diff --git a/packages/domain/src/domain/ml/tests/_fixtures.py b/packages/domain/src/domain/ml/tests/_fixtures.py
index 171af42b..f653fee6 100644
--- a/packages/domain/src/domain/ml/tests/_fixtures.py
+++ b/packages/domain/src/domain/ml/tests/_fixtures.py
@@ -164,8 +164,20 @@ def make_window(
     permanent_shutters_present: Union[bool, str] = False,
     frame_material: Optional[str] = "PVC",
     window_transmission_details: Optional[WindowTransmissionDetails] = None,
+    solar_transmittance: Optional[float] = None,
+    u_value: float = 2.8,
 ) -> SapWindow:
-    """Build a SapWindow with SAP10 defaults; override the fields the test cares about."""
+    """Build a SapWindow with SAP10 defaults; override the fields the test cares about.
+
+    `solar_transmittance` is a shortcut that builds a
+    `WindowTransmissionDetails(u_value, data_source=1, solar_transmittance)`
+    when `window_transmission_details` isn't supplied directly — keeps
+    Elmhurst §6 fixture literals tight.
+    """
+    if window_transmission_details is None and solar_transmittance is not None:
+        window_transmission_details = WindowTransmissionDetails(
+            u_value=u_value, data_source=1, solar_transmittance=solar_transmittance,
+        )
     return SapWindow(
         frame_material=frame_material,
         glazing_gap=glazing_gap,
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
index 3e237527..8e539a81 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py
@@ -26,8 +26,10 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
     SapBuildingPart,
     SapFloorDimension,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap.worksheet.ventilation import MechanicalVentilationKind
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
@@ -275,3 +277,29 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     446.3699, 444.6484, 431.0234, 405.3696, 380.1328, 350.1810,
     336.5702, 339.0214, 352.0014, 382.2118, 410.3693, 434.9896,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 5 wall windows across 2 glazing types (Manufacturer-declared g⊥):
+#   "Double between 2002" g=0.72: E 3.74, SE 0.50, NW 1.98
+#   "Double with unknown" g=0.76: E 3.74, NW 1.76
+# All PVC frame. No roof windows, no rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=3, width=3.74, height=1.0, solar_transmittance=0.72),
+    make_window(orientation=4, width=0.50, height=1.0, solar_transmittance=0.72),
+    make_window(orientation=8, width=1.98, height=1.0, solar_transmittance=0.72),
+    make_window(orientation=3, width=3.74, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=8, width=1.76, height=1.0, solar_transmittance=0.76),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = ()
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+     74.2861, 144.8943, 240.3371, 357.4289, 446.8944, 462.0286,
+    437.9571, 369.7981, 281.3970, 172.1314,  92.4825,  61.2179,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    520.6560, 589.5427, 671.3605, 762.7985, 827.0272, 812.2096,
+    774.5274, 708.8195, 633.3985, 554.3432, 502.8518, 496.2075,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
index a9f030ea..6cbf4021 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py
@@ -23,8 +23,10 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
     SapFloorDimension,
     SapRoomInRoof,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap.worksheet.ventilation import MechanicalVentilationKind
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
@@ -212,3 +214,27 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     537.2284, 535.1563, 517.9364, 486.2482, 454.3577, 418.6634,
     401.8987, 404.7492, 421.3137, 457.4575, 492.3897, 522.7868,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 3 wall windows across 2 glazing types (Manufacturer-declared g⊥):
+#   "Double between 2002" g=0.72: E 1.28, W 6.76
+#   "Double with unknown" g=0.76: W 1.17
+# All PVC frame. No roof windows, no rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=3, width=1.28, height=1.0, solar_transmittance=0.72),
+    make_window(orientation=7, width=6.76, height=1.0, solar_transmittance=0.72),
+    make_window(orientation=7, width=1.17, height=1.0, solar_transmittance=0.76),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = ()
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+     63.6246, 124.4632, 204.9732, 298.9411, 366.3636, 375.0383,
+    357.0518, 306.7022, 238.3923, 147.6861,  79.3324,  52.3218,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    600.8530, 659.6195, 722.9096, 785.1894, 820.7212, 793.7017,
+    758.9505, 711.4514, 659.7060, 605.1436, 571.7221, 575.1085,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
index dd14e6ae..f25c049e 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py
@@ -24,8 +24,10 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
     SapFloorDimension,
     SapRoomInRoof,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap.worksheet.ventilation import MechanicalVentilationKind
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
@@ -244,3 +246,24 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     574.7298, 572.6278, 554.4769, 521.1014, 487.3662, 450.0743,
     432.3496, 435.2074, 452.6494, 490.4110, 527.2724, 559.3723,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 2 wall windows, both Manufacturer g⊥=0.76 / PVC: NE 8.74, SW 1.80.
