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 1e6dd84e..38df234e 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -383,10 +383,15 @@ def _space_heating_fuel_cost_gbp_per_kwh(main: Optional[MainHeatingDetail]) -> f
 def _hot_water_fuel_cost_gbp_per_kwh(
     main: Optional[MainHeatingDetail], water_heating_fuel: Optional[int]
 ) -> float:
-    """Hot water bills at the *water-heating* fuel's rate — distinct from
-    the main heating fuel for gas-heated dwellings whose DHW runs off an
-    electric immersion. Falls back to the main fuel when the cert
-    doesn't lodge a separate water fuel."""
+    """Hot water bills at the *water-heating* fuel's rate. Special case:
+    an E7-tariff dwelling (electric storage / direct-electric main
+    heating) running an electric immersion HW cylinder bills HW at the
+    7h-low rate too, since these households typically run the immersion
+    on the off-peak timer — RdSAP convention. Falls back to the main
+    fuel when the cert doesn't lodge a separate water fuel."""
+    is_e7 = _is_electric_storage_or_direct(main)
+    if is_e7 and (water_heating_fuel is None or fuel_unit_price_p_per_kwh(water_heating_fuel) > _E7_LOW_RATE_P_PER_KWH):
+        return _E7_LOW_RATE_P_PER_KWH * _PENCE_TO_GBP
     if water_heating_fuel is not None:
         return fuel_unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP
     return _fuel_cost_gbp_per_kwh(main)
diff --git a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
index c06ec41b..87e6c68a 100644
--- a/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py
@@ -335,9 +335,11 @@ def test_electric_storage_heater_space_heating_at_off_peak_rate() -> None:
     # Act
     inputs = cert_to_inputs(epc)
 
-    # Assert
+    # Assert — RdSAP convention: when an E7 dwelling's HW runs on
+    # electric immersion, the immersion is presumed to be on the
+    # off-peak timer, so HW bills at the 7h-low rate too.
     assert inputs.space_heating_fuel_cost_gbp_per_kwh == 0.055
-    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.1319
+    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.055
     assert inputs.other_fuel_cost_gbp_per_kwh == 0.1319