diff --git a/packages/domain/src/domain/sap/calculator.py b/packages/domain/src/domain/sap/calculator.py
index f17d23fc..8ef3e429 100644
--- a/packages/domain/src/domain/sap/calculator.py
+++ b/packages/domain/src/domain/sap/calculator.py
@@ -328,19 +328,23 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
     sap_cont = sap_rating(ecf=ecf)
     co2 = delivered_fuel_kwh * inputs.co2_factor_kg_per_kwh
 
-    primary_energy_kwh = (
-        main_fuel_kwh * inputs.space_heating_primary_factor
-        + secondary_fuel_kwh * inputs.space_heating_primary_factor
-        + inputs.hot_water_kwh_per_yr * inputs.hot_water_primary_factor
-        + (inputs.pumps_fans_kwh_per_yr + inputs.lighting_kwh_per_yr)
-        * inputs.other_primary_factor
-    )
+    space_heating_primary_kwh = (
+        main_fuel_kwh + secondary_fuel_kwh
+    ) * inputs.space_heating_primary_factor
+    hot_water_primary_kwh = inputs.hot_water_kwh_per_yr * inputs.hot_water_primary_factor
+    other_primary_kwh = (
+        inputs.pumps_fans_kwh_per_yr + inputs.lighting_kwh_per_yr
+    ) * inputs.other_primary_factor
     # PV offsets primary energy at the same PEF (Appendix M: export PEF =
     # standard-electricity PEF for ratings, since the displaced grid kWh
     # would have been imported electricity).
+    pv_primary_offset_kwh = inputs.pv_generation_kwh_per_yr * inputs.other_primary_factor
     primary_energy_kwh = max(
         0.0,
-        primary_energy_kwh - inputs.pv_generation_kwh_per_yr * inputs.other_primary_factor,
+        space_heating_primary_kwh
+        + hot_water_primary_kwh
+        + other_primary_kwh
+        - pv_primary_offset_kwh,
     )
     primary_energy_per_m2 = primary_energy_kwh / tfa if tfa > 0 else 0.0
 
@@ -373,6 +377,10 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         "deflator": ENERGY_COST_DEFLATOR,
         "delivered_fuel_kwh_per_yr": delivered_fuel_kwh,
         "co2_factor_kg_per_kwh": inputs.co2_factor_kg_per_kwh,
+        "space_heating_primary_kwh_per_yr": space_heating_primary_kwh,
+        "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,
     }
 
     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 22915dd2..86abc224 100644
--- a/packages/domain/src/domain/sap/tests/test_calculator.py
+++ b/packages/domain/src/domain/sap/tests/test_calculator.py
@@ -308,6 +308,46 @@ def test_calculate_exposes_co2_chain() -> None:
     ) == pytest.approx(result.co2_kg_per_yr, rel=1e-9)
 
 
+def test_calculate_exposes_primary_energy_breakdown() -> None:
+    # Arrange — P5 trace mode: primary energy splits across three PEFs
+    # (space-heating, hot-water, other) and a PV offset at the other-PEF
+    # (Appendix M). Exposing the four components makes the top-level
+    # primary_energy_kwh_per_yr auditable, including whether the
+    # floor-at-zero was hit when PV exceeded gross primary.
+    inputs = _baseline_inputs()
+
+    # Act
+    result = calculate_sap_from_inputs(inputs)
+
+    # Assert
+    space_heating_primary = (
+        result.main_heating_fuel_kwh_per_yr + result.secondary_heating_fuel_kwh_per_yr
+    ) * inputs.space_heating_primary_factor
+    hot_water_primary = inputs.hot_water_kwh_per_yr * inputs.hot_water_primary_factor
+    other_primary = (
+        inputs.pumps_fans_kwh_per_yr + inputs.lighting_kwh_per_yr
+    ) * inputs.other_primary_factor
+    pv_offset = inputs.pv_generation_kwh_per_yr * inputs.other_primary_factor
+
+    assert result.intermediate["space_heating_primary_kwh_per_yr"] == pytest.approx(
+        space_heating_primary, rel=1e-9
+    )
+    assert result.intermediate["hot_water_primary_kwh_per_yr"] == pytest.approx(
+        hot_water_primary, rel=1e-9
+    )
+    assert result.intermediate["other_primary_kwh_per_yr"] == pytest.approx(
+        other_primary, rel=1e-9
+    )
+    assert result.intermediate["pv_primary_offset_kwh_per_yr"] == pytest.approx(
+        pv_offset, rel=1e-9
+    )
+    expected_total = max(
+        0.0,
+        space_heating_primary + hot_water_primary + other_primary - pv_offset,
+    )
+    assert result.primary_energy_kwh_per_yr == pytest.approx(expected_total, 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.