From 44b1d0d923587a65caf0b0fc8275e7a63bd3207c Mon Sep 17 00:00:00 2001
From: Khalim Conn-Kowlessar <kconnkowlessar@gmail.com>
Date: Tue, 19 May 2026 10:22:53 +0000
Subject: [PATCH] P5.5: SapResult.intermediate exposes
 useful_space_heating_kwh_per_yr
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§9 / Table 9c step 10 output keyed by worksheet name on `intermediate`.
Mirrors the top-level `space_heating_kwh_per_yr` field so spec sweep
slices refer to the worksheet name regardless of field renames.

135 SAP tests pass.
---
 packages/domain/src/domain/sap/calculator.py    |  1 +
 .../src/domain/sap/tests/test_calculator.py     | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
index 8f1ae621..97ea331e 100644
--- a/packages/domain/src/domain/sap/calculator.py
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -351,6 +351,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "time_constant_h": tau_h,
         "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,
     }
 
     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 3572674d..98e2b3fd 100644
--- a/packages/domain/src/domain/sap/tests/test_calculator.py
+++ b/packages/domain/src/domain/sap/tests/test_calculator.py
@@ -212,6 +212,23 @@ def test_calculate_exposes_hlc_hlp_and_annual_averages() -> None:
     )
 
 
+def test_calculate_exposes_useful_space_heating_kwh() -> None:
+    # Arrange — P5 trace mode: useful space heating kWh/yr (§9 / Table 9c
+    # step 10) surfaces on `intermediate` keyed by worksheet name. Mirrors
+    # `space_heating_kwh_per_yr` on the top-level result so spec sweep
+    # slices can refer to the worksheet name regardless of `SapResult`
+    # field renames.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    assert result.intermediate["useful_space_heating_kwh_per_yr"] == pytest.approx(
+        result.space_heating_kwh_per_yr, 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.