for the Appendix U fallback tables, or a `PostcodeClimate`
+    record for postcode-specific demand-cascade values from PCDB Table 172."""
+    if isinstance(region_or_climate, PostcodeClimate):
+        _validate_month(month)
+        return region_or_climate.monthly_external_temp_c[month - 1]
+    _validate(region_or_climate, month)
+    return _TABLE_U1[region_or_climate][month - 1]
 
 
-def wind_speed_m_per_s(region: int, month: int) -> float:
+def wind_speed_m_per_s(
-    """Mean wind speed (m/s) for a SAP climate region in a month."""
+    region_or_climate: "int | PostcodeClimate", month: int
-    _validate(region, month)
+) -> float:
-    return _TABLE_U2[region][month - 1]
+    """Mean wind speed (m/s) per month. Accepts either a SAP region index
+    (0..21) or a `PostcodeClimate` record."""
+    if isinstance(region_or_climate, PostcodeClimate):
+        _validate_month(month)
+        return region_or_climate.monthly_wind_speed_m_per_s[month - 1]
+    _validate(region_or_climate, month)
+    return _TABLE_U2[region_or_climate][month - 1]
 
 
 # Table U3 — Mean global solar irradiance on a horizontal plane (W/m²),
@@ -136,12 +151,18 @@ _TABLE_U3: Final[tuple[tuple[float, ...], ...]] = (
 )
 
 
-def horizontal_solar_irradiance_w_per_m2(region: int, month: int) -> float:
+def horizontal_solar_irradiance_w_per_m2(
-    """Mean global solar irradiance on a horizontal plane (W/m²) for a SAP
+    region_or_climate: "int | PostcodeClimate", month: int,
-    climate region in a month. The starting point for the per-orientation
+) -> float:
-    surface-flux calculation in SAP 10.2 §6.1."""
+    """Mean global solar irradiance on a horizontal plane (W/m²). Accepts
-    _validate(region, month)
+    either a SAP region index (0..21) or a `PostcodeClimate` record. The
-    return float(_TABLE_U3[region][month - 1])
+    starting point for the per-orientation surface-flux calculation in
+    SAP 10.2 §6.1."""
+    if isinstance(region_or_climate, PostcodeClimate):
+        _validate_month(month)
+        return region_or_climate.monthly_horizontal_solar_w_per_m2[month - 1]
+    _validate(region_or_climate, month)
+    return float(_TABLE_U3[region_or_climate][month - 1])
 
 
 # Table U3 footer — Solar declination (°), region-independent (function of

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

@@ -55,6 +55,10 @@ from domain.ml.sap_efficiencies import (
 from domain.sap.calculator import CalculatorInputs
 from domain.sap.tables.pcdb import gas_oil_boiler_record
 from domain.sap.tables.pcdb.parser import GasOilBoilerRecord
+from domain.sap.tables.pcdb.postcode_weather import (
+    PostcodeClimate,
+    postcode_climate,
+)
 from domain.sap.tables.table_12 import (
     co2_monthly_factors_kg_per_kwh,
     co2_factor_kg_per_kwh,
@@ -340,21 +344,24 @@ def _dwelling_exposure(dwelling_type: Optional[str]) -> DwellingExposure:
 
 
 def _region_index(region_code: Optional[str]) -> int:
-    """SAP rating must be computed with UK-average weather per Appendix U:
+    """SAP rating must be computed with UK-average weather per Appendix U
-    "Calculations for fabric energy efficiency (FEE), regulation compliance
+    (p.124). Always returns region 0 (UK average); the demand cascade
-    (TER and DER, TPER and DPER) and for ratings (SAP rating and environmental
+    (Current Carbon / Current Primary Energy / Fuel Bill) uses the
-    impact rating) are done with UK average weather. Other calculations (such
+    `postcode_climate` parameter on `cert_to_inputs` instead — see
-    as for energy use and costs on EPCs) are done using local weather."
+    `cert_to_demand_inputs`."""
-
-    Since our calculator's primary output is the SAP rating, we always return
-    region 0 (UK average) regardless of the cert's actual region_code. A
-    future slice may add a `compute_local_weather` flag to also produce the
-    energy-use kWh totals at local weather.
-    """
     _ = region_code
     return 0
 
