diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index b08b718c..b2fa1e63 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -365,27 +365,39 @@ def _fuel_cost_gbp_per_kwh(
     return prices.unit_price_p_per_kwh(_main_fuel_code(main)) * _PENCE_TO_GBP
 
 
+# RdSAP energy_tariff enum (per datatypes/epc/domain/epc_codes.csv):
+#   1 = dual                  (off-peak / Economy-7)
+#   2 = Single                (standard tariff)
+#   3 = Unknown               (treated as Single — verified against
+#                              Elmhurst assessor software with one
+#                              real property; SAP assessor behavior is
+#                              to default Unknown tariffs to Single)
+#   4 = dual (24 hour)        (off-peak / 24h heating)
+#   5 = off-peak 18 hour      (off-peak)
+#
+# Different from the SAP-Schema enum which is 1=standard, 2=off-peak.
+# Our corpus is RdSAP so we use RdSAP codes.
+_RDSAP_OFF_PEAK_METER_CODES: Final[frozenset[int]] = frozenset({1, 4, 5})
+
+
 def _is_off_peak_meter(meter_type: object) -> bool:
-    """The cert's `sap_energy_source.meter_type` reports the electricity
-    tariff. SAP10 enum (verified empirically on the 250k corpus —
-    distribution: 75% type 2, 14% type 1, 11% type 3):
-      1 = Off-peak (Economy-7 / dual)
-      2 = Single (Standard)
-      3 = Off-peak (24h heating)
-    The transform.py docstring describes 1=Standard / 2=Off-peak but the
-    corpus distribution rules that out (75% on off-peak would not match
-    UK demographics). Per user guidance we trust whatever meter_type the
-    cert lodges, but the mapping is to-be-confirmed against the SAP10
-    schema definition."""
+    """True when the cert's `sap_energy_source.meter_type` lodges an
+    off-peak / dual-rate tariff per the RdSAP enum (codes 1, 4, 5).
+    Unknown (3) defaults to standard since that's the cert software's
+    behaviour. Trusts whatever meter_type the cert lodges regardless of
+    whether the heating system makes that tariff "natural" — per user
+    guidance the cert's lodged meter_type is the source of truth."""
     if meter_type is None:
         return False
     if isinstance(meter_type, int):
-        return meter_type != 2
+        return meter_type in _RDSAP_OFF_PEAK_METER_CODES
     if isinstance(meter_type, str):
         stripped = meter_type.strip().lower()
-        if stripped in {"single", "standard", "2"}:
-            return False
-        return stripped not in {"", "unknown", "not specified"}
+        if stripped in {"dual", "off-peak", "1", "4", "5"}:
+            return True
+        if stripped in {"dual (24 hour)", "off-peak 18 hour"}:
+            return True
+        return False
     return False