diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 58a08efd..93af44e6 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -2138,6 +2138,16 @@ def _table_12a_system_for_main(
         main.main_heating_index_number is not None
         and heat_pump_record(main.main_heating_index_number) is not None
     )
+    # Electric room heaters (RdSAP main_heating_category 10) are direct-
+    # acting electric → SAP 10.2 Table 12a Grid 1 (PDF p.191) "Other
+    # systems including direct-acting electric" row (7-hour high-rate
+    # fraction 1.00, 10-hour 0.50). Distinct from electric STORAGE
+    # heaters (category 7), which charge off-peak and correctly fall
+    # through to None here (→ 100% low rate). Gated on `_is_electric_main`
+    # so a non-electric room heater (gas / solid-fuel cat 10) is excluded;
+    # all callers already pre-gate on electric, this is belt-and-braces.
+    if main.main_heating_category == 10 and _is_electric_main(main):
+        return Table12aSystem.OTHER_DIRECT_ACTING_ELECTRIC
     # ASHP — Table 4a rows 211-217 (earlier generations) + 221-227
     # (2013+) cover the air-source space. Warm-air ASHPs are 521-524.
     if code is not None and (
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 fc95112c..c0d9d685 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -2938,6 +2938,55 @@ def test_space_heating_off_peak_fallback_uses_actual_tariff_low_rate_not_e7() ->
     assert abs(cost_eighteen_hour - 0.0741) <= 1e-6
 
 
+def test_space_heating_electric_room_heater_off_peak_bills_at_direct_acting_high_rate() -> None:
+    # Arrange — an ELECTRIC room heater (RdSAP main_heating_category 10,
+    # e.g. SAP code 691) is direct-acting electric, so SAP 10.2 Table 12a
+    # Grid 1 (PDF p.191) puts it on the "Other systems including direct-
+    # acting electric" row: 7-hour high-rate fraction 1.00, 10-hour 0.50.
+    # Unlike STORAGE heaters (category 7), which charge off-peak and so
+    # correctly bill 100% at the low rate, a room heater runs on demand —
+    # mostly at the HIGH rate. `_table_12a_system_for_main` only mapped
+    # ASHP, so a room heater fell through to the "100% low-rate" fallback
+    # (5.50 p, £0.0550), under-charging space heating by ~9.79 p/kWh and
+    # systematically OVER-rating the cat-10 cluster (1,000-cert API sample:
+    # 48 certs, mean |err| 9.49, signed +5.08). The fix maps electric
+    # cat-10 mains to OTHER_DIRECT_ACTING_ELECTRIC. Mirror of S0380.228
+    # (which fixed the same fallback for electric SECONDARY heating).
+    from domain.sap10_calculator.tables.table_12a import Tariff
+    electric_room_heater_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=30,  # standard electricity
+        heat_emitter_type=2,
+        emitter_temperature=1,
+        main_heating_control=2602,
+        main_heating_category=10,  # electric room heaters
+        sap_main_heating_code=691,
+    )
+    gas_room_heater_main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=1,  # mains gas — NOT electric
+        heat_emitter_type=2,
+        emitter_temperature=1,
+        main_heating_control=2602,
+        main_heating_category=10,  # gas room heater (also cat 10)
+        sap_main_heating_code=631,
+    )
+
+    # Act — 7-hour off-peak tariff.
+    electric_rate = _space_heating_fuel_cost_gbp_per_kwh(
+        electric_room_heater_main, Tariff.SEVEN_HOUR, prices=SAP_10_2_SPEC_PRICES,
+    )
+    gas_rate = _space_heating_fuel_cost_gbp_per_kwh(
+        gas_room_heater_main, Tariff.SEVEN_HOUR, prices=SAP_10_2_SPEC_PRICES,
+    )
+
+    # Assert — electric room heater: 1.00 × 15.29 p = £0.1529 (high rate).
+    # Gas room heater is unaffected (non-electric → single Table 32 rate,
+    # not the off-peak electric split).
+    assert abs(electric_rate - 0.1529) <= 1e-6
+    assert abs(gas_rate - 0.0550) > 1e-6
+
+
 def test_heat_network_dlf_full_table_12c_age_band_coverage() -> None:
     # Arrange — SAP 10.2 Table 12c (page 193) heat-network Distribution
     # Loss Factor by dwelling age band A..M. None → K-or-newer