 
+def _climate_source(
+    postcode_climate_override: Optional[PostcodeClimate],
+) -> "int | PostcodeClimate":
+    """Pick the climate source for downstream lookups. None → region 0
+    (UK-average, ratings cascade); a `PostcodeClimate` → postcode-district
+    PCDB Table 172 data (demand cascade)."""
+    return postcode_climate_override if postcode_climate_override is not None else 0
+
+
 def _is_timber_or_steel_frame(parts: list[SapBuildingPart]) -> bool:
     """RdSAP 10 §5: wall_construction codes 5 (timber frame) and 6 (system
     build steel frame) get the lower 0.25 structural ACH; everything else
@@ -953,6 +960,8 @@ def _roof_windows_for_solar_gains(
 
 def mean_internal_temperature_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[MeanInternalTemperatureResult]:
     """SAP 10.2 §7 cert→inputs cascade for `mean_internal_temperature_monthly`.
 
@@ -968,10 +977,10 @@ def mean_internal_temperature_section_from_cert(
     if epc.total_floor_area_m2 is None:
         return None
     dim = dimensions_from_cert(epc)
-    ventilation = ventilation_from_cert(epc)
+    ventilation = ventilation_from_cert(epc, postcode_climate=postcode_climate)
     ht = heat_transmission_section_from_cert(epc)
     ig = internal_gains_section_from_cert(epc)
-    sg = solar_gains_section_from_cert(epc)
+    sg = solar_gains_section_from_cert(epc, postcode_climate=postcode_climate)
     assert ig is not None, "internal_gains None despite TFA present"
     internal_gains_monthly_w = ig.total_internal_gains_monthly_w
     solar_gains_monthly_w = sg.total_solar_gains_monthly_w
@@ -983,10 +992,10 @@ def mean_internal_temperature_section_from_cert(
         for m in range(12)
     )
     main = _first_main_heating(epc)
-    region = _region_index(epc.region_code)
+    climate = _climate_source(postcode_climate)
     return mean_internal_temperature_monthly(
         monthly_external_temp_c=tuple(
-            external_temperature_c(region, m) for m in range(1, 13)
+            external_temperature_c(climate, m) for m in range(1, 13)
         ),
         monthly_total_gains_w=monthly_total_gains_w,
         monthly_heat_transfer_coefficient_w_per_k=monthly_htc_w_per_k,
@@ -1003,6 +1012,8 @@ def mean_internal_temperature_section_from_cert(
 
 def space_heating_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[SpaceHeatingResult]:
     """SAP 10.2 §8 cert→inputs cascade for `space_heating_monthly_kwh`.
 
@@ -1012,16 +1023,21 @@ def space_heating_section_from_cert(
     (95)..(99) line ref) so cascade pin tests can assert each §8 line
     ref against the U985 PDF.
 
+    `postcode_climate` selects the demand cascade (postcode wind/temp/solar
+    via PCDB Table 172); None uses UK-average rating climate.
+
     Returns `None` when TFA is missing (matches other section helpers).
     """
     if epc.total_floor_area_m2 is None:
         return None
     dim = dimensions_from_cert(epc)
-    ventilation = ventilation_from_cert(epc)
+    ventilation = ventilation_from_cert(epc, postcode_climate=postcode_climate)
     ht = heat_transmission_section_from_cert(epc)
     ig = internal_gains_section_from_cert(epc)
-    sg = solar_gains_section_from_cert(epc)
+    sg = solar_gains_section_from_cert(epc, postcode_climate=postcode_climate)
-    mit = mean_internal_temperature_section_from_cert(epc)
+    mit = mean_internal_temperature_section_from_cert(
+        epc, postcode_climate=postcode_climate
+    )
     assert ig is not None, "internal_gains None despite TFA present"
     assert mit is not None, "mit None despite TFA present"
     monthly_total_gains_w = tuple(
@@ -1032,9 +1048,9 @@ def space_heating_section_from_cert(
         ht.total_w_per_k + 0.33 * dim.volume_m3 * ventilation.effective_monthly_ach[m]
         for m in range(12)
     )
-    region = _region_index(epc.region_code)
+    climate = _climate_source(postcode_climate)
     monthly_external_temp_c = tuple(
-        external_temperature_c(region, m) for m in range(1, 13)
+        external_temperature_c(climate, m) for m in range(1, 13)
     )
     return space_heating_monthly_kwh(
         monthly_heat_transfer_coefficient_w_per_k=monthly_htc_w_per_k,
@@ -1048,6 +1064,8 @@ def space_heating_section_from_cert(
 
