fix(tariff): Unknown meter + storage/CPSU main → off-peak (§12)

Electric storage heaters (and CPSU) charge overnight and cannot run
economically on a single rate, so their presence is physical evidence the
dwelling is on an off-peak tariff. RdSAP 10 §12 (PDF p.62) applied Rules
1-4 only for a Dual meter; an "Unknown" (code 3) meter returned STANDARD
without consulting the heating type, so a cat-7 storage main billed its
overnight charge at the standard 13.19 p/kWh instead of the 7-hour low
rate (5.50 p/kWh) — ~2.4x too high → large under-rate.

Two coupled fixes:
- `rdsap_tariff_for_cert`: for an Unknown meter, infer the off-peak tariff
  from a Rule-1 CPSU (→10-hour) or Rule-2 storage (→7-hour) main; keep
  STANDARD otherwise. Direct-acting/room heaters/heat pumps (Rule 3) are
  NOT off-peak evidence (run on demand, exist on single-rate meters) so
  they stay STANDARD — billing them 100% at the low rate over-credits.
- `_fuel_cost` now resolves its tariff via the §12-aware `_rdsap_tariff`
  (not the raw `tariff_from_meter_type`), so the off-peak branch fires for
  these storage certs and the legacy scalar fields bill the low rate.

Mirrors `_is_off_peak_meter`'s existing Unknown+electric heuristic (which
already routes HW/secondary off-peak), closing the main-space-heating gap.
Meter-3 electric cluster: mean |err| 11.18 → 6.52, within-1.0 3 → 5 (cert
7336 -26.1 → -0.16, 0380 -19.9 → +1.0). Eval headline 44.9% → 45.0%, mean
|err| 1.82 → 1.76, mean signed -0.08 → +0.02. A few storage certs overshoot
(other residuals the standard rate was masking).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -6114,8 +6114,14 @@ def _fuel_cost(
     is the natural extension point for the Table 12a `_SH_HIGH_RATE_
     FRACTION` lookup + `Table12aSystem` mapping (deferred per slice 3
     docs `Q11` follow-ups)."""
-    meter_type = epc.sap_energy_source.meter_type
-    tariff = tariff_from_meter_type(meter_type)
+    # Use the §12-Rules-aware tariff (not the raw meter→tariff): it routes
+    # an "Unknown" (code 3) meter with an electric storage / heat-pump /
+    # room-heater main to its off-peak tariff (storage heaters can't run on
+    # a single rate), so the off-peak branch below fires and the legacy
+    # scalar fields bill the overnight charge at the low rate instead of
+    # the standard 13.19 p/kWh. A non-electric Unknown-meter dwelling still
+    # resolves STANDARD here, keeping the full §10a precompute.
+    tariff = _rdsap_tariff(epc)
     if tariff is not Tariff.STANDARD:
         # Off-peak path defers to the legacy scalar fuel-cost fields on
         # CalculatorInputs (the pre-§10a `_space_heating_fuel_cost_gbp_

domain/sap10_calculator/tables/table_12a.py

@@ -263,6 +263,21 @@ _RULE_3_TEN_HOUR_CODES: Final[frozenset[int]] = frozenset(
 )
 
 
+def _meter_is_unknown(meter_type: object) -> bool:
+    """True when the meter is the RdSAP "Unknown" sentinel (code 3 / the
+    "unknown" / "" / "3" string aliases) — the assessor did not record the
+    tariff. Distinct from Single (code 2), an explicit single-rate
+    lodgement. Mirrors `_is_off_peak_meter`'s code extraction so the main-
+    heating tariff inference stays consistent with the HW/secondary path."""
+    if isinstance(meter_type, bool):
+        return False
+    if isinstance(meter_type, int):
+        return meter_type == 3
+    if isinstance(meter_type, str):
+        return meter_type.strip().lower() in {"unknown", "3", ""}
+    return False
+
+
 def rdsap_tariff_for_cert(
     meter_type: object,
     *,
@@ -297,23 +312,46 @@ def rdsap_tariff_for_cert(
     TEN_HOUR, matching the worksheet's "10 Hour Off Peak" lodging.
     """
     base = tariff_from_meter_type(meter_type)
-    # Non-Dual meters resolve straight from the meter type.
-    if base is not Tariff.SEVEN_HOUR:
-        return base
     main_codes = {
         c for c in (main_1_sap_code, main_2_sap_code) if c is not None
     }
-    # Rule 1
-    if main_codes & _RULE_1_CPSU_CODES:
-        return Tariff.TEN_HOUR
-    # Rule 2 — checked BEFORE rule 3 per §12 ordering (storage takes
-    # precedence over the broader Rule 3 electric set).
-    if main_codes & _RULE_2_STORAGE_CODES:
-        return Tariff.SEVEN_HOUR
-    # Rule 3
-    if main_codes & _RULE_3_TEN_HOUR_CODES:
-        return Tariff.TEN_HOUR
-    if main_1_is_heat_pump_database or main_2_is_heat_pump_database:
-        return Tariff.TEN_HOUR
-    # Rule 4 — default
-    return Tariff.SEVEN_HOUR
+
+    def _rules_1_to_3() -> Optional[Tariff]:
+        """§12 Rules 1-3 — the explicit electric-system tariff matches.
+        Returns None when no electric storage / CPSU / heat-pump / room-
+        heater main is present (i.e. Rule 4 territory)."""
+        # Rule 1
+        if main_codes & _RULE_1_CPSU_CODES:
+            return Tariff.TEN_HOUR
+        # Rule 2 — checked BEFORE rule 3 per §12 ordering (storage takes
+        # precedence over the broader Rule 3 electric set).
+        if main_codes & _RULE_2_STORAGE_CODES:
+            return Tariff.SEVEN_HOUR
+        # Rule 3
+        if main_codes & _RULE_3_TEN_HOUR_CODES:
+            return Tariff.TEN_HOUR
+        if main_1_is_heat_pump_database or main_2_is_heat_pump_database:
+            return Tariff.TEN_HOUR
+        return None
+
+    # Dual meter — §12 Rules 1-4, where Rule 4 is the 7-hour default.
+    if base is Tariff.SEVEN_HOUR:
+        return _rules_1_to_3() or Tariff.SEVEN_HOUR
+    # "Unknown" meter (code 3): the assessor didn't record the tariff, but
+    # an electric CPSU (Rule 1) or STORAGE (Rule 2) main is physical
+    # evidence the dwelling is on an off-peak tariff — these charge
+    # overnight at the low rate and cannot run economically on a single
+    # rate, so the tariff is implied. Direct-acting electric / room heaters
+    # / heat pumps (Rule 3) are NOT off-peak evidence (they run on demand
+    # and exist on single-rate meters too), so they keep STANDARD here
+    # rather than being mis-billed 100% at the off-peak low rate. A
+    # non-electric main also keeps STANDARD (no Rule 4 default — Unknown
+    # must not force off-peak on a gas dwelling).
+    if _meter_is_unknown(meter_type):
+        if main_codes & _RULE_1_CPSU_CODES:
+            return Tariff.TEN_HOUR
+        if main_codes & _RULE_2_STORAGE_CODES:
+            return Tariff.SEVEN_HOUR
+        return Tariff.STANDARD
+    # Single (code 2) or any other explicit non-off-peak meter.
+    return base

