diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index ec27d6fcb..e89062610 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -2966,8 +2966,9 @@ def _sap_17_1_meter_type(electricity_tariff: Optional[int]) -> str:
 
     The two code spaces *differ* (epc_codes.csv `energy_tariff` vs
     `_METER_INT_TO_TARIFF`): full-SAP 1=standard / 2=off-peak-7hr / 3=off-peak-
-    10hr / 4=24-hour, whereas RdSAP meter 1=dual-7hr / 2=single / 3=unknown /
-    4=24-hour. Passing the full-SAP code straight through (the prior bug) read a
+    10hr / 4=24-hour / 5=off-peak-18hr, whereas RdSAP meter 1=dual-7hr /
+    2=single / 3=unknown / 4=24-hour. Passing the full-SAP code straight
+    through (the prior bug) read a
     standard-tariff cert as Economy 7 (over-rated) and an Economy-7 cert as
     single (under-rated). Map onto the RdSAP word aliases so the resolved tariff
     is correct; absent/ND → "" (the unknown→standard sentinel)."""
@@ -2985,6 +2986,7 @@ _SAP_TARIFF_TO_RDSAP_METER_TYPE: dict[int, str] = {
     2: "dual",            # off-peak 7 hour (Economy 7)
     3: "dual",            # off-peak 10 hour (§12 dispatch resolves 7/10hr)
     4: "dual (24 hour)",  # 24-hour tariff
+    5: "off-peak 18 hour",  # off-peak 18 hour (Table 12a EIGHTEEN_HOUR)
 }
 
 
diff --git a/datatypes/epc/domain/tests/test_from_sap_schema.py b/datatypes/epc/domain/tests/test_from_sap_schema.py
index 7a682c8d7..957cc4a44 100644
--- a/datatypes/epc/domain/tests/test_from_sap_schema.py
+++ b/datatypes/epc/domain/tests/test_from_sap_schema.py
@@ -181,6 +181,7 @@ class TestFullSapElectricityTariffTranslation:
             (2, "SEVEN_HOUR"),       # off-peak 7-hour → Economy 7
             (3, "SEVEN_HOUR"),       # off-peak 10-hour → dual (§12 resolves 7/10hr)
             (4, "TWENTY_FOUR_HOUR"),  # 24-hour tariff (e.g. property 709874 — unchanged)
+            (5, "EIGHTEEN_HOUR"),    # off-peak 18-hour — was dropped to "" → STANDARD
             (None, "STANDARD"),      # absent/ND → unknown → standard
         ],
     )
@@ -869,6 +870,27 @@ class TestFullSapSchema16xRouting:
         assert epc.door_count == 1
         assert epc.dwelling_type == "Detached house"
 
+    def test_full_sap_16_x_electricity_tariff_is_translated_not_passed_through(
+        self,
+    ) -> None:
+        # A full-SAP 16.x cert lodges the SAP `energy_tariff` code space, not the
+        # RdSAP `meter_type` one. Routing it through the full-SAP mapper (via the
+        # assessment_type=SAP gate) must apply `_sap_17_1_meter_type`: tariff 1 =
+        # standard → STANDARD. Were it raw-passed to the RdSAP normaliser, code 1
+        # = "dual" → SEVEN_HOUR (the prime-lead over-rating bug this guards).
+        from domain.sap10_calculator.tables.table_12a import (
+            Tariff,
+            tariff_from_meter_type,
+        )
+
+        data = load("sap_16_0_full.json")  # electricity_tariff = 1 (standard)
+
+        epc = EpcPropertyDataMapper.from_api_response(data)
+
+        assert tariff_from_meter_type(epc.sap_energy_source.meter_type) is (
+            Tariff.STANDARD
+        )
+
     def test_reduced_16_x_cert_unaffected_by_full_sap_routing(self) -> None:
         # Arrange — a reduced 16.2 cert (assessment_type RdSAP) must stay on the
         # RdSAP path, keeping its top-level property_type.