 def space_cooling_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[SpaceCoolingResult]:
     """SAP 10.2 §8c cert→inputs cascade for `space_cooling_monthly_kwh`.
 
@@ -1058,20 +1076,22 @@ def space_cooling_section_from_cert(
     full `SpaceCoolingResult` (every (100)..(108) line ref) so cascade pin
     tests can assert each §8c line ref against the U985 PDF.
 
+    `postcode_climate` selects the demand cascade; None uses UK-average.
+
     Returns `None` when TFA is missing (matches other section helpers).
     """
     if epc.total_floor_area_m2 is None:
         return None
     dim = dimensions_from_cert(epc)
-    ventilation = ventilation_from_cert(epc)
+    ventilation = ventilation_from_cert(epc, postcode_climate=postcode_climate)
     ht = heat_transmission_section_from_cert(epc)
     monthly_htc_w_per_k = tuple(
         ht.total_w_per_k + 0.33 * dim.volume_m3 * ventilation.effective_monthly_ach[m]
         for m in range(12)
     )
-    region = _region_index(epc.region_code)
+    climate = _climate_source(postcode_climate)
     monthly_external_temp_c = tuple(
-        external_temperature_c(region, m) for m in range(1, 13)
+        external_temperature_c(climate, m) for m in range(1, 13)
     )
     return space_cooling_monthly_kwh(
         monthly_heat_transfer_coefficient_w_per_k=monthly_htc_w_per_k,
@@ -1139,16 +1159,23 @@ class EnvironmentalSection:
 
 def environmental_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[EnvironmentalSection]:
     """SAP 10.2 §12 cert→inputs cascade. Composes §9a per-system fuel kWh +
     §4 water heating + §5 lighting + Table 12d monthly electricity CO2 +
     Table 12 annual fuel CO2 into per-end-use CO2 line refs.
 
+    `postcode_climate` selects the demand cascade (postcode climate via
+    PCDB Table 172 — used for EPC Current Carbon); None uses UK-average.
+
     Returns None when TFA missing."""
     if epc.total_floor_area_m2 is None:
         return None
     dim = dimensions_from_cert(epc)
-    er = energy_requirements_section_from_cert(epc)
+    er = energy_requirements_section_from_cert(
+        epc, postcode_climate=postcode_climate,
+    )
     assert er is not None, "energy_requirements None despite TFA present"
 
     main = _first_main_heating(epc)
@@ -1169,7 +1196,7 @@ def environmental_section_from_cert(
     )
     water_co2 = er.main_1_fuel_kwh_per_yr  # placeholder, replaced below
     # Hot water kWh: derived from wh_result. Recompute via cert_to_inputs path.
-    full_inputs = cert_to_inputs(epc)
+    full_inputs = cert_to_inputs(epc, postcode_climate=postcode_climate)
     water_co2 = full_inputs.hot_water_kwh_per_yr * water_factor
 
     # Electric shower (264a) — distinct line ref when present.
@@ -1248,32 +1275,39 @@ def sap_rating_section_from_cert(
 
 def fuel_cost_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[FuelCostResult]:
     """SAP 10.2 §10a cert→inputs cascade for `fuel_cost`. Off-peak certs
     return the zero sentinel (Table 12a high-rate-fraction split deferred).
     For STANDARD-tariff certs returns the full (240)..(255) FuelCostResult.
 
     Composes via `cert_to_inputs(epc)` — `_fuel_cost` is invoked there with
-    all upstream §4/§5/§6/§7/§8/§9a values plumbed in. Returns None when
+    all upstream §4/§5/§6/§7/§8/§9a values plumbed in. `postcode_climate`
+    selects the demand cascade (EPC Fuel Bill). Returns None when
     TFA missing.
     """
     if epc.total_floor_area_m2 is None:
         return None
-    return cert_to_inputs(epc).fuel_cost
+    return cert_to_inputs(epc, postcode_climate=postcode_climate).fuel_cost
 
 
 def energy_requirements_section_from_cert(
     epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> Optional[EnergyRequirementsResult]:
     """SAP 10.2 §9a cert→inputs cascade for `space_heating_fuel_monthly_kwh`.
 
     Composes §8 (98c)m + Table 11 secondary fraction + per-system
     efficiencies into the (201)..(221) line refs. Single-main scope A
-    (no (203)/(207)/(213)/(209)/(221)). Returns None when TFA missing.
+    (no (203)/(207)/(213)/(209)/(221)). `postcode_climate` selects the
+    demand cascade (Current Carbon / Current PE on EPC); None uses UK-avg.
+    Returns None when TFA missing.
     """
     if epc.total_floor_area_m2 is None:
         return None
-    sh = space_heating_section_from_cert(epc)
+    sh = space_heating_section_from_cert(epc, postcode_climate=postcode_climate)
     assert sh is not None, "space_heating None despite TFA present"
     main = _first_main_heating(epc)
     main_code = main.sap_main_heating_code if main is not None else None
@@ -1298,28 +1332,39 @@ def energy_requirements_section_from_cert(
     )
 
 
-def solar_gains_section_from_cert(epc: EpcPropertyData) -> SolarGainsResult:
+def solar_gains_section_from_cert(
+    epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
+) -> SolarGainsResult:
     """SAP 10.2 §6 cert→inputs cascade for `solar_gains_from_cert`.
 
     Returns the full `SolarGainsResult` (every (74)..(83) per-orientation
     line ref + (82)/(82a) roof-window/rooflight monthly tuples) computed
     from the cert's `sap_windows` (vertical wall windows) and
     `sap_roof_windows` (pitched roof windows for line (82)) at default
-    AVERAGE overshading and UK-average region (matches cert_to_inputs'
+    AVERAGE overshading.
-    internal cascade for the SAP-rating pass).
+
+    `postcode_climate` selects the demand cascade (postcode horizontal
+    solar irradiance + latitude via PCDB Table 172); None uses UK-average
+    region 0 — the SAP-rating pass.
 
     Rooflights (horizontal Z=1.0 glazing) are not yet lodged on the cert
     datatype distinct from roof windows — they pass through as empty.
     """
     return solar_gains_from_cert(
         epc=epc,
-        region=_region_index(epc.region_code),
+        region=_climate_source(postcode_climate),
         overshading=_INTERNAL_GAINS_DEFAULT_OVERSHADING,
         roof_windows=_roof_windows_for_solar_gains(epc),
     )
 
 
-def ventilation_from_cert(epc: EpcPropertyData) -> VentilationResult:
+def ventilation_from_cert(
+    epc: EpcPropertyData,
+    *,
+    postcode_climate: Optional[PostcodeClimate] = None,
+) -> VentilationResult:
     """SAP 10.2 §2 cert→inputs cascade for `ventilation_from_inputs`.
 
     Reads dimensions + sap_ventilation lodgement from `epc` and produces
@@ -1327,6 +1372,10 @@ def ventilation_from_cert(epc: EpcPropertyData) -> VentilationResult:
     exact same call cert_to_inputs makes internally. Exposed so cascade
     pin tests can assert every §2 line ref against the U985 PDF.
 
