diff --git a/datatypes/epc/domain/mapper.py b/datatypes/epc/domain/mapper.py
index f9f56143b..b9d93ecba 100644
--- a/datatypes/epc/domain/mapper.py
+++ b/datatypes/epc/domain/mapper.py
@@ -6093,6 +6093,11 @@ def _map_sap_heating(
                 condensing=main.condensing,
                 weather_compensator=main.weather_compensator,
                 central_heating_pump_age_str=main.central_heating_pump_age,
+                # PCDB product id → the appliance's SEDBUK efficiency source
+                # (`gas_oil_boiler_record`); 0 means no product lodged (#1563).
+                main_heating_index_number=(
+                    main.product_id if main.product_id > 0 else None
+                ),
             )
         ],
         has_fixed_air_conditioning=ventilation.has_fixed_air_conditioning,
diff --git a/datatypes/epc/domain/tests/test_from_site_notes.py b/datatypes/epc/domain/tests/test_from_site_notes.py
index fa9d0fb1b..21de94ee7 100644
--- a/datatypes/epc/domain/tests/test_from_site_notes.py
+++ b/datatypes/epc/domain/tests/test_from_site_notes.py
@@ -314,6 +314,35 @@ class TestWaterHeatingCoding:
             )
 
 
+class TestMainHeatingEfficiencySource:
+    """`from_site_notes` must carry the surveyed PCDB `product_id` onto
+    `main_heating_index_number`, so the calculator reads the actual appliance's
+    SEDBUK efficiency (`gas_oil_boiler_record`) instead of a generic Table-4b
+    default. The site note lodges the boiler by PCDB id (`summer_efficiency` is
+    always 0), so the index IS the efficiency source (issue #1563).
+    """
+
+    def _survey(self, **main: object) -> PasHubRdSapSiteNotes:
+        data = load("pashub_rdsap_site_notes_example1.json")
+        data["heating_and_hot_water"]["main_heating"].update(main)
+        return from_dict(PasHubRdSapSiteNotes, data)
+
+    def test_product_id_maps_to_index_number(self) -> None:
+        result = EpcPropertyDataMapper.from_site_notes(self._survey(product_id=18400))
+        assert (
+            result.sap_heating.main_heating_details[0].main_heating_index_number
+            == 18400
+        )
+
+    def test_zero_product_id_leaves_index_none(self) -> None:
+        # No PCDB product lodged (e.g. selected by efficiency) → no index.
+        result = EpcPropertyDataMapper.from_site_notes(self._survey(product_id=0))
+        assert (
+            result.sap_heating.main_heating_details[0].main_heating_index_number
+            is None
+        )
+
+
 class TestFromSiteNotesExample1:
     """
     Fixture: pashub_rdsap_site_notes_example1.json
@@ -666,6 +695,7 @@ class TestFromSiteNotesExample1:
                         condensing=True,
                         weather_compensator=False,
                         central_heating_pump_age_str="Unknown",
+                        main_heating_index_number=18400,  # PCDB product id → efficiency source
                     )
                 ],
                 has_fixed_air_conditioning=False,