+# No roof windows, no rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=2, width=8.74, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=6, width=1.80, height=1.0, solar_transmittance=0.76),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = ()
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+     60.7732, 115.5953, 190.2388, 289.4797, 373.3152, 392.2015,
+    369.1410, 303.2958, 224.0851, 136.4060,  74.9915,  50.5862,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    635.5030, 688.2231, 744.7156, 810.5810, 860.6814, 842.2758,
+    801.4906, 738.5032, 676.7345, 626.8170, 602.2639, 609.9585,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
index deb6a15e..bde9b043 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py
@@ -22,8 +22,10 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
     SapFloorDimension,
     SapRoomInRoof,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
 # RdSAP wall_construction code seen in the cert→worksheet mapping. The
@@ -259,3 +261,26 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     548.8580, 546.8173, 529.7869, 499.9224, 468.4056, 433.5905,
     417.8752, 420.2523, 436.8085, 472.1723, 505.4898, 534.4324,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 2 wall windows, both South-facing, 2 glazing types (Manufacturer g⊥):
+#   "Double with unknown" g=0.76: S 0.77
+#   "Double between 2002" g=0.72: S 6.69
+# All PVC frame. No roof windows, no rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=5, width=0.77, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=5, width=6.69, height=1.0, solar_transmittance=0.72),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = ()
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+    122.5143, 200.6468, 255.5884, 288.8704, 301.0208, 289.6916,
+    283.0463, 274.8772, 266.9923, 216.4164, 145.2212, 105.8636,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    671.3723, 747.4641, 785.3754, 788.7928, 769.4264, 723.2821,
+    700.9215, 695.1295, 703.8008, 688.5887, 650.7110, 640.2960,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
index 721447c0..554caf90 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py
@@ -28,8 +28,10 @@ from datatypes.epc.domain.epc_property_data import (
     EpcPropertyData,
     SapBuildingPart,
     SapFloorDimension,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import RoofWindowInput, RooflightInput
 from domain.sap.worksheet.ventilation import MechanicalVentilationKind
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
@@ -255,3 +257,25 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     505.8067, 503.7906, 488.2293, 459.2325, 430.5641, 397.6412,
     382.3822, 385.1801, 400.0449, 433.4120, 465.2242, 492.9448,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 3 wall windows: NE 0.81, SE 5.52, NW 2.70 — all DG pre-2002 / PVC / 12 mm
+# with manufacturer g⊥=0.76. No roof windows, no rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=2, width=0.81, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=4, width=5.52, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=8, width=2.70, height=1.0, solar_transmittance=0.76),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = ()
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+     89.4795, 157.2665, 228.0608, 304.1703, 360.4042, 366.4669,
+    349.7056, 306.4273, 254.2093, 177.2863, 108.0591,  76.0043,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    595.2863, 661.0571, 716.2901, 763.4028, 790.9684, 764.1081,
+    732.0878, 691.6074, 654.2542, 610.6983, 573.2833, 568.9492,
+)
diff --git a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
index 05c460a3..9067eb6d 100644
--- a/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
+++ b/packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py
@@ -29,8 +29,10 @@ from datatypes.epc.domain.epc_property_data import (
     SapBuildingPart,
     SapFloorDimension,
     SapRoomInRoof,
+    SapWindow,
 )
-from domain.ml.tests._fixtures import make_minimal_sap10_epc
+from domain.ml.tests._fixtures import make_minimal_sap10_epc, make_window
+from domain.sap.worksheet.solar_gains import Orientation, RoofWindowInput, RooflightInput
 from domain.sap.worksheet.ventilation import MechanicalVentilationKind
 from domain.sap.worksheet.water_heating import TABLE_J1_TCOLD_FROM_MAINS_C
 
@@ -222,3 +224,30 @@ LINE_73_M_TOTAL_INTERNAL_GAINS_W: tuple[float, ...] = (
     600.1430, 597.7026, 578.1360, 542.0517, 505.9694, 466.5482,
     447.5794, 450.9272, 469.7096, 509.7947, 549.4610, 583.9178,
 )
+
+# ============================================================================
+# §6 Solar gains — cert-derived inputs + expected outputs
+# ============================================================================
+# 2 wall windows (Manufacturer g⊥=0.76 / PVC): NE 3.88, SW 4.43.
+# 1 roof window (NE 1.18, Manufacturer g⊥=0.76, pitch 45° per RdSAP10
+# Table 24 default, Wood frame FF=0.70). No rooflights.
+SECTION_6_VERTICAL_WINDOWS: tuple[SapWindow, ...] = (
+    make_window(orientation=2, width=3.88, height=1.0, solar_transmittance=0.76),
+    make_window(orientation=6, width=4.43, height=1.0, solar_transmittance=0.76),
+)
+SECTION_6_ROOF_WINDOWS: tuple[RoofWindowInput, ...] = (
+    RoofWindowInput(
+        area_m2=1.18, orientation=Orientation.NE,
+        g_perpendicular=0.76, frame_factor=0.70,
+    ),
+)
+SECTION_6_ROOFLIGHTS: tuple[RooflightInput, ...] = ()
+
+LINE_83_M_TOTAL_SOLAR_W: tuple[float, ...] = (
+     85.4797, 154.2449, 234.3467, 329.9943, 406.2034, 419.4674,
+    397.6609, 338.2558, 267.0293, 176.7227, 103.9502,  72.1445,
+)
+LINE_84_M_TOTAL_GAINS_W: tuple[float, ...] = (
+    685.6226, 751.9476, 812.4827, 872.0460, 912.1729, 886.0156,
+    845.2404, 789.1829, 736.7389, 686.5174, 653.4111, 656.0623,
+)
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 becdaa4f..ad464894 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
@@ -9,9 +9,15 @@ Appendix U §U3.2 (pages 127-129), Table U4 (latitudes), Table U5
 (k1-k9 constants per orientation).
 """
 
+from types import ModuleType
+
 import pytest
 
-from datatypes.epc.domain.epc_property_data import SapWindow, WindowTransmissionDetails
+from datatypes.epc.domain.epc_property_data import (
+    EpcPropertyData,
+    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 (
@@ -23,6 +29,7 @@ from domain.sap.worksheet.solar_gains import (
     window_solar_gain_w,
     z_solar_for_overshading,
 )
+from domain.sap.worksheet.tests._elmhurst_fixtures import ALL_FIXTURES, fixture_id
 
 
 # Worksheet U985-0001-000490 reference (UK-avg weather, region 0):
@@ -240,3 +247,55 @@ def test_out_of_range_region_raises_value_error() -> None:
         surface_solar_flux_w_per_m2(
             orientation=Orientation.S, pitch_deg=90.0, region=22, month=7,
         )
+
+
+def _build_section_6_epc(fixture: ModuleType) -> EpcPropertyData:
+    """Wrap a fixture's base build_epc() with the §6-relevant fields it
+    doesn't yet carry: per-orientation vertical wall windows lodged on the
+    epc. Roof windows + rooflights pass through as orchestrator args
+    because RdSAP cert summaries don't lodge them distinctly. Kept local
+    to the §6 test so legacy build_epc()s stay pinned for §1-§4 +
+    e2e SAP-score regressions (handover §11)."""
+    base = fixture.build_epc()
+    base.sap_windows = list(fixture.SECTION_6_VERTICAL_WINDOWS)
+    return base
+
+
+@pytest.mark.parametrize("fixture", ALL_FIXTURES, ids=[fixture_id(f) for f in ALL_FIXTURES])
+def test_solar_gains_from_cert_matches_elmhurst_worksheet_all_fixtures(
+    fixture: ModuleType,
+) -> None:
+    """End-to-end §6 orchestrator against every Elmhurst conformance fixture.
+
+    Each fixture pins its own §6 input constants
+    (SECTION_6_VERTICAL_WINDOWS, SECTION_6_ROOF_WINDOWS,
+    SECTION_6_ROOFLIGHTS) + worksheet outputs (LINE_83_M, LINE_84_M). The
+    test composes an EPC from the cert-shape windows and drives
+    solar_gains_from_cert at region 0 (UK-avg, the SAP rating pass).
+    (84) reconciles via the §5 LINE_73_M total internal gains plus our
+    new (83), checking both halves of the worksheet sum.
+    """
+    # Arrange
+    epc = _build_section_6_epc(fixture)
+
+    # Act
+    result = solar_gains_from_cert(
+        epc=epc,
+        region=0,
+        overshading=OvershadingCategory.AVERAGE,
+        roof_windows=fixture.SECTION_6_ROOF_WINDOWS,
+        rooflights=fixture.SECTION_6_ROOFLIGHTS,
+    )
+
+    # Assert
+    for m in range(12):
+        assert result.total_solar_gains_monthly_w[m] == pytest.approx(
+            fixture.LINE_83_M_TOTAL_SOLAR_W[m], abs=5e-3
+        ), f"(83) month {m+1}"
+        line_84 = (
+            fixture.LINE_73_M_TOTAL_INTERNAL_GAINS_W[m]
+            + result.total_solar_gains_monthly_w[m]
+        )
+        assert line_84 == pytest.approx(
+            fixture.LINE_84_M_TOTAL_GAINS_W[m], abs=5e-3
+        ), f"(84) month {m+1}"