diff --git a/domain/modelling/generators/heating_recommendation.py b/domain/modelling/generators/heating_recommendation.py
index 410eb5e6..b10e1b76 100644
--- a/domain/modelling/generators/heating_recommendation.py
+++ b/domain/modelling/generators/heating_recommendation.py
@@ -64,6 +64,10 @@ _HHR_STORAGE_OVERLAY = HeatingOverlay(
     cylinder_insulation_type=1,
     cylinder_insulation_thickness_mm=120,
     cylinder_thermostat="Y",
+    # Single off-peak electric immersion — drives the SAP 10.2 Table 13 HW
+    # high-rate split (matches the relodged after-cert; without it the HW
+    # bills 100% at the low rate, +1.26 SAP over the reference).
+    immersion_heating_type=1,
     has_hot_water_cylinder=True,
     meter_type="Dual",
 )
diff --git a/domain/modelling/scoring/overlay_applicator.py b/domain/modelling/scoring/overlay_applicator.py
index c11285ca..d47d84a7 100644
--- a/domain/modelling/scoring/overlay_applicator.py
+++ b/domain/modelling/scoring/overlay_applicator.py
@@ -98,6 +98,7 @@ _SAP_HEATING_FIELDS: tuple[str, ...] = (
     "cylinder_insulation_type",
     "cylinder_insulation_thickness_mm",
     "cylinder_thermostat",
+    "immersion_heating_type",
 )
 _ENERGY_SOURCE_FIELDS: tuple[str, ...] = ("meter_type", "mains_gas")
 
diff --git a/domain/modelling/simulation.py b/domain/modelling/simulation.py
index 083f8898..7d951ac5 100644
--- a/domain/modelling/simulation.py
+++ b/domain/modelling/simulation.py
@@ -133,6 +133,11 @@ class HeatingOverlay:
     cylinder_insulation_type: Optional[Union[int, str]] = None
     cylinder_insulation_thickness_mm: Optional[int] = None
     cylinder_thermostat: Optional[str] = None
+    # The cylinder's immersion-heater arrangement (e.g. single off-peak
+    # immersion = 1). Drives the SAP 10.2 Table 13 HW high-rate fraction on
+    # off-peak tariffs — without it the calculator cannot resolve
+    # `immersion_single` and bills HW 100% at the low rate.
+    immersion_heating_type: Optional[Union[int, str]] = None
     # EpcPropertyData (top-level)
     has_hot_water_cylinder: Optional[bool] = None
     # sap_energy_source
diff --git a/tests/domain/modelling/test_heating_recommendation.py b/tests/domain/modelling/test_heating_recommendation.py
index 5e8e1576..81b9f692 100644
--- a/tests/domain/modelling/test_heating_recommendation.py
+++ b/tests/domain/modelling/test_heating_recommendation.py
@@ -67,6 +67,7 @@ def test_electric_storage_dwelling_yields_an_hhr_storage_bundle() -> None:
         cylinder_insulation_type=1,
         cylinder_insulation_thickness_mm=120,
         cylinder_thermostat="Y",
+        immersion_heating_type=1,
         has_hot_water_cylinder=True,
         meter_type="Dual",
     )