diff --git a/scripts/backfill_boiler_efficiency_band.py b/scripts/backfill_boiler_efficiency_band.py
new file mode 100644
index 000000000..2d95309dd
--- /dev/null
+++ b/scripts/backfill_boiler_efficiency_band.py
@@ -0,0 +1,175 @@
+"""Backfill the ``boiler_efficiency_band`` override for already-ingested boilers.
+
+The SEDBUK band (ADR-0068) is losslessly present in every ``main_heating_system``
+override's ``original_spreadsheet_description`` (``Boiler: C rated Combi``) but was
+dropped at classification. This one-off populates the new ``boiler_efficiency_band``
+override rows for the existing stock by re-parsing that text with the SAME guard the
+live classifier uses (``boiler_efficiency_band_guard``), so the backfill and the
+forward path cannot drift.
+
+One ``boiler_efficiency_band`` row per ``main_heating_system`` row whose description
+carries a band (``A``–``G``), keyed to the same ``(property_id, building_part)``.
+Descriptions with no band (a plain boiler, a non-boiler heating system) get no row.
+The modelling gate (gas/oil boilers only, ADR-0068) is applied at overlay time, not
+here — so a band parsed off an electric ``Boiler: A rated NA`` is stored for
+fidelity but ignored by the calculator, exactly as the live classifier stores it.
+
+GATED: the ``boiler_efficiency_band`` value of the FE-owned ``override_component``
+pgEnum must exist first (the Drizzle migration in the assessment-model repo). Until
+then this writes nothing in prod — the Class-A/B deferred pattern.
+
+DRY-RUN BY DEFAULT: prints the counts it would write and writes nothing. Pass
+``--apply`` to execute inside a transaction. Idempotent — a row already carrying the
+target band is left untouched, so re-running is a no-op.
+"""
+
+from __future__ import annotations
+
+import argparse
+from collections.abc import Iterable
+from dataclasses import dataclass
+
+from sqlalchemy import Connection, text
+
+from domain.epc.property_overrides.boiler_efficiency_band import BoilerEfficiencyBand
+from domain.epc.property_overrides.boiler_efficiency_band_guard import (
+    boiler_efficiency_band_guard,
+)
+from scripts.e2e_common import build_engine, load_env
+
+_OVERRIDE_COMPONENT = "boiler_efficiency_band"
+
+
+@dataclass(frozen=True)
+class MainHeatingRow:
+    """The fields of a ``main_heating_system`` ``property_overrides`` row the
+    backfill reads."""
+
+    property_id: int
+    portfolio_id: int
+    building_part: int
+    original_spreadsheet_description: str
+
+
+@dataclass(frozen=True)
+class BandBackfillRow:
+    """A ``boiler_efficiency_band`` override row to upsert for a boiler."""
+
+    property_id: int
+    portfolio_id: int
+    building_part: int
+    override_value: str
+    original_spreadsheet_description: str
+
+
+def band_backfill_rows(rows: Iterable[MainHeatingRow]) -> list[BandBackfillRow]:
+    """The ``boiler_efficiency_band`` rows to write for a set of
+    ``main_heating_system`` rows — one per row whose description carries a
+    parseable SEDBUK band, keyed to the same property + building part. Rows with
+    no band (or the ``UNKNOWN`` sentinel) produce nothing.
+
+    Pure and DB-free, so the parse/decide logic is unit-tested without a
+    database; the connection wrapper below just feeds it rows and upserts."""
+    backfill: list[BandBackfillRow] = []
+    for row in rows:
+        band = boiler_efficiency_band_guard(row.original_spreadsheet_description)
+        if band is None or band is BoilerEfficiencyBand.UNKNOWN:
+            continue
+        backfill.append(
+            BandBackfillRow(
+                property_id=row.property_id,
+                portfolio_id=row.portfolio_id,
+                building_part=row.building_part,
+                override_value=band.value,
+                original_spreadsheet_description=row.original_spreadsheet_description,
+            )
+        )
+    return backfill
+
+
+_SELECT_MAIN_HEATING = text(
+    """
+    SELECT property_id, portfolio_id, building_part, original_spreadsheet_description
+    FROM property_overrides
+    WHERE override_component = 'main_heating_system'
+    """
+)
+# Idempotent upsert on the (property, component, part) unique constraint — a row
+# already carrying the target band is left untouched (updated_at unchanged).
