diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 93af44e6..a2b988c9 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -2183,6 +2183,41 @@ def _space_heating_fuel_cost_gbp_per_kwh(
     return blended * _PENCE_TO_GBP
 
 
+def _main_space_heating_high_rate_fraction(
+    main: Optional[MainHeatingDetail],
+    tariff: Tariff,
+) -> float:
+    """SAP 10.2 Appendix M1 §3a (PDF p.93) — the fraction of the main
+    space-heating fuel that is billed at the HIGH rate in Section 10a,
+    i.e. carries an "electricity not at the low-rate" fuel code (30, 32,
+    34, 35 or 38). Only this high-rate portion of E_space,m may enter the
+    PV-eligible demand D_PV,m; the low-rate portion (code 31/33/36/37/39)
+    is excluded.
+
+    Mirrors `_space_heating_fuel_cost_gbp_per_kwh`'s rate split exactly so
+    the D_PV inclusion and the §10a billing stay consistent:
+      - non-electric main, or STANDARD tariff → 1.0 (no off-peak split;
+        the eligible-code gate in `_pv_eligible_demand_monthly_kwh`
+        already excludes non-electric fuels, and a STANDARD-tariff
+        electric main bills 100% at code 30).
+      - electric main on an off-peak tariff whose Table 12a Grid 1 SH row
+        is wired → the published high-rate fraction. Electric STORAGE
+        heaters (Table 12a `_table_12a_system_for_main` → None, charged
+        wholly off-peak) and any system whose Grid 1 SH row is not yet
+        wired bill 100% at the low rate → fraction 0.0, so E_space,m is
+        excluded from D_PV entirely (worksheet (240) high-rate cost = 0).
+    """
+    if not _is_electric_main(main) or tariff is Tariff.STANDARD:
+        return 1.0
+    system = _table_12a_system_for_main(main)
+    if system is None:
+        return 0.0
+    try:
+        return space_heating_high_rate_fraction(system, tariff)
+    except NotImplementedError:
+        return 0.0
+
+
 def _hot_water_fuel_cost_gbp_per_kwh(
     water_heating_fuel: Optional[int],
     main: Optional[MainHeatingDetail],
@@ -2474,6 +2509,7 @@ def _pv_eligible_demand_monthly_kwh(
     main_fuel_code_table_12: Optional[int],
     secondary_fuel_code_table_12: Optional[int],
     water_heating_fuel_code_table_12: Optional[int],
+    main_space_high_rate_fraction: float = 1.0,
 ) -> tuple[float, ...]:
     """SAP 10.2 Appendix M1 §3a (p.93) — monthly PV-eligible demand
     D_PV,m. Always includes lighting + appliances + cooking + electric
@@ -2482,6 +2518,18 @@ def _pv_eligible_demand_monthly_kwh(
     (codes 30, 32, 34, 35, 38 per spec). Includes E_water,m only when
     the water heating fuel code is 30 (standard electricity) per spec.
 
+    `main_space_high_rate_fraction` scales the main-heating contribution
+    by the portion billed at the HIGH rate (code 30) in Section 10a.
+    Per the §3a inclusion rule "(211) should be included only where the
+    fuel code applied to it in Section 10a is 30, 32, 34, 35 or 38 (i.e.
+    electricity not at the low-rate)", off-peak electric mains (e.g.
+    storage heaters charged wholly at the low rate, fraction 0.0) must
+    NOT add their (211) to D_PV. Defaults to 1.0 → unchanged for
+    STANDARD-tariff electric mains and the gas-main / electric-secondary
+    cohort. Without this, off-peak storage-heater dwellings over-counted
+    D_PV by the full (211) in winter, inflating R_PV,m → β → the onsite
+    PV split (case 19: β_Jan 0.894 → 0.792, matching worksheet 0.791).
+
     Secondary space heating is included on the same footing as main:
     Appendix M1 §3a counts E_space,m as the dwelling's total electric
     space-heating demand, which for a gas-main / electric-secondary
@@ -2516,7 +2564,7 @@ def _pv_eligible_demand_monthly_kwh(
             + pumps_fans_monthly_kwh[m]
         )
         if include_main_space:
-            d += main_1_fuel_monthly_kwh[m]
+            d += main_space_high_rate_fraction * main_1_fuel_monthly_kwh[m]
         if include_secondary_space:
             d += secondary_fuel_monthly_kwh[m]
         if include_water:
@@ -6721,6 +6769,12 @@ def cert_to_inputs(
             )
             if epc.sap_heating.water_heating_fuel is not None else None
         ),
+        # SAP 10.2 Appendix M1 §3a — exclude the low-rate portion of an
+        # off-peak electric main from D_PV (the §10a high/low split that
+        # `_space_heating_fuel_cost_gbp_per_kwh` already bills).
+        main_space_high_rate_fraction=_main_space_heating_high_rate_fraction(
+            main, _rdsap_tariff(epc),
+        ),
     )
     pv_split = pv_split_monthly(
         epv_monthly_kwh=pv_monthly_kwh,
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index c0d9d685..bde4bb44 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -59,7 +59,9 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _is_electric_water,  # pyright: ignore[reportPrivateUsage]
     _is_off_peak_meter,  # pyright: ignore[reportPrivateUsage]
     _main_floor_u_value,  # pyright: ignore[reportPrivateUsage]
+    _main_space_heating_high_rate_fraction,  # pyright: ignore[reportPrivateUsage]
     _other_fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    _pv_eligible_demand_monthly_kwh,  # pyright: ignore[reportPrivateUsage]
     _primary_loss_applies,  # pyright: ignore[reportPrivateUsage]
     _rdsap_extract_fans_default,  # pyright: ignore[reportPrivateUsage]
     _pv_overshading_factor,  # pyright: ignore[reportPrivateUsage]
@@ -1766,6 +1768,90 @@ def test_other_fuel_cost_for_18_hour_tariff_uses_18_hour_high_rate() -> None:
     )
 
 
+def test_main_space_high_rate_fraction_zero_for_off_peak_storage_heaters() -> None:
+    # Arrange — SAP 10.2 Appendix M1 §3a (PDF p.93): E_space,m (211) is
+    # included in D_PV "only where the fuel code applied to it in Section
+    # 10a is 30, 32, 34, 35 or 38 (i.e. electricity not at the low-rate)".
+    # Electric STORAGE heaters (code 402) on a 7-hour off-peak tariff are
+    # charged wholly at the low rate (Table 12a Grid 1 SH fraction 0.00 /
+    # `_table_12a_system_for_main` → None) — worksheet (240) high-rate
+    # cost = 0 — so none of (211) may enter D_PV.
+    from domain.sap10_calculator.tables.table_12a import Tariff
+
+    storage_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2402,
+        main_heating_category=7,
+        sap_main_heating_code=402,
+    )
+
+    # Act
+    off_peak = _main_space_heating_high_rate_fraction(
+        storage_main, Tariff.SEVEN_HOUR
+    )
+    standard = _main_space_heating_high_rate_fraction(
+        storage_main, Tariff.STANDARD
+    )
+    gas = _main_space_heating_high_rate_fraction(
+        _gas_boiler_detail(sap_main_heating_code=102), Tariff.SEVEN_HOUR
+    )
+
+    # Assert
+    assert abs(off_peak - 0.0) <= 1e-9
+    # STANDARD tariff has no high/low split → 100% high rate.
+    assert abs(standard - 1.0) <= 1e-9
+    # Non-electric main never carries an off-peak split.
+    assert abs(gas - 1.0) <= 1e-9
+
+
+def test_pv_eligible_demand_excludes_low_rate_main_space_heating() -> None:
+    # Arrange — SAP 10.2 Appendix M1 §3a (PDF p.93). A main billed wholly
+    # at the low rate (high-rate fraction 0.0) must contribute zero to
+    # D_PV even though its Table-12 code (30) is in the eligible set; the
+    # secondary (also code 30) at its full high-rate fraction stays in.
+    main_1 = tuple(float(100 + m) for m in range(12))
+    secondary = tuple(float(10 + m) for m in range(12))
+    base = tuple(float(5) for _ in range(12))  # lighting et al.
+
+    # Act
+    excluded = _pv_eligible_demand_monthly_kwh(
+        lighting_monthly_kwh=base,
+        appliances_monthly_kwh=(0.0,) * 12,
+        cooking_monthly_kwh=(0.0,) * 12,
+        electric_shower_monthly_kwh=(0.0,) * 12,
+        pumps_fans_monthly_kwh=(0.0,) * 12,
+        main_1_fuel_monthly_kwh=main_1,
+        secondary_fuel_monthly_kwh=secondary,
+        hot_water_monthly_kwh=(0.0,) * 12,
+        main_fuel_code_table_12=30,
+        secondary_fuel_code_table_12=30,
+        water_heating_fuel_code_table_12=26,  # gas → no E_water
+        main_space_high_rate_fraction=0.0,
+    )
+    included = _pv_eligible_demand_monthly_kwh(
+        lighting_monthly_kwh=base,
+        appliances_monthly_kwh=(0.0,) * 12,
+        cooking_monthly_kwh=(0.0,) * 12,
+        electric_shower_monthly_kwh=(0.0,) * 12,
+        pumps_fans_monthly_kwh=(0.0,) * 12,
+        main_1_fuel_monthly_kwh=main_1,
+        secondary_fuel_monthly_kwh=secondary,
+        hot_water_monthly_kwh=(0.0,) * 12,
+        main_fuel_code_table_12=30,
+        secondary_fuel_code_table_12=30,
+        water_heating_fuel_code_table_12=26,
+        main_space_high_rate_fraction=1.0,
+    )
+
+    # Assert — excluded drops the full (211); secondary stays in both.
+    for m in range(12):
+        assert abs(excluded[m] - (base[m] + secondary[m])) <= 1e-9
+        assert abs(included[m] - (base[m] + secondary[m] + main_1[m])) <= 1e-9
+
+
 def test_is_off_peak_meter_recognises_bare_18_hour_lodging() -> None:
     # Arrange — RdSAP 10 §17 page 85 row 10-2 lodges 18-hour meter
     # as the bare "18-hour" or "18 Hour" form (Elmhurst Summary §14.2