tests/domain/sap10_calculator/test_table_12a.py

@@ -46,6 +46,53 @@ def test_dual_meter_electric_room_heater_resolves_to_ten_hour_tariff() -> None:
     assert rdsap_tariff_for_cert(1, main_1_sap_code=601) is Tariff.SEVEN_HOUR
 
 
+def test_unknown_meter_infers_off_peak_from_electric_storage_main() -> None:
+    # Arrange — RdSAP 10 §12 (PDF p.62). An "Unknown" meter (code 3) was
+    # not recorded by the assessor, but an electric STORAGE main (SAP
+    # 401-409, Rule 2) or CPSU (192, Rule 1) is physical evidence the
+    # dwelling is on an off-peak tariff — these charge overnight at the low
+    # rate and cannot run economically on a single rate. So infer the §12
+    # off-peak tariff rather than billing the overnight charge at the
+    # standard rate. Certs 7336/2080 (cat-7 storage, meter 3) under-rated
+    # ~25 SAP from standard-rate space heating.
+
+    # Act / Assert — storage (Rule 2) → 7-hour; CPSU (Rule 1) → 10-hour.
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=402) is Tariff.SEVEN_HOUR
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=192) is Tariff.TEN_HOUR
+
+
+def test_unknown_meter_does_not_infer_off_peak_for_room_heater_or_heat_pump() -> None:
+    # Arrange — direct-acting electric room heaters (Rule 3, SAP 691) and
+    # heat pumps run ON DEMAND and exist on single-rate meters too, so they
+    # are NOT evidence of an off-peak tariff. On an Unknown meter they keep
+    # STANDARD — billing them 100% at the off-peak low rate would
+    # over-credit (room heaters draw mostly at the high rate).
+
+    # Act / Assert
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=691) is Tariff.STANDARD
+    assert rdsap_tariff_for_cert(3, main_1_is_heat_pump_database=True) is Tariff.STANDARD
+
+
+def test_unknown_meter_with_non_electric_main_stays_standard() -> None:
+    # Arrange — an "Unknown" meter on a GAS-heated dwelling (SAP 102) has
+    # no off-peak evidence, so it must NOT pick up the Rule-4 Dual default
+    # (7-hour); it stays STANDARD. (The off-peak inference fires only when
+    # a Rule 1/2 storage/CPSU system is present.)
+
+    # Act / Assert
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=102) is Tariff.STANDARD
+    assert rdsap_tariff_for_cert(3, main_1_sap_code=None) is Tariff.STANDARD
+
+
+def test_single_meter_with_storage_stays_standard() -> None:
+    # Arrange — code 2 (Single) is an EXPLICIT single-rate lodgement, not
+    # "unknown", so it is NOT overridden even with a storage main: the
+    # off-peak inference is only for the Unknown (code 3) sentinel.
+
+    # Act / Assert
+    assert rdsap_tariff_for_cert(2, main_1_sap_code=402) is Tariff.STANDARD
+
+
 def test_tariff_enum_has_five_members() -> None:
     """Table 12a columns: standard (no off-peak split), 7-hour, 10-hour,
     18-hour, 24-hour. Worksheet-shape fidelity: TEN_HOUR is included for