Assume a dual Economy-7 meter for all-electric room-heater dwellings 🟥

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/domain/epc/test_main_heating_system_overlay.py

@@ -46,6 +46,19 @@ def test_electric_room_heaters_overlay_the_direct_acting_room_heater_code() -> N
     assert simulation.heating.sap_main_heating_code == 691
 
 
+def test_electric_room_heaters_assume_a_dual_economy7_meter() -> None:
+    # A dwelling on electric room heaters is all-electric and realistically
+    # billed on Economy 7 (its immersion hot water charges overnight; §12 Rule 3
+    # gives the room heaters a 10-hour off-peak window). When the landlord names
+    # only the system, the coherent meter to assume is Dual — the §12 dispatch
+    # then applies the realistic high/low split, not a single-rate over-penalty.
+    simulation = main_heating_overlay_for("Electric room heaters", 0)
+
+    assert simulation is not None
+    assert simulation.heating is not None
+    assert simulation.heating.meter_type == "Dual"
+
+
 @pytest.mark.parametrize(
     ("main_heating_value", "code"),
     [