P5.10: SapResult.intermediate exposes rating-equation spec constants

Promotes _FLOOR_AREA_OFFSET_M2 → FLOOR_AREA_OFFSET_M2 (§13 ECF
denominator, Table 12) and _ECF_LOG_THRESHOLD → ECF_LOG_THRESHOLD
(SAP rating linear/log regime boundary at ECF = 3.5). Together with
the deflator (P5.7) they fully document the §13 rating curve in
trace mode.

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

@@ -44,7 +44,9 @@ from domain.sap.worksheet.heat_transmission import HeatTransmission
 from domain.sap.worksheet.internal_gains import internal_gains_w
 from domain.sap.worksheet.mean_internal_temperature import mean_internal_temperature_c
 from domain.sap.worksheet.rating import (
+    ECF_LOG_THRESHOLD,
     ENERGY_COST_DEFLATOR,
+    FLOOR_AREA_OFFSET_M2,
     energy_cost_factor,
     sap_rating,
     sap_rating_integer,
@@ -381,6 +383,8 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "hot_water_primary_kwh_per_yr": hot_water_primary_kwh,
         "other_primary_kwh_per_yr": other_primary_kwh,
         "pv_primary_offset_kwh_per_yr": pv_primary_offset_kwh,
+        "floor_area_offset_m2": FLOOR_AREA_OFFSET_M2,
+        "ecf_log_threshold": ECF_LOG_THRESHOLD,
     }
 
     return SapResult(

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

@@ -348,6 +348,22 @@ def test_calculate_exposes_primary_energy_breakdown() -> None:
     assert result.primary_energy_kwh_per_yr == pytest.approx(expected_total, rel=1e-9)
 
 
+def test_calculate_exposes_rating_equation_spec_constants() -> None:
+    # Arrange — P5 trace mode: the §13 ECF denominator carries a 45 m²
+    # floor-area offset (Table 12) and the SAP rating splits between a
+    # linear and a log regime at ECF = 3.5. Surfacing both on
+    # `intermediate` documents the equation alongside the already-exposed
+    # ecf + deflator (P5.7), so the SAP rating curve is fully auditable.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    assert result.intermediate["floor_area_offset_m2"] == pytest.approx(45.0, rel=1e-12)
+    assert result.intermediate["ecf_log_threshold"] == pytest.approx(3.5, rel=1e-12)
+
+
 def test_higher_main_heating_efficiency_reduces_fuel_use() -> None:
     # Arrange — Direction check: doubling the boiler efficiency must halve
     # the main-heating fuel kWh, holding everything else constant.

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

@@ -19,8 +19,8 @@ from typing import Final
 
 
 ENERGY_COST_DEFLATOR: Final[float] = 0.36
-_FLOOR_AREA_OFFSET_M2: Final[float] = 45.0
+FLOOR_AREA_OFFSET_M2: Final[float] = 45.0
-_ECF_LOG_THRESHOLD: Final[float] = 3.5
+ECF_LOG_THRESHOLD: Final[float] = 3.5
 _SAP_LINEAR_INTERCEPT: Final[float] = 100.0
 _SAP_LINEAR_SLOPE: Final[float] = 16.21
 _SAP_LOG_INTERCEPT: Final[float] = 108.8
@@ -38,13 +38,13 @@ def energy_cost_factor(
     total_floor_area_m2: float,
 ) -> float:
     """SAP 10.3 §13 equation (7): ECF = 0.36 × cost / (TFA + 45)."""
-    return ENERGY_COST_DEFLATOR * total_cost_gbp / (total_floor_area_m2 + _FLOOR_AREA_OFFSET_M2)
+    return ENERGY_COST_DEFLATOR * total_cost_gbp / (total_floor_area_m2 + FLOOR_AREA_OFFSET_M2)
 
 
 def sap_rating(*, ecf: float) -> float:
     """SAP 10.3 §13 equations (8)/(9). Un-rounded result so callers can
     inspect the continuous value; `sap_rating_integer` rounds and clamps."""
-    if ecf >= _ECF_LOG_THRESHOLD:
+    if ecf >= ECF_LOG_THRESHOLD:
         return _SAP_LOG_INTERCEPT - _SAP_LOG_SLOPE * log10(ecf)
     return _SAP_LINEAR_INTERCEPT - _SAP_LINEAR_SLOPE * ecf
 
@@ -64,7 +64,7 @@ def environmental_impact_rating(
 ) -> float:
     """SAP 10.3 §14 equations (10)-(12). Un-rounded EI rating; mirrors the
     SAP rating curve but uses CO2 emissions per (TFA + 45) as the input."""
-    cf = co2_emissions_kg_per_yr / (total_floor_area_m2 + _FLOOR_AREA_OFFSET_M2)
+    cf = co2_emissions_kg_per_yr / (total_floor_area_m2 + FLOOR_AREA_OFFSET_M2)
     if cf >= _CF_LOG_THRESHOLD:
         return _EI_LOG_INTERCEPT - _EI_LOG_SLOPE * log10(cf)
     return _EI_LINEAR_INTERCEPT - _EI_LINEAR_SLOPE * cf