diff --git a/packages/domain/src/domain/sap/worksheet/solar_gains.py b/packages/domain/src/domain/sap/worksheet/solar_gains.py
index 041ba179..caca48d5 100644
--- a/packages/domain/src/domain/sap/worksheet/solar_gains.py
+++ b/packages/domain/src/domain/sap/worksheet/solar_gains.py
@@ -258,7 +258,12 @@ class SolarGainsResult:
 
 
 def _g_perpendicular(w: SapWindow) -> float:
-    """Table 6b g⊥ by glazing_type code, defaulting to 0.76 for unknowns."""
+    """Table 6b g⊥. Prefer the cert's measured value
+    (`window_transmission_details.solar_transmittance`) when present —
+    Elmhurst lodges it for every window; Table 6b is the cascade fallback.
+    """
+    if w.window_transmission_details is not None:
+        return float(w.window_transmission_details.solar_transmittance)
     if isinstance(w.glazing_type, int) and w.glazing_type in _G_PERPENDICULAR_BY_GLAZING_TYPE:
         return _G_PERPENDICULAR_BY_GLAZING_TYPE[w.glazing_type]
     return _G_PERPENDICULAR_DEFAULT
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_solar_gains.py b/packages/domain/src/domain/sap/worksheet/tests/test_solar_gains.py
index 3fa8d631..becdaa4f 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/test_solar_gains.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/test_solar_gains.py
@@ -11,7 +11,7 @@ Appendix U §U3.2 (pages 127-129), Table U4 (latitudes), Table U5
 
 import pytest
 
-from datatypes.epc.domain.epc_property_data import SapWindow
+from datatypes.epc.domain.epc_property_data import SapWindow, WindowTransmissionDetails
 from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
 from domain.sap.worksheet.internal_gains import OvershadingCategory
 from domain.sap.worksheet.solar_gains import (
@@ -63,6 +63,35 @@ def test_solar_gains_from_cert_reproduces_000490_line_83_total() -> None:
         ), f"(83) month {m+1}"
 
 
+def test_solar_gains_from_cert_prefers_manufacturer_g_perpendicular_over_table_6b() -> None:
+    # Arrange — SAP 10.2 §6.1 + Table 6b note 2: when the manufacturer
+    # supplies a measured solar transmittance via the cert's
+    # `window_transmission_details`, it overrides the Table 6b code lookup.
+    # Elmhurst lodges this for every window (Source = "Manufacturer"). Same
+    # 5.52 m² SE window as 000490 SE, but glazing_type=4 (low-E soft, table
+    # default 0.63) with manufacturer g⊥=0.76 — the manufacturer value must
+    # win, reproducing the worksheet gain.
+    window = make_window(
+        orientation=4, width=5.52, height=1.0, glazing_type=4,
+    )
+    window.window_transmission_details = WindowTransmissionDetails(
+        u_value=2.8, data_source=1, solar_transmittance=0.76,
+    )
+    epc = make_minimal_sap10_epc(
+        total_floor_area_m2=53.0,
+        sap_windows=[window],
+    )
+
+    # Act
+    result = solar_gains_from_cert(
+        epc=epc, region=0, overshading=OvershadingCategory.AVERAGE,
+    )
+
+    # Assert — single SE window, Jan UK-avg flux 36.7938, gain 74.8788 W
+    assert result.southeast_monthly_w[0] == pytest.approx(74.8788, abs=5e-3)
+    assert result.total_solar_gains_monthly_w[0] == pytest.approx(74.8788, abs=5e-3)
+
+
 def test_z_solar_for_overshading_returns_table_6d_first_column() -> None:
     # Arrange — SAP 10.2 Table 6d "Winter solar access factor (for calculation
     # of solar gains for heating)" — first numeric column on p178. Used for