+    `postcode_climate` overrides the UK-average wind tuple (Table U2 row 0)
+    with PCDB Table 172 postcode-district wind for the demand cascade
+    (Current Carbon / Current Primary Energy on the EPC).
+
     Defaults track the same conventions as cert_to_inputs (sheltered
     sides → 2 when missing, MV kind → NATURAL until cert→MV mapping is
     documented).
@@ -1336,6 +1385,10 @@ def ventilation_from_cert(epc: EpcPropertyData) -> VentilationResult:
     storeys = max(1, dim.storey_count)
     vc = _ventilation_counts(epc.sap_ventilation)
     sv = epc.sap_ventilation
+    wind_kwargs: dict[str, tuple[float, ...]] = (
+        {"monthly_wind_speed_m_s": postcode_climate.monthly_wind_speed_m_per_s}
+        if postcode_climate is not None else {}
+    )
     return ventilation_from_inputs(
         volume_m3=vol,
         storey_count=storeys,
@@ -1355,6 +1408,7 @@ def ventilation_from_cert(epc: EpcPropertyData) -> VentilationResult:
         window_pct_draught_proofed=float(epc.percent_draughtproofed or 0),
         sheltered_sides=int(sv.sheltered_sides) if sv is not None and sv.sheltered_sides is not None else 2,
         mv_kind=MechanicalVentilationKind.NATURAL,
+        **wind_kwargs,
     )
 
 
@@ -1702,7 +1756,10 @@ def _fuel_cost(
 
 
 def cert_to_inputs(
-    epc: EpcPropertyData, *, prices: PriceTable = SAP_10_2_SPEC_PRICES
+    epc: EpcPropertyData,
+    *,
+    prices: PriceTable = SAP_10_2_SPEC_PRICES,
+    postcode_climate: Optional[PostcodeClimate] = None,
 ) -> CalculatorInputs:
     """Build a typed `CalculatorInputs` aggregate from an `EpcPropertyData`.
 
@@ -1717,7 +1774,7 @@ def cert_to_inputs(
     # SAP §3 heat transmission + §2 ventilation cascades — see the
     # respective `_from_cert` helpers for cert→inputs mapping rules.
     ht = heat_transmission_section_from_cert(epc)
-    ventilation = ventilation_from_cert(epc)
+    ventilation = ventilation_from_cert(epc, postcode_climate=postcode_climate)
 
     main = _first_main_heating(epc)
     main_code = main.sap_main_heating_code if main is not None else None
@@ -1818,9 +1875,10 @@ def cert_to_inputs(
             )
         )
 
+    climate: "int | PostcodeClimate" = _climate_source(postcode_climate)
     solar_gains_monthly_w = solar_gains_from_cert(
         epc=epc,
-        region=_region_index(epc.region_code),
+        region=climate,
         overshading=_INTERNAL_GAINS_DEFAULT_OVERSHADING,
         roof_windows=_roof_windows_for_solar_gains(epc),
     ).total_solar_gains_monthly_w
@@ -1842,7 +1900,7 @@ def cert_to_inputs(
     )
     mit_result = mean_internal_temperature_monthly(
         monthly_external_temp_c=tuple(
-            external_temperature_c(_region_index(epc.region_code), m)
+            external_temperature_c(climate, m)
             for m in range(1, 13)
         ),
         monthly_total_gains_w=monthly_total_gains_w,
@@ -1859,8 +1917,7 @@ def cert_to_inputs(
     # HTC + total-gains tuples already computed for §7 and adds T_int + η
     # from the MIT result. Includes the Table 9c step 10 summer clamp.
     monthly_external_temp_c = tuple(
-        external_temperature_c(_region_index(epc.region_code), m)
+        external_temperature_c(climate, m) for m in range(1, 13)
-        for m in range(1, 13)
     )
     space_heating_result = space_heating_monthly_kwh(
         monthly_heat_transfer_coefficient_w_per_k=monthly_htc_w_per_k,
@@ -1978,6 +2035,7 @@ def cert_to_inputs(
         # SAP10.2 (109) — Fabric Energy Efficiency precomputed above.
         fabric_energy_efficiency_kwh_per_m2_yr=fee_kwh_per_m2,
         region=_region_index(epc.region_code),
+        monthly_external_temp_c_override=monthly_external_temp_c,
         control_type=control_type_value,
         responsiveness=responsiveness_value,
         living_area_fraction=living_area_fraction_value,
@@ -2074,3 +2132,31 @@ def cert_to_inputs(
             cooling_kwh=energy_requirements_result.cooling_fuel_kwh_per_yr,
         ),
     )
+
+
+def local_climate_for_cert(epc: EpcPropertyData) -> Optional[PostcodeClimate]:
+    """Per SAP 10.2 Appendix U (p.124), the demand cascade (Current Carbon,
+    Current Primary Energy, Fuel Bill on the EPC) uses postcode-specific
+    weather data from PCDB Table 172. Returns the PostcodeClimate for the
+    cert's lodged postcode, or None when the postcode is missing or not in
+    Table 172 (callers fall back to UK-average / cert_to_inputs default).
+    """
+    return postcode_climate(epc.postcode)
+
+
+def cert_to_demand_inputs(
+    epc: EpcPropertyData, *, prices: PriceTable = SAP_10_2_SPEC_PRICES,
+) -> CalculatorInputs:
+    """Demand-cascade variant of cert_to_inputs (postcode climate from PCDB
+    Table 172). Used for EPC-displayed Current Carbon / Current Primary
+    Energy / Fuel Bill. Falls back to UK-average climate when the cert's
+    postcode is missing or absent from Table 172.
+
+    Reference: SAP 10.2 Appendix U paragraph 1 (p.124) — "Other
+    calculations (such as for energy use and costs on EPCs) are done using
+    local weather. Weather data for each postcode district are taken from
+    the PCDB and are used when the postcode district is known".
+    """
+    return cert_to_inputs(
+        epc, prices=prices, postcode_climate=local_climate_for_cert(epc),
+    )

packages/domain/src/domain/sap/worksheet/solar_gains.py

@@ -35,6 +35,7 @@ from math import cos, radians, sin
 from typing import Final
 
 from datatypes.epc.domain.epc_property_data import EpcPropertyData, SapWindow
+from domain.sap.tables.pcdb.postcode_weather import PostcodeClimate
 from domain.sap.climate.appendix_u import (
     horizontal_solar_irradiance_w_per_m2,
     solar_declination_deg,
@@ -101,23 +102,29 @@ _ORIENTATION_TO_K: Final[dict[Orientation, tuple[float, ...]]] = {
 }
 
 
-def _latitude_deg(region: int) -> float:
+def _latitude_deg(region_or_climate: "int | PostcodeClimate") -> float:
-    if not 0 <= region < len(_LATITUDE_DEG):
+    if isinstance(region_or_climate, PostcodeClimate):
-        raise ValueError(f"region must be 0..{len(_LATITUDE_DEG) - 1}, got {region}")
+        return region_or_climate.latitude_deg
-    return _LATITUDE_DEG[region]
+    if not 0 <= region_or_climate < len(_LATITUDE_DEG):
+        raise ValueError(
+            f"region must be 0..{len(_LATITUDE_DEG) - 1}, got {region_or_climate}"
+        )
+    return _LATITUDE_DEG[region_or_climate]
 
 
 def surface_solar_flux_w_per_m2(
     *,
     orientation: Orientation,
     pitch_deg: float,
-    region: int,
+    region: "int | PostcodeClimate",
     month: int,
 ) -> float:
     """Per-orientation per-pitch monthly solar flux on a surface (W/m²).
 
     SAP 10.2 Appendix U §U3.2 polynomial conversion from the horizontal
-    irradiance in Table U3 to any orientation/tilt combination.
+    irradiance in Table U3 to any orientation/tilt combination. Accepts
+    either a SAP region index (0..21) or a `PostcodeClimate` record from
+    PCDB Table 172 (demand cascade).
     """
     s_h = horizontal_solar_irradiance_w_per_m2(region, month)
     declination = solar_declination_deg(month)
@@ -292,7 +299,7 @@ def _vertical_window_gain_monthly_w(
     *,
     w: SapWindow,
     orientation: Orientation,
-    region: int,
+    region: "int | PostcodeClimate",
     z_solar: float,
 ) -> tuple[float, ...]:
     """Compute the 12-tuple of monthly solar gain (W) for one vertical wall
@@ -324,7 +331,7 @@ def _sum_tuples(*tuples: tuple[float, ...]) -> tuple[float, ...]:
 def solar_gains_from_cert(
     *,
     epc: EpcPropertyData,
-    region: int,
+    region: "int | PostcodeClimate",
     overshading: OvershadingCategory = OvershadingCategory.AVERAGE,
     roof_windows: tuple[RoofWindowInput, ...] = (),
     rooflights: tuple[RooflightInput, ...] = (),