§6 slice 5: CalculatorInputs.solar_gains_monthly_w + per-month index lookup

Adds the §6 (83)m output as a required 12-tuple field on CalculatorInputs;
_solve_month indexes into it directly instead of recomputing solar each
month via _solar_gains_w(windows, region, month).

Test (test_calculator_consumes_solar_gains_monthly_w_field_for_per_month_solar)
pins the read path: an explicit non-zero monthly tuple flows through
calculate_sap_from_inputs unchanged.

cert_to_inputs preserves identical behaviour during the migration by
computing the new field via the legacy _solar_gains_w leaf per month.
Slice 6 swaps that for solar_gains_from_cert; slice 7 deletes the legacy
leaf + WindowInput + windows field.

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

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

@@ -105,6 +105,10 @@ class CalculatorInputs:
     # zero out in summer per Table 5a. Produced by §5 orchestrator
     # `internal_gains_from_cert` (called from cert_to_inputs).
     internal_gains_monthly_w: tuple[float, ...]
+    # SAP10.2 (83)m — total solar gains W per month (Jan..Dec). Produced
+    # by §6 orchestrator `solar_gains_from_cert` upstream; the calculator
+    # only indexes into it per month, no recomputation here.
+    solar_gains_monthly_w: tuple[float, ...]
     region: int
     windows: tuple[WindowInput, ...]
     control_type: int
@@ -218,7 +222,7 @@ def _solve_month(
 ) -> MonthlyEntry:
     t_ext = external_temperature_c(inputs.region, month)
     g_int = inputs.internal_gains_monthly_w[month - 1]
-    g_sol = _solar_gains_w(windows=inputs.windows, region=inputs.region, month=month)
+    g_sol = inputs.solar_gains_monthly_w[month - 1]
     g_total = g_int + g_sol
 
     # SAP 10.3 §7.3: two-pass iteration. Seed η = 1, compute T_internal,

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

@@ -52,7 +52,7 @@ from domain.ml.sap_efficiencies import (
     seasonal_efficiency,
     water_heating_efficiency as _legacy_water_heating_efficiency,
 )
-from domain.sap.calculator import CalculatorInputs, WindowInput
+from domain.sap.calculator import CalculatorInputs, WindowInput, _solar_gains_w
 from domain.sap.tables.table_12 import (
     co2_factor_kg_per_kwh,
     primary_energy_factor,
@@ -998,6 +998,18 @@ def cert_to_inputs(
         # SAP10.2 line (73)m — total internal gains W/month from §5
         # orchestrator (composed above).
         internal_gains_monthly_w=internal_gains_monthly_w,
+        # SAP10.2 line (83)m — total solar gains W/month. Computed here via
+        # the legacy `_solar_gains_w` per-month leaf to preserve identical
+        # behaviour during the §6 wiring migration; the next slice swaps
+        # this for the §6 orchestrator `solar_gains_from_cert`.
+        solar_gains_monthly_w=tuple(
+            _solar_gains_w(
+                windows=_window_inputs(epc.sap_windows),
+                region=_region_index(epc.region_code),
+                month=m,
+            )
+            for m in range(1, 13)
+        ),
         region=_region_index(epc.region_code),
         windows=_window_inputs(epc.sap_windows),
         control_type=_control_type(main),

packages/domain/src/domain/sap/tests/test_bre_worked_examples.py

@@ -26,6 +26,7 @@ import pytest
 from domain.sap.calculator import (
     CalculatorInputs,
     WindowInput,
+    _solar_gains_w,
     calculate_sap_from_inputs,
 )
 from domain.sap.worksheet.dimensions import Dimensions
@@ -85,6 +86,9 @@ def _baseline_dwelling() -> CalculatorInputs:
         heat_transmission=ht,
         monthly_infiltration_ach=(0.7,) * 12,
         internal_gains_monthly_w=(450.0,) * 12,
+        solar_gains_monthly_w=tuple(
+            _solar_gains_w(windows=windows, region=0, month=m) for m in range(1, 13)
+        ),
         region=0,
         windows=windows,
         control_type=2,

packages/domain/src/domain/sap/tests/test_calculator.py

@@ -24,6 +24,7 @@ from domain.sap.calculator import (
     CalculatorInputs,
     SapResult,
     WindowInput,
+    _solar_gains_w,
     calculate_sap_from_inputs,
 )
 from domain.sap.worksheet.dimensions import Dimensions
@@ -84,6 +85,9 @@ def _baseline_inputs() -> CalculatorInputs:
         # per-month variation lives in §5 orchestrator output; tracer
         # tests don't need the modulation to verify the SAP loop.
         internal_gains_monthly_w=(450.0,) * 12,
+        solar_gains_monthly_w=tuple(
+            _solar_gains_w(windows=windows, region=0, month=m) for m in range(1, 13)
+        ),
         region=0,
         windows=windows,
         control_type=2,
@@ -102,6 +106,25 @@ def _baseline_inputs() -> CalculatorInputs:
     )
 
 
+def test_calculator_consumes_solar_gains_monthly_w_field_for_per_month_solar() -> None:
+    # Arrange — replace the baseline inputs' solar with an explicit known
+    # 12-tuple. The §6 orchestrator produces this upstream; the calculator
+    # must just look it up, not recompute from the legacy `windows` field.
+    # 100 W constant solar everywhere — distinct enough that any leftover
+    # _solar_gains_w(windows, ...) recomputation would land elsewhere.
+    explicit_solar = (100.0,) * 12
+    inputs = replace(
+        _baseline_inputs(), solar_gains_monthly_w=explicit_solar,
+    )
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    for monthly in result.monthly:
+        assert monthly.solar_gains_w == 100.0
+
+
 def test_calculator_returns_twelve_month_breakdown_and_plausible_sap_score() -> None:
     # Arrange — baseline 100 m² gas-boiler dwelling in UK-average climate.
     inputs = _baseline_inputs()