diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 31709822..15f14149 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -566,6 +566,12 @@ _MV_DUCT_TYPE_RIGID: Final[int] = 2
 # 5, 6 = through-wall (use no-duct IUF independent of duct type)
 _MEV_THROUGH_WALL_CONFIG_CODES: Final[frozenset[int]] = frozenset({5, 6})
 
+# SAP 10.2 Table 4g (PDF p.176) / §2.6.3 note 1 — default specific fan
+# power (W per litre/sec) for an MEV system whose fan(s) are not in the
+# PCDB. Used directly as the IUF-adjusted SFPav in the (230a) formula
+# (SFPav × 1.22 × V) when no Table 322 record resolves.
+_TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S: Final[float] = 0.8
+
 
 def _mev_decentralised_kwh_per_yr_from_cert(epc: EpcPropertyData) -> float:
     """Compose the SAP 10.2 §5 Table 4f line (230a) MEV decentralised
@@ -601,11 +607,28 @@ def _mev_decentralised_kwh_per_yr_from_cert(epc: EpcPropertyData) -> float:
     decentralised fixture; the rule above fits cert 000565 alone.
     """
     pcdf_id = epc.mechanical_ventilation_index_number
-    if pcdf_id is None:
-        return 0.0
-    record = decentralised_mev_record(pcdf_id)
+    record = decentralised_mev_record(pcdf_id) if pcdf_id is not None else None
     if record is None:
-        return 0.0
+        # No PCDB Table 322 record (index absent, or lodged index not in
+        # the table). For a mechanical-EXTRACT system, SAP 10.2 §2.6.3 /
+        # Table 4g note 1 prescribes the default SFP (0.8 W/(l/s)) used
+        # directly as SFPav. Natural / balanced (MVHR / MV) systems
+        # contribute no (230a) decentralised-MEV fan electricity here —
+        # the gate mirrors `_has_balanced_mechanical_ventilation`'s
+        # MEV / PIV-from-outside vs balanced split. Closes the +2.2 SAP
+        # over-rate on the index-less MEV cohort (mostly gas houses), which
+        # previously billed zero fan electricity.
+        sv = epc.sap_ventilation
+        if (
+            sv is None
+            or sv.mechanical_ventilation_kind
+            != MechanicalVentilationKind.EXTRACT_OR_PIV_OUTSIDE.name
+        ):
+            return 0.0
+        return mev_decentralised_kwh_per_yr(
+            sfp_av_w_per_l_per_s=_TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,
+            dwelling_volume_m3=dimensions_from_cert(epc).volume_m3,
+        )
     iuf_record = mv_in_use_factors_record(_MEV_DECENTRALISED_SYSTEM_TYPE)
     if iuf_record is None:
         return 0.0
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 b10fb12f..516a5e9e 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -1032,6 +1032,49 @@ def test_ventilation_from_cert_routes_mev_decentralised_to_extract_or_piv_outsid
         assert abs(w25 - expected) <= 1e-4
 
 
+def test_index_less_mev_uses_table_4g_default_sfp_for_fan_electricity() -> None:
+    # Arrange — an MEV system with NO PCDB record (index absent / not in
+    # Table 322). SAP 10.2 §2.6.3 / Table 4g note 1 prescribes a default
+    # specific fan power of 0.8 W/(l/s), used directly as the SFPav in the
+    # §5 Table 4f line (230a) `SFPav × 1.22 × V`. Without it the cascade
+    # billed ZERO fan electricity for index-less MEV (the +2.2 SAP
+    # over-rate on the index-less MEV cohort, mostly gas houses). A natural
+    # dwelling contributes nothing.
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,  # pyright: ignore[reportPrivateUsage]
+        _mev_decentralised_kwh_per_yr_from_cert,  # pyright: ignore[reportPrivateUsage]
+        dimensions_from_cert,
+    )
+
+    base = _typical_semi_detached_epc()
+    mev_no_index = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, habitable_rooms_count=4,
+        region_code="1", sap_building_parts=base.sap_building_parts,
+        sap_windows=base.sap_windows, sap_heating=base.sap_heating,
+        sap_ventilation=SapVentilation(
+            mechanical_ventilation_kind="EXTRACT_OR_PIV_OUTSIDE"
+        ),
+    )
+    natural = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2, habitable_rooms_count=4,
+        region_code="1", sap_building_parts=base.sap_building_parts,
+        sap_windows=base.sap_windows, sap_heating=base.sap_heating,
+        sap_ventilation=SapVentilation(extract_fans_count=0),
+    )
+
+    # Act
+    mev_kwh = _mev_decentralised_kwh_per_yr_from_cert(mev_no_index)
+    natural_kwh = _mev_decentralised_kwh_per_yr_from_cert(natural)
+
+    # Assert — default SFP 0.8 × 1.22 × V for the index-less MEV; zero for
+    # the naturally-ventilated dwelling.
+    volume = dimensions_from_cert(mev_no_index).volume_m3
+    expected = _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S * 1.22 * volume
+    assert abs(mev_kwh - expected) <= 1e-6
+    assert mev_kwh > 0.0
+    assert natural_kwh == 0.0
+
+
 def test_open_chimneys_raise_infiltration_ach() -> None:
     # Arrange — Direction check: chimneys add Table 2.1 volume to the
     # infiltration calc, so an otherwise identical dwelling with 2 open