From 2104c8c2da8412d5739aa7d23e0d157fcfd59489 Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Tue, 19 May 2026 10:24:27 +0000
Subject: [PATCH] P5.6: SapResult.intermediate exposes per-end-use fuel costs
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Per-end-use £/yr costs (main heating, secondary heating, hot water,
pumps_fans, lighting) lifted from the inlined total_cost sum into named
locals and populated on `intermediate`. §12 sweep slices can now diff
each line against the spec (Table 12 unit prices, future Table 12a
fractional blending, Table 12c heat-network DLF) without re-deriving
the cost decomposition.

Behaviour-preserving — `total_fuel_cost_gbp` reconciles bit-for-bit.

136 SAP tests pass.
---
 packages/domain/src/domain/sap/calculator.py  | 22 ++++++++++---
 .../src/domain/sap/tests/test_calculator.py   | 33 +++++++++++++++++++
 2 files changed, 50 insertions(+), 5 deletions(-)

diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
index 97ea331e..0c156691 100644
--- a/packages/domain/src/domain/sap/calculator.py
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -302,13 +302,20 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         + inputs.lighting_kwh_per_yr
     )
     pv_credit = inputs.pv_generation_kwh_per_yr * inputs.pv_export_credit_gbp_per_kwh
+    main_heating_cost = main_fuel_kwh * inputs.space_heating_fuel_cost_gbp_per_kwh
+    secondary_heating_cost = (
+        secondary_fuel_kwh * inputs.secondary_heating_fuel_cost_gbp_per_kwh
+    )
+    hot_water_cost = inputs.hot_water_kwh_per_yr * inputs.hot_water_fuel_cost_gbp_per_kwh
+    pumps_fans_cost = inputs.pumps_fans_kwh_per_yr * inputs.other_fuel_cost_gbp_per_kwh
+    lighting_cost = inputs.lighting_kwh_per_yr * inputs.other_fuel_cost_gbp_per_kwh
     total_cost = max(
         0.0,
-        main_fuel_kwh * inputs.space_heating_fuel_cost_gbp_per_kwh
-        + secondary_fuel_kwh * inputs.secondary_heating_fuel_cost_gbp_per_kwh
-        + inputs.hot_water_kwh_per_yr * inputs.hot_water_fuel_cost_gbp_per_kwh
-        + (inputs.pumps_fans_kwh_per_yr + inputs.lighting_kwh_per_yr)
-        * inputs.other_fuel_cost_gbp_per_kwh
+        main_heating_cost
+        + secondary_heating_cost
+        + hot_water_cost
+        + pumps_fans_cost
+        + lighting_cost
         - pv_credit,
     )
     ecf = energy_cost_factor(total_cost_gbp=total_cost, total_floor_area_m2=tfa)
@@ -352,6 +359,11 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "internal_gains_annual_avg_w": sum(e.internal_gains_w for e in monthly) / 12.0,
         "mean_internal_temp_annual_avg_c": sum(e.internal_temp_c for e in monthly) / 12.0,
         "useful_space_heating_kwh_per_yr": space_heating_kwh,
+        "main_heating_cost_gbp": main_heating_cost,
+        "secondary_heating_cost_gbp": secondary_heating_cost,
+        "hot_water_cost_gbp": hot_water_cost,
+        "pumps_fans_cost_gbp": pumps_fans_cost,
+        "lighting_cost_gbp": lighting_cost,
     }
 
     return SapResult(
diff --git a/packages/domain/src/domain/sap/tests/test_calculator.py b/packages/domain/src/domain/sap/tests/test_calculator.py
index 98e2b3fd..609b30cb 100644
--- a/packages/domain/src/domain/sap/tests/test_calculator.py
+++ b/packages/domain/src/domain/sap/tests/test_calculator.py
@@ -229,6 +229,39 @@ def test_calculate_exposes_useful_space_heating_kwh() -> None:
     )
 
 
+def test_calculate_exposes_per_end_use_fuel_costs() -> None:
+    # Arrange — P5 trace mode: per-end-use fuel costs (§12 / Table 12) break
+    # out on `intermediate` so the §12 sweep can diff main vs hot water vs
+    # pumps/fans vs lighting individually rather than against the bundled
+    # `total_fuel_cost_gbp`. Secondary heating cost is also surfaced even
+    # though §11 omitted it — the field exists on the calculator and is a
+    # named worksheet variable.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    main_cost = (
+        result.main_heating_fuel_kwh_per_yr * inputs.space_heating_fuel_cost_gbp_per_kwh
+    )
+    secondary_cost = (
+        result.secondary_heating_fuel_kwh_per_yr
+        * inputs.secondary_heating_fuel_cost_gbp_per_kwh
+    )
+    hot_water_cost = inputs.hot_water_kwh_per_yr * inputs.hot_water_fuel_cost_gbp_per_kwh
+    pumps_cost = inputs.pumps_fans_kwh_per_yr * inputs.other_fuel_cost_gbp_per_kwh
+    lighting_cost = inputs.lighting_kwh_per_yr * inputs.other_fuel_cost_gbp_per_kwh
+
+    assert result.intermediate["main_heating_cost_gbp"] == pytest.approx(main_cost, rel=1e-9)
+    assert result.intermediate["secondary_heating_cost_gbp"] == pytest.approx(
+        secondary_cost, rel=1e-9
+    )
+    assert result.intermediate["hot_water_cost_gbp"] == pytest.approx(hot_water_cost, rel=1e-9)
+    assert result.intermediate["pumps_fans_cost_gbp"] == pytest.approx(pumps_cost, rel=1e-9)
+    assert result.intermediate["lighting_cost_gbp"] == pytest.approx(lighting_cost, rel=1e-9)
+
+
 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.