From 0fa39e859cb1621734f3daf51cadd0261276a754 Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Tue, 19 May 2026 12:24:59 +0000
Subject: [PATCH] P5.13: SapResult.intermediate exposes per-end-use CO2
 breakdown
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Closes the second §11-sketch gap noted in HANDOVER_SYSTEMATIC_REVIEW
("primary energy AND CO2 per end-use"). Lifts the single co2 = total
× factor expression into five named locals (main_heating, secondary,
hot_water, pumps_fans, lighting) and exposes them on `intermediate`.
The five components sum exactly to the top-level co2_kg_per_yr — no
PV deduction in the current implementation.
---
 packages/domain/src/domain/sap/calculator.py  | 21 +++++++++-
 .../src/domain/sap/tests/test_calculator.py   | 38 +++++++++++++++++++
 2 files changed, 57 insertions(+), 2 deletions(-)

diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
index 3fe92e5a..57a1b14b 100644
--- a/packages/domain/src/domain/sap/calculator.py
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -328,7 +328,19 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
     ecf = energy_cost_factor(total_cost_gbp=total_cost, total_floor_area_m2=tfa)
     sap_int = sap_rating_integer(ecf=ecf)
     sap_cont = sap_rating(ecf=ecf)
-    co2 = delivered_fuel_kwh * inputs.co2_factor_kg_per_kwh
+    co2_factor = inputs.co2_factor_kg_per_kwh
+    main_heating_co2 = main_fuel_kwh * co2_factor
+    secondary_heating_co2 = secondary_fuel_kwh * co2_factor
+    hot_water_co2 = inputs.hot_water_kwh_per_yr * co2_factor
+    pumps_fans_co2 = inputs.pumps_fans_kwh_per_yr * co2_factor
+    lighting_co2 = inputs.lighting_kwh_per_yr * co2_factor
+    co2 = (
+        main_heating_co2
+        + secondary_heating_co2
+        + hot_water_co2
+        + pumps_fans_co2
+        + lighting_co2
+    )
 
     space_heating_primary_kwh = (
         main_fuel_kwh + secondary_fuel_kwh
@@ -379,7 +391,12 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "ecf": ecf,
         "deflator": ENERGY_COST_DEFLATOR,
         "delivered_fuel_kwh_per_yr": delivered_fuel_kwh,
-        "co2_factor_kg_per_kwh": inputs.co2_factor_kg_per_kwh,
+        "co2_factor_kg_per_kwh": co2_factor,
+        "main_heating_co2_kg_per_yr": main_heating_co2,
+        "secondary_heating_co2_kg_per_yr": secondary_heating_co2,
+        "hot_water_co2_kg_per_yr": hot_water_co2,
+        "pumps_fans_co2_kg_per_yr": pumps_fans_co2,
+        "lighting_co2_kg_per_yr": lighting_co2,
         "space_heating_pe_kwh_per_m2": space_heating_primary_kwh / tfa if tfa > 0 else 0.0,
         "hot_water_pe_kwh_per_m2": hot_water_primary_kwh / tfa if tfa > 0 else 0.0,
         "other_pe_kwh_per_m2": other_primary_kwh / tfa if tfa > 0 else 0.0,
diff --git a/packages/domain/src/domain/sap/tests/test_calculator.py b/packages/domain/src/domain/sap/tests/test_calculator.py
index 1582cf8c..89c1e483 100644
--- a/packages/domain/src/domain/sap/tests/test_calculator.py
+++ b/packages/domain/src/domain/sap/tests/test_calculator.py
@@ -352,6 +352,44 @@ def test_calculate_exposes_primary_energy_breakdown() -> None:
     )
 
 
+def test_calculate_exposes_per_end_use_co2() -> None:
+    # Arrange — P5 trace mode: §11 sketch lists "primary energy AND CO2
+    # per end-use". The calculator applies a single co2_factor_kg_per_kwh
+    # to total delivered fuel (no PV deduction on CO2 in the current
+    # implementation), so per-end-use CO2 is fuel_kwh × factor and the
+    # five components sum exactly to the top-level co2_kg_per_yr.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    factor = inputs.co2_factor_kg_per_kwh
+    assert result.intermediate["main_heating_co2_kg_per_yr"] == pytest.approx(
+        result.main_heating_fuel_kwh_per_yr * factor, rel=1e-9
+    )
+    assert result.intermediate["secondary_heating_co2_kg_per_yr"] == pytest.approx(
+        result.secondary_heating_fuel_kwh_per_yr * factor, rel=1e-9
+    )
+    assert result.intermediate["hot_water_co2_kg_per_yr"] == pytest.approx(
+        result.hot_water_kwh_per_yr * factor, rel=1e-9
+    )
+    assert result.intermediate["pumps_fans_co2_kg_per_yr"] == pytest.approx(
+        result.pumps_fans_kwh_per_yr * factor, rel=1e-9
+    )
+    assert result.intermediate["lighting_co2_kg_per_yr"] == pytest.approx(
+        result.lighting_kwh_per_yr * factor, rel=1e-9
+    )
+    breakdown_sum = (
+        result.intermediate["main_heating_co2_kg_per_yr"]
+        + result.intermediate["secondary_heating_co2_kg_per_yr"]
+        + result.intermediate["hot_water_co2_kg_per_yr"]
+        + result.intermediate["pumps_fans_co2_kg_per_yr"]
+        + result.intermediate["lighting_co2_kg_per_yr"]
+    )
+    assert breakdown_sum == pytest.approx(result.co2_kg_per_yr, rel=1e-9)
+
+
 def test_calculate_exposes_pv_export_credit() -> None:
     # Arrange — P5 trace mode: total_fuel_cost_gbp = sum(per-end-use
     # costs) − pv_export_credit, floored at 0. The PV credit is the only