fix(storage-heaters): Table 12a code-408 integrated-storage high-rate fraction

SAP 10.2 Table 12a Grid 1 (PDF p.191): electric storage heater SAP code 408
is an "Integrated (storage + direct-acting) system" with a 0.20 space-heating
high-rate fraction on a 7-hour tariff — NOT the 0.00 of "other storage
heaters". `_table_12a_system_for_main` returned None for all storage codes (an
explicit TODO), so code 408 fell back to the 100%-low-rate path and billed
space heating at the bare 7-hour low rate (5.50 p/kWh) — under-costing →
over-rating. Mapped cat-7 storage: 408 -> INTEGRATED_STORAGE_DIRECT (0.20),
others -> OTHER_STORAGE_HEATERS (0.00, unchanged behaviour). The enum +
fraction rows already existed; this only wires the dispatch, so the split
flows self-consistently to both the §10a cost and the Appendix-M1 D_PV
high-rate fraction.

Corpus: sap408 over-raters +14.6/+12.9/+12.7 -> +7.1/+5.1/+3.4 (two crossed
into within-0.5). Gauge 65.9% -> 66.1%, MAE 1.160 -> 1.128. Floor 0.63 -> 0.64
/ MAE ceiling 1.22 -> 1.18. Worksheet harness 47/47 0 diverge. The residual
+3..+7 is the "all other uses" 0.90 high-rate fraction (lighting/pumps/HW
still billed 100%-low on the off-peak legacy path) — the next slice.

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2408,7 +2408,8 @@ def _table_12a_system_for_main(
 
     Coverage as fixtures land:
     - ASHP / GSHP (codes 211-224, 521-524, PCDB index) — wired
-    - Storage heaters (401-409) — TODO
+    - Storage heaters (cat 7): 408 → INTEGRATED_STORAGE_DIRECT (0.20),
+      all others → OTHER_STORAGE_HEATERS (0.00) — wired
     - Underfloor heating (421-422) — TODO
     - Direct-acting electric (191) / CPSU (192) / electric storage
       boiler (193, 195) — TODO
@@ -2451,6 +2452,16 @@ def _table_12a_system_for_main(
         211 <= code <= 217 or 221 <= code <= 227 or 521 <= code <= 524
     ):
         return Table12aSystem.ASHP_OTHER
+    # Electric STORAGE heaters (RdSAP main_heating_category 7) — SAP 10.2
+    # Table 12a Grid 1 (PDF p.191). Code 408 is an "Integrated (storage +
+    # direct-acting) system" → 0.20 SH high-rate fraction at 7-hour; every
+    # other storage code is "Other storage heaters" → 0.00 (charged wholly
+    # off-peak, the same 100%-low-rate the None fallback already gave).
+    # Gated on `_is_electric_main` belt-and-braces (all callers pre-gate).
+    if main.main_heating_category == 7 and _is_electric_main(main):
+        if code == 408:
+            return Table12aSystem.INTEGRATED_STORAGE_DIRECT
+        return Table12aSystem.OTHER_STORAGE_HEATERS
     return None
 
 

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -624,6 +624,78 @@ def test_cylinder_size_inaccessible_code_5_off_peak_dual_immersion_uses_210l() -
     assert volume_l is not None and abs(volume_l - 210.0) <= 1e-9
 
 
+def test_integrated_storage_heater_408_bills_table_12a_grid1_high_rate_fraction() -> None:
+    # Arrange — electric storage heaters (cat 7), SAP code 408, on an
+    # off-peak 7-hour (dual / Economy-7) meter. SAP 10.2 Table 12a Grid 1
+    # (PDF p.191): code 408 is an "Integrated (storage + direct-acting)
+    # system" with a 0.20 space-heating high-rate fraction at 7-hour (NOT
+    # the 0.00 of "other storage heaters"). The scalar SH rate is therefore
+    # the blend 0.20 × high (15.29) + 0.80 × low (5.50) = 7.458 p/kWh.
+    from dataclasses import replace
+
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,  # electricity
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,  # electric storage heaters
+        sap_main_heating_code=408,
+    )
+    base = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part(construction_age_band="E")],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — blended scalar rate 0.20×15.29 + 0.80×5.50 = 7.458 p/kWh.
+    assert abs(inputs.space_heating_fuel_cost_gbp_per_kwh - 0.07458) <= 1e-9
+
+
+def test_non_integrated_storage_heater_bills_100_percent_low_rate() -> None:
+    # Arrange — same off-peak storage cert but SAP code 401 ("other storage
+    # heaters"): Table 12a Grid 1 gives a 0.00 high-rate fraction → the heat
+    # is charged wholly at the 7-hour low rate (5.50 p/kWh). Guards that the
+    # 408 integrated-system 0.20 fraction does NOT leak to other codes.
+    from dataclasses import replace
+
+    main = MainHeatingDetail(
+        has_fghrs=False,
+        main_fuel_type=29,
+        heat_emitter_type=1,
+        emitter_temperature=1,
+        main_heating_control=2401,
+        main_heating_category=7,
+        sap_main_heating_code=401,
+    )
+    base = make_minimal_sap10_epc(
+        total_floor_area_m2=_TYPICAL_TFA_M2,
+        habitable_rooms_count=4,
+        country_code="ENG",
+        sap_building_parts=[make_building_part(construction_age_band="E")],
+        sap_heating=make_sap_heating(main_heating_details=[main]),
+    )
+    epc = replace(
+        base,
+        sap_energy_source=replace(base.sap_energy_source, meter_type="1"),
+    )
+
+    # Act
+    inputs = cert_to_