+_UPSERT_BAND = text(
+    """
+    INSERT INTO property_overrides
+        (id, property_id, portfolio_id, building_part, override_component,
+         override_value, original_spreadsheet_description, created_at, updated_at)
+    VALUES
+        (gen_random_uuid(), :property_id, :portfolio_id, :building_part,
+         'boiler_efficiency_band', :override_value, :original_spreadsheet_description,
+         now(), now())
+    ON CONFLICT (property_id, override_component, building_part)
+    DO UPDATE SET override_value = EXCLUDED.override_value,
+                  original_spreadsheet_description =
+                      EXCLUDED.original_spreadsheet_description,
+                  updated_at = now()
+    WHERE property_overrides.override_value <> EXCLUDED.override_value
+    """
+)
+
+
+def backfill(conn: Connection, *, apply: bool) -> int:
+    """Backfill the ``boiler_efficiency_band`` rows off the existing
+    ``main_heating_system`` descriptions. Returns the number of band rows the run
+    writes (or would write, in dry-run)."""
+    rows = [
+        MainHeatingRow(
+            property_id=r.property_id,
+            portfolio_id=r.portfolio_id,
+            building_part=r.building_part,
+            original_spreadsheet_description=r.original_spreadsheet_description or "",
+        )
+        for r in conn.execute(_SELECT_MAIN_HEATING)
+    ]
+    to_write = band_backfill_rows(rows)
+    if apply:
+        for band_row in to_write:
+            conn.execute(
+                _UPSERT_BAND,
+                {
+                    "property_id": band_row.property_id,
+                    "portfolio_id": band_row.portfolio_id,
+                    "building_part": band_row.building_part,
+                    "override_value": band_row.override_value,
+                    "original_spreadsheet_description": (
+                        band_row.original_spreadsheet_description
+                    ),
+                },
+            )
+    return len(to_write)
+
+
+def main() -> None:
+    load_env()
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--apply",
+        action="store_true",
+        help="execute the writes (default: dry-run, writes nothing)",
+    )
+    args = parser.parse_args()
+
+    engine = build_engine()
+    with engine.begin() as conn:
+        conn.execute(text("SET statement_timeout = 120000"))
+        total = backfill(conn, apply=args.apply)
+
+    verb = "backfilled" if args.apply else "would backfill"
+    print(
+        f"{verb} {total} boiler_efficiency_band override row(s) from the existing "
+        "main_heating_system descriptions (Boiler: <A-G> rated ...). The modelling "
+        "gate (gas/oil boilers only) is applied at overlay time (ADR-0068)."
+    )
+    if not args.apply:
+        print("\nDRY-RUN — nothing written. Re-run with --apply to execute.")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tests/scripts/test_backfill_boiler_efficiency_band.py b/tests/scripts/test_backfill_boiler_efficiency_band.py
new file mode 100644
index 000000000..c94220201
--- /dev/null
+++ b/tests/scripts/test_backfill_boiler_efficiency_band.py
@@ -0,0 +1,54 @@
+from __future__ import annotations
+
+from scripts.backfill_boiler_efficiency_band import (
+    BandBackfillRow,
+    MainHeatingRow,
+    band_backfill_rows,
+)
+
+
+def _row(desc: str, *, property_id: int = 1, building_part: int = 0) -> MainHeatingRow:
+    return MainHeatingRow(
+        property_id=property_id,
+        portfolio_id=796,
+        building_part=building_part,
+        original_spreadsheet_description=desc,
+    )
+
+
+def test_backfills_a_band_row_per_boiler_with_a_parseable_band() -> None:
+    rows = [
+        _row("Boiler: D rated Regular Boiler", property_id=10),
+        _row("Boiler: G rated Combi", property_id=11, building_part=1),
+    ]
+
+    result = band_backfill_rows(rows)
+
+    assert result == [
+        BandBackfillRow(10, 796, 0, "D", "Boiler: D rated Regular Boiler"),
+        BandBackfillRow(11, 796, 1, "G", "Boiler: G rated Combi"),
+    ]
+
+
+def test_leaves_descriptions_without_a_band_untouched() -> None:
+    rows = [
+        _row("Gas boiler"),
+        _row("Community Heating Systems: Community boilers only (RdSAP)"),
+        _row(""),
+    ]
+
+    assert band_backfill_rows(rows) == []
+
+
+def test_uses_the_same_guard_as_the_live_path_for_multi_system_and_electric() -> None:
+    rows = [
+        # Multi-system: primary (system 1) band wins — matches the guard.
+        _row("Boiler: A rated Combi, System 2: Boiler: C rated Combi", property_id=20),
+        # Band letter on an electric "NA" boiler is stored for fidelity; the
+        # gas/oil modelling gate ignores it at overlay time (ADR-0068).
+        _row("Boiler: A rated NA", property_id=21),
+    ]
+
+    result = band_backfill_rows(rows)
+
+    assert [(r.property_id, r.override_value) for r in result] == [(20, "A"), (21, "A")]