Bill secondary electric heating at the dwelling's §12 tariff, not the meter default 🟩

An electric room/panel-heater dwelling (main SAP code 691) on a Dual meter is a
10-hour tariff by RdSAP 10 §12 Rule 3, so its main heating bills the secondary's
identical electricity at a 0.50-blended 11.09 p/kWh. Its SECONDARY heating,
however, resolved its tariff from the bare meter string — where a Dual meter
defaults to 7-hour (`tariff_from_meter_type`) — and billed at the 7-hour
all-high-rate 15.29 p/kWh. The ~10% secondary fraction was over-priced by ~4 p/kWh,
under-rating SAP by ~1 point across the LRHA WAVE 3 code-691 cohort.

Route the dwelling's resolved `_rdsap_tariff(epc)` into `_secondary_fuel_cost_gbp_
per_kwh` / `_secondary_off_peak_rate_gbp_per_kwh`, exactly as the main-heating cost
path (`_space_heating_fuel_cost_gbp_per_kwh`) and the secondary CO2 path
(`_secondary_heating_co2_factor_kg_per_kwh`) already do — removing the internal
meter-only resolution that diverged from the dwelling's single §12 tariff.

Storage-heater dwellings (7-hour by §12) are unchanged — their secondary correctly
stays at 15.29 p (regression-pinned in test_cert_to_inputs). LRHA WAVE 3 within-0.5
49.5% → 52.4%, MAE 0.977; gov-API, Elmhurst RealCert, Guinness and Wythenshawe
corpora unaffected.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/documents_parser/tests/test_pashub_sap_accuracy_lrha_wave3.py

@@ -88,11 +88,19 @@ _MANIFEST_PATH = _FIXTURES_DIR / "manifest.json"
 # 461178278096, which lodges no main heating at all (`MissingMainFuelType`,
 # tolerated above pending a modelling decision). Floor/ceiling re-baselined to
 # the corrected cohort.
+# 2026-07-24 (accuracy tail audit): the secondary-heating COST path resolved its
+# tariff from the bare meter string (a Dual meter defaulting to 7-hour, all
+# high-rate) instead of the dwelling's SAP §12 tariff, so an electric room/panel-
+# heater dwelling (10-hour by §12) billed its secondary at 15.29 p/kWh instead of
+# the 11.09 p its main heating bills — under-rating SAP by ~1 point across the
+# code-691 dwellings. Routed the resolved `_rdsap_tariff` into the secondary cost
+# path (mirroring the CO2 path, which already did this) → within-0.5 49.5% →
+# 52.4%, MAE 0.977. Storage-heater dwellings (7-hour by §12) are unchanged.
 # Treat the constants as a regression tripwire, NOT a cohort accuracy figure;
 # re-baseline (coverage growth is not loosening) as further mapper fixes unblock
 # fixtures.
-_MIN_WITHIN_HALF: float = 0.49
-_MAX_SAP_MAE: float = 1.05
+_MIN_WITHIN_HALF: float = 0.52
+_MAX_SAP_MAE: float = 0.98
 
 _KNOWN_GAP_REASON = (
     "pashub `from_site_notes` mapper does not int-code a main-heating "
@@ -153,6 +161,23 @@ def _evaluate(deal_id: str) -> Outcome:
     )
 
 
+@pytest.mark.skipif(
+    "462024626384" not in _BY_ID, reason="fixture 462024626384 not in manifest"
+)
+def test_electric_panel_secondary_prices_at_dwelling_tariff() -> None:
+    """Regression: an electric room/panel-heater dwelling (main SAP code 691) on
+    a dual meter must bill its SECONDARY electric heating at the dwelling's SAP
+    §12 tariff (10-hour, Table 12a SH high-rate fraction 0.50 → ~11.09 p/kWh) —
+    the same rate as its main heating — not the 7-hour all-high-rate (~15.29
+    p/kWh) that the bare meter string defaulted to. The mispriced secondary
+    under-rated SAP by ~1.4 (fixture 462024626384 scored 44.5 vs PasHub's 46);
+    the CO2 path already resolves the tariff via `_rdsap_tariff`, the cost path
+    now matches it."""
+    outcome = _evaluate("462024626384")
+    assert outcome.diff is not None
+    assert outcome.diff < 0.5, f"secondary mis-priced: diff {outcome.diff:.2f}"
+
+
 @pytest.mark.skipif(not _FIXTURES, reason="no pashub_accuracy_lrha_wave3 fixtures/manifest")
 @pytest.mark.parametrize("deal_id", _IDS)
 def test_pashub_fixture_computes(deal_id: str) -> None:

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -3237,7 +3237,7 @@ def _secondary_efficiency(
     return seasonal_efficiency(code, None, None)
 
 
-def _secondary_off_peak_rate_gbp_per_kwh(meter_type: object) -> float:
+def _secondary_off_peak_rate_gbp_per_kwh(tariff: Tariff) -> float:
     """SAP 10.2 Table 12a Grid 1 (PDF p.191) blended rate for an electric
     secondary heater on an off-peak tariff. The secondary is a direct-
     acting electric room heater (RdSAP 10 §A.2.2 default), so it sits on
@@ -3247,12 +3247,14 @@ def _secondary_off_peak_rate_gbp_per_kwh(meter_type: object) -> float:
     rate. Worksheet evidence — simulated case 19 (242): "Space heating -
     secondary (1.00*15.29 + 0.00*5.50)" → all at the 7-hour HIGH rate.
 
-    Mirrors `_space_heating_fuel_cost_gbp_per_kwh`: the meter resolves to
-    a tariff (the `_is_off_peak_meter` Unknown-code-3 heuristic falls
-    through to 7-hour, as in `_off_peak_low_rate_gbp_per_kwh_via_meter_
-    heuristic`); 18-/24-hour tariffs (absent from the Grid 1 direct-acting
-    row) fall back to the tariff's Table 32 low rate."""
-    tariff = tariff_from_meter_type(meter_type)
+    Takes the dwelling's resolved SAP §12 tariff (`_rdsap_tariff`), the
+    SAME tariff the main heating bills on — a room-heater dwelling on a
+    Dual meter is 10-hour by §12 Rule 3, so its secondary must blend at
+    0.50 (11.09 p) like the main, not 1.00 (15.29 p). Resolving from the
+    bare meter string instead defaulted a Dual meter to 7-hour and
+    over-priced the secondary (mirrors the CO2 path, which already keys
+    on `_rdsap_tariff`). 18-/24-hour tariffs (absent from the Grid 1
+    direct-acting row) fall back to the tariff's Table 32 low rate."""
     if tariff is Tariff.STANDARD:
         tariff = Tariff.SEVEN_HOUR
     try:
@@ -3271,23 +3273,27 @@ def _secondary_fuel_cost_gbp_per_kwh(
     main: Optional[MainHeatingDetail],
     meter_type: object,
     prices: PriceTable,
+    tariff: Tariff,
 ) -> float:
     """Secondary fuel cost. When secondary_fuel_type is missing, default
     to portable-electric (code 30 standard electricity, or off-peak
     under E7-eligible meter). The cert's secondary is an electric room
-    heater per the §A.2.2 default."""
+    heater per the §A.2.2 default. `tariff` is the dwelling's resolved
+    SAP §12 tariff, so an off-peak secondary bills at the same tariff as
+    the main heating rather than a meter-string default (see
+    `_secondary_off_peak_rate_gbp_per_kwh`)."""
     sec_fuel = sap_heating.secondary_fuel_type
     if sec_fuel is None:
         # Default to electricity since the default secondary system is
         # portable electric heaters (code 693).
         if _is_off_peak_meter(meter_type, fuel_is_electric=True):
-            return _secondary_off_peak_rate_gbp_per_kwh(meter_type)
+            return _secondary_off_peak_rate_gbp_per_kwh(tariff)
         return prices.standard_electricity_p_per_kwh * _PENCE_TO_GBP
     # When secondary_fuel_type is electricity, apply off-peak if applicable.
     if _is_electric_water(sec_fuel) and _is_off_peak_meter(
         meter_type, fuel_is_electric=True
     ):
-        return _secondary_off_peak_rate_gbp_per_kwh(meter_type)
+        return _secondary_off_peak_rate_gbp_per_kwh(tariff)
     # Normalise colliding gov-API enum codes (e.g. 9 dual fuel, whose
     # value collides with Table-32 9 = LPG SC11F) before the price lookup,
     # exactly as the main-fuel boundary does — otherwise the same-value
@@ -8943,7 +8949,8 @@ def cert_to_inputs(
         secondary_heating_efficiency=secondary_efficiency_value,
         energy_requirements=energy_requirements_result,
         secondary_heating_fuel_cost_gbp_per_kwh=_secondary_fuel_cost_gbp_per_kwh(
-            epc.sap_heating, main, epc.sap_energy_source.meter_type, prices
+            epc.sap_heating, main, epc.sap_energy_source.meter_type, prices,
+            _rdsap_tariff(epc),
         ),
         space_heating_primary_factor=_main_heating_primary_factor(
             main, _rdsap_tariff(epc),

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -89,6 +89,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _pv_eligible_demand_monthly_kwh,  # pyright: ignore[reportPrivateUsage]
     _primary_loss_applies,  # pyright: ignore[reportPrivateUsage]
     _rdsap_extract_fans_default,  # pyright: ignore[reportPrivateUsage]
+    _rdsap_tariff,  # pyright: ignore[reportPrivateUsage]
     _pv_overshading_factor,  # pyright: ignore[reportPrivateUsage]
     _pv_pitch_deg,  # pyright: ignore[reportPrivateUsage]
     _responsiveness,  # pyright: ignore[reportPrivateUsage]
@@ -3185,6 +3186,7 @@ def test_dual_fuel_secondary_api_enum_9_prices_as_dual_fuel_not_lpg() -> None:
         gas_boiler_main,
         2,  # standard (single-rate) meter
         SAP_10_2_SPEC_PRICES,
+        _rdsap_tariff(dual_fuel_secondary_epc),
     )
     secondary_factor_code = _secondary_fuel_code(dual_fuel_secondary_epc)
 
@@ -4080,6 +4082,8 @@ def test_secondary_electric_off_peak_bills_at_table_12a_direct_acting_high_rate(
         main_heating_category=None,
         sap_main_heating_code=402,  # electric storage heaters
     )
+    from dataclasses import replace
+
     dual_meter_off_peak_epc = make_minimal_sap10_epc(
         total_floor_area_m2=_TYPICAL_TFA_M2,
         habitable_rooms_count=4,
@@ -4089,16 +4093,28 @@ def test_secondary_electric_off_peak_bills_at_table_12a_direct_acting_high_rate(
             # secondary_fuel_type omitted → §A.2.2 portable electric default
         ),
     )
+    # A storage-heater dwelling on a Dual meter resolves to the 7-hour tariff
+    # by SAP §12; set it on the cert so the cost path reads the dwelling's
+    # tariff (`_rdsap_tariff`), the same source the main heating bills on —
+    # NOT the bare meter string. (A room/panel-heater dwelling on a Dual meter
+    # is instead 10-hour, so its secondary blends 0.50 → 11.09 p, not 15.29.)
+    dual_meter_off_peak_epc = replace(
+        dual_meter_off_peak_epc,
+        sap_energy_source=replace(
+            dual_meter_off_peak_epc.sap_energy_source, meter_type="1"
+        ),
+    )
 
     # Act
     secondary_rate_gbp_per_kwh = _secondary_fuel_cost_gbp_per_kwh(
         dual_meter_off_peak_epc.sap_heating,
         storage_heater_main,
-        1,  # Dual meter → 7-hour off-peak tariff
+        dual_meter_off_peak_epc.sap_energy_source.meter_type,
         SAP_10_2_SPEC_PRICES,
+        _rdsap_tariff(dual_meter_off_peak_epc),
     )
 
-    # Assert — 1.00 × 15.29 p + 0.00 × 5.50 p = 15.29 p/kWh = £0.1529.
+    # Assert — storage main → 7-hour: 1.00 × 15.29 p + 0.00 × 5.50 p = £0.1529.
     assert abs(secondary_rate_gbp_per_kwh - 0.1529) <= 1e-6