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)
 }