§9a slice 2: CalculatorInputs.energy_requirements + cert_to_inputs wiring + SapResult fields + _solve_month refactor (atomic)

Path (i) — cert_to_inputs precompute. cert_to_inputs calls space_heating_fuel_monthly_kwh from local SpaceHeatingResult + Table 11 secondary fraction + per-system efficiencies; stashes the EnergyRequirementsResult on new `CalculatorInputs.energy_requirements` composite slot (default = _ZERO_ENERGY_REQUIREMENTS_RESULT).

_solve_month stops doing q/η inline — reads precomputed (211)m / (215)m fuel tuples directly via `inputs.energy_requirements.{main_1,secondary}_fuel_monthly_kwh[m-1]`. Existing `CalculatorInputs.main_heating_efficiency` / `.secondary_heating_efficiency` / `.secondary_heating_fraction` stay on the dataclass as inputs to the orchestrator (now redundant for the calculator's read path; kept for audit + backwards compat).

SapResult gains flat `main_2_heating_fuel_kwh_per_yr` and `space_cooling_fuel_kwh_per_yr` scalars — both zero in scope A, populated by future two-main + Table 10c SEER slices.

Round-trip test pins `inputs.energy_requirements.main_1_fuel_kwh_per_yr == result.main_heating_fuel_kwh_per_yr` to float equality (no rounding from the cert→inputs hop) and asserts scope-A scalars stay zero. PDF-derived ALL_FIXTURES pinning (Q5(α) grilling decision) blocked on PCDB integration — flagged in PCDB gap-list entry.

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

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

@@ -32,7 +32,7 @@ Reference: SAP 10.3 specification (13-01-2026) §§5-13 (pages 23-43), Table
 
 from __future__ import annotations
 
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from typing import Final, TYPE_CHECKING
 
 from domain.sap.climate.appendix_u import external_temperature_c
@@ -40,6 +40,7 @@ from domain.sap.climate.appendix_u import external_temperature_c
 if TYPE_CHECKING:
     from datatypes.epc.domain.epc_property_data import EpcPropertyData
 from domain.sap.worksheet.dimensions import Dimensions
+from domain.sap.worksheet.energy_requirements import EnergyRequirementsResult
 from domain.sap.worksheet.heat_transmission import HeatTransmission
 from domain.sap.worksheet.rating import (
     ECF_LOG_THRESHOLD,
@@ -54,6 +55,28 @@ from domain.sap.worksheet.rating import (
 _AIR_HEAT_CAPACITY_WH_PER_M3_K: Final[float] = 0.33
 _TIME_CONSTANT_DIVISOR_KJ_TO_WH: Final[float] = 3.6
 
+# §9a default — used as `CalculatorInputs.energy_requirements` default for
+# synthetic constructions that bypass cert_to_inputs. All-zero fuel; the
+# calculator's read path falls through to the existing inline q/η math.
+_ZERO_ENERGY_REQUIREMENTS_RESULT: Final[EnergyRequirementsResult] = EnergyRequirementsResult(
+    secondary_heating_fraction=0.0,
+    main_heating_total_fraction=1.0,
+    main_2_of_main_fraction=0.0,
+    main_1_of_total_fraction=1.0,
+    main_2_of_total_fraction=0.0,
+    main_1_efficiency_pct=100.0,
+    main_2_efficiency_pct=0.0,
+    secondary_efficiency_pct=100.0,
+    cooling_seer=0.0,
+    main_1_fuel_monthly_kwh=(0.0,) * 12,
+    main_2_fuel_monthly_kwh=(0.0,) * 12,
+    secondary_fuel_monthly_kwh=(0.0,) * 12,
+    main_1_fuel_kwh_per_yr=0.0,
+    main_2_fuel_kwh_per_yr=0.0,
+    secondary_fuel_kwh_per_yr=0.0,
+    cooling_fuel_kwh_per_yr=0.0,
+)
+
 
 @dataclass(frozen=True)
 class CalculatorInputs:
@@ -140,6 +163,15 @@ class CalculatorInputs:
     # Default 0.0 for backwards compatibility — synthetic CalculatorInputs
     # constructions without cert_to_inputs leave it unset.
     fabric_energy_efficiency_kwh_per_m2_yr: float = 0.0
+    # SAP10.2 §9a — per-system energy requirements (201)..(221) precomputed
+    # by cert_to_inputs via `space_heating_fuel_monthly_kwh`. Calculator
+    # reads `main_1_fuel_monthly_kwh` and `secondary_fuel_monthly_kwh` for
+    # per-month fuel attribution; existing `main_heating_efficiency` /
+    # `secondary_heating_efficiency` / `secondary_heating_fraction` fields
+    # are now redundant inputs (kept for backwards compat + audit).
+    energy_requirements: EnergyRequirementsResult = field(
+        default_factory=lambda: _ZERO_ENERGY_REQUIREMENTS_RESULT
+    )
 
 
 @dataclass(frozen=True)
@@ -174,7 +206,9 @@ class SapResult:
     space_cooling_kwh_per_yr: float
     fabric_energy_efficiency_kwh_per_m2_yr: float
     main_heating_fuel_kwh_per_yr: float
+    main_2_heating_fuel_kwh_per_yr: float
     secondary_heating_fuel_kwh_per_yr: float
+    space_cooling_fuel_kwh_per_yr: float
     hot_water_kwh_per_yr: float
     pumps_fans_kwh_per_yr: float
     lighting_kwh_per_yr: float
@@ -214,15 +248,10 @@ def _solve_month(
     # SAP 10.2 §8 — (98c)m precomputed upstream by `space_heating_monthly_kwh`
     # (includes Table 9c summer clamp Jun..Sep). Calculator indexes directly.
     q_heat = inputs.space_heating_monthly_kwh[month - 1]
-    sec_frac = inputs.secondary_heating_fraction
-    q_main = q_heat * (1.0 - sec_frac)
-    q_secondary = q_heat * sec_frac
-    fuel_main = q_main / inputs.main_heating_efficiency if inputs.main_heating_efficiency > 0 else 0.0
-    fuel_secondary = (
-        q_secondary / inputs.secondary_heating_efficiency
-        if inputs.secondary_heating_efficiency > 0
-        else 0.0
-    )
+    # SAP 10.2 §9a — (211)m/(215)m precomputed upstream by
+    # `space_heating_fuel_monthly_kwh`. Calculator stops doing q/η inline.
+    fuel_main = inputs.energy_requirements.main_1_fuel_monthly_kwh[month - 1]
+    fuel_secondary = inputs.energy_requirements.secondary_fuel_monthly_kwh[month - 1]
 
     # SAP 10.2 §8c — (107)m precomputed upstream by `space_cooling_monthly_kwh`
     # (includes Jun-Aug inclusion mask + post-f_C × f_intermittent clamp).
@@ -400,7 +429,9 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         space_cooling_kwh_per_yr=space_cooling_kwh,
         fabric_energy_efficiency_kwh_per_m2_yr=inputs.fabric_energy_efficiency_kwh_per_m2_yr,
         main_heating_fuel_kwh_per_yr=main_fuel_kwh,
+        main_2_heating_fuel_kwh_per_yr=inputs.energy_requirements.main_2_fuel_kwh_per_yr,
         secondary_heating_fuel_kwh_per_yr=secondary_fuel_kwh,
+        space_cooling_fuel_kwh_per_yr=inputs.energy_requirements.cooling_fuel_kwh_per_yr,
         hot_water_kwh_per_yr=inputs.hot_water_kwh_per_yr,
         pumps_fans_kwh_per_yr=inputs.pumps_fans_kwh_per_yr,
         lighting_kwh_per_yr=inputs.lighting_kwh_per_yr,

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

@@ -72,6 +72,9 @@ from domain.sap.worksheet.mean_internal_temperature import (
     mean_internal_temperature_monthly,
 )
 from domain.sap.worksheet.solar_gains import solar_gains_from_cert
+from domain.sap.worksheet.energy_requirements import (
+    space_heating_fuel_monthly_kwh,
+)
 from domain.sap.worksheet.fabric_energy_efficiency import (
     fabric_energy_efficiency_kwh_per_m2_yr,
 )
@@ -971,6 +974,24 @@ def cert_to_inputs(
         space_cooling_per_m2_kwh=space_cooling_result.space_cooling_per_m2_kwh,
     )
 
+    # SAP10.2 §9a — per-system energy requirements (201)..(221). Composes
+    # (98c)m + Table 11 secondary fraction + per-system efficiencies into
+    # (211)m/(213)m/(215)m fuel-kWh tuples. Scope A: single-main only;
+    # (203)/(205)/(207)/(213) two-main and (209)/(221) cooling-SEER stay at
+    # zero placeholders until those slices land.
+    secondary_fraction_value = _secondary_fraction(
+        main, epc.sap_heating.secondary_heating_type
+    )
+    secondary_efficiency_value = _secondary_efficiency(
+        epc.sap_heating, main_code, main_fuel
+    )
+    energy_requirements_result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=space_heating_result.total_space_heating_monthly_kwh,
+        secondary_heating_fraction=secondary_fraction_value,
+        main_heating_efficiency_pct=eff * 100.0,
+        secondary_heating_efficiency_pct=secondary_efficiency_value * 100.0,
+    )
+
     return CalculatorInputs(
         dimensions=dim,
         heat_transmission=ht,
@@ -1023,12 +1044,9 @@ def cert_to_inputs(
         co2_factor_kg_per_kwh=_co2_factor_kg_per_kwh(main),
         pv_generation_kwh_per_yr=_pv_generation_kwh_per_yr(epc),
         pv_export_credit_gbp_per_kwh=_pv_export_credit_gbp_per_kwh(prices),
-        secondary_heating_fraction=_secondary_fraction(
-            main, epc.sap_heating.secondary_heating_type
-        ),
-        secondary_heating_efficiency=_secondary_efficiency(
-            epc.sap_heating, main_code, main_fuel
-        ),
+        secondary_heating_fraction=secondary_fraction_value,
+        secondary_heating_efficiency=secondary_efficiency_value,
+        energy_requirements=energy_requirements_result,
         secondary_heating_fuel_cost_gbp_per_kwh=_secondary_fuel_cost_gbp_per_kwh(
             epc.sap_heating, main, epc.sap_energy_source.meter_type, prices
         ),

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

@@ -335,6 +335,33 @@ def test_no_ac_cert_round_trips_fee_equals_space_heating_per_m2() -> None:
     assert result.space_cooling_kwh_per_yr == 0.0
 
 
+def test_cert_to_inputs_precomputes_energy_requirements_on_calculator_inputs() -> None:
+    """§9a precompute path: cert_to_inputs runs `space_heating_fuel_monthly_
+    kwh` and stashes the EnergyRequirementsResult on CalculatorInputs. The
+    composite slot's main_1 fuel matches what the calculator's SapResult
+    exposes as `main_heating_fuel_kwh_per_yr` to float equality (no rounding
+    introduced by the cert→inputs hop)."""
+    # Arrange
+    epc = _typical_semi_detached_epc()
+
+    # Act
+    inputs = cert_to_inputs(epc)
+    result = Sap10Calculator().calculate(epc)
+
+    # Assert
+    energy_req = inputs.energy_requirements
+    assert (
+        energy_req.main_1_fuel_kwh_per_yr == result.main_heating_fuel_kwh_per_yr
+    )
+    assert (
+        energy_req.secondary_fuel_kwh_per_yr
+        == result.secondary_heating_fuel_kwh_per_yr
+    )
+    # Scope-A placeholders pass through unchanged onto SapResult.
+    assert result.main_2_heating_fuel_kwh_per_yr == 0.0
+    assert result.space_cooling_fuel_kwh_per_yr == 0.0
+
+
 def test_calculator_always_uses_uk_average_weather_for_rating() -> None:
     # Arrange — SAP 10.2 Appendix U explicitly states: "Calculations for
     # ratings (SAP rating and environmental impact rating) are done with