P5.7: SapResult.intermediate exposes ECF and energy-cost deflator

Promotes `_ENERGY_COST_DEFLATOR` to `ENERGY_COST_DEFLATOR` so the
§13 Table 12 constant can be referenced in trace mode alongside the
ECF it scales. ECF mirrors the top-level field; the deflator is the
only fixed worksheet constant the SAP rating depends on.

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

@@ -43,7 +43,12 @@ from domain.sap.worksheet.dimensions import Dimensions
 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 energy_cost_factor, sap_rating, sap_rating_integer
+from domain.sap.worksheet.rating import (
+    ENERGY_COST_DEFLATOR,
+    energy_cost_factor,
+    sap_rating,
+    sap_rating_integer,
+)
 from domain.sap.worksheet.solar_gains import (
     Orientation,
     surface_solar_flux_w_per_m2,
@@ -364,6 +369,8 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "hot_water_cost_gbp": hot_water_cost,
         "pumps_fans_cost_gbp": pumps_fans_cost,
         "lighting_cost_gbp": lighting_cost,
+        "ecf": ecf,
+        "deflator": ENERGY_COST_DEFLATOR,
     }
 
     return SapResult(

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

@@ -262,6 +262,22 @@ def test_calculate_exposes_per_end_use_fuel_costs() -> None:
     assert result.intermediate["lighting_cost_gbp"] == pytest.approx(lighting_cost, rel=1e-9)
 
 
+def test_calculate_exposes_ecf_and_deflator() -> None:
+    # Arrange — P5 trace mode: ECF (the rating denominator) and the §13
+    # Table 12 deflator (0.36) surface on `intermediate`. ECF mirrors the
+    # top-level field; deflator is the only fixed worksheet constant the
+    # SAP rating depends on, so naming it lets future rating-equation
+    # sweep slices reference it explicitly.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    assert result.intermediate["ecf"] == pytest.approx(result.ecf, rel=1e-9)
+    assert result.intermediate["deflator"] == pytest.approx(0.36, 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

@@ -18,7 +18,7 @@ from math import log10
 from typing import Final
 
 
-_ENERGY_COST_DEFLATOR: Final[float] = 0.36
+ENERGY_COST_DEFLATOR: Final[float] = 0.36
 _FLOOR_AREA_OFFSET_M2: Final[float] = 45.0
 _ECF_LOG_THRESHOLD: Final[float] = 3.5
 _SAP_LINEAR_INTERCEPT: Final[float] = 100.0
@@ -38,7 +38,7 @@ 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: