diff --git a/domain/epc/property_overlays/main_heating_system_overlay.py b/domain/epc/property_overlays/main_heating_system_overlay.py
index 904a774bf..466466594 100644
--- a/domain/epc/property_overlays/main_heating_system_overlay.py
+++ b/domain/epc/property_overlays/main_heating_system_overlay.py
@@ -71,6 +71,13 @@ _ASSUMED_DUAL_METER_CODES = OFF_PEAK_IMPLYING_HEATING_CODES | _ROOM_HEATER_CODES
 # systems that take a charge control.
 _MANUAL_CHARGE_CONTROL = 2401
 _STORAGE_HEATER_CODES = frozenset(range(401, 410))
+# SAP Table 4a "electric storage heaters" category. Set on a storage-system
+# override so the overlaid cert is internally consistent (code 401-409 ↔ category
+# 7) — mirroring the control/fuel/meter companions the overlay already drags. A
+# storage code with a stale non-storage category is a latent trap: the SAP 10.2
+# Table 12a resolver classifies by category, so a leftover room-heater category
+# 10 would price the off-peak storage heating at the peak rate.
+_STORAGE_HEATER_CATEGORY = 7
 # High heat retention storage heaters (SAP Table 4a 409) take a SINGLE intrinsic
 # charge control — Table 4e 2404 (HHR, 0 C adjustment). It is named by the
 # archetype, not unobserved, so 409 drags 2404 rather than the manual default
@@ -266,10 +273,17 @@ def _control_for(code: int) -> Optional[int]:
 
 def _category_for(code: int) -> Optional[int]:
     """The SAP Table 4a heating category a code implies, where the archetype
-    fixes it. Room heaters are category 10 — set so a system-only override reads
-    as a room-heater system, not whatever category the replaced system carried."""
+    fixes it. Set so a system-only override reads as the new system's category,
+    not whatever category the replaced system carried — room heaters are 10,
+    electric storage heaters (incl. HHR 409) are 7, heat pumps 4, heat networks
+    6. Setting storage → 7 keeps the overlaid cert consistent (code 401-409 ↔
+    category 7); without it a storage override left a stale category behind,
+    which the SAP 10.2 Table 12a resolver (keyed on category) could misread as a
+    non-storage system and price the off-peak storage heating at the peak rate."""
     if code in _CATEGORY_10_ROOM_HEATER_CODES:
         return _ROOM_HEATER_CATEGORY
+    if code in _STORAGE_HEATER_CODES:
+        return _STORAGE_HEATER_CATEGORY
     if code in _HEAT_PUMP_CODES:
         return _HEAT_PUMP_CATEGORY
     if code in _HEAT_NETWORK_CODES:
diff --git a/tests/domain/epc/test_main_heating_system_overlay.py b/tests/domain/epc/test_main_heating_system_overlay.py
index 9f83c9b3f..dae992cba 100644
--- a/tests/domain/epc/test_main_heating_system_overlay.py
+++ b/tests/domain/epc/test_main_heating_system_overlay.py
@@ -229,6 +229,9 @@ def test_high_heat_retention_storage_drags_its_intrinsic_companions() -> None:
     assert simulation.heating.main_heating_control == 2404
     assert simulation.heating.main_fuel_type == 29
     assert simulation.heating.meter_type == "Dual"
+    # Storage category (7) — consistent with storage code 409, so the SAP 10.2
+    # Table 12a resolver prices the off-peak storage heating off-peak, not peak.
+    assert simulation.heating.main_heating_category == 7
 
 
 @pytest.mark.parametrize(
@@ -288,6 +291,9 @@ def test_storage_heaters_drag_along_conservative_manual_charge_control(
     assert simulation is not None
     assert simulation.heating is not None
     assert simulation.heating.main_heating_control == 2401
+    # ...and the storage category (7), consistent with the storage code, so the
+    # SAP 10.2 Table 12a resolver prices these off-peak, not at the peak rate.
+    assert simulation.heating.main_heating_category == 7
 
 
 def test_direct_acting_electric_does_not_drag_along_a_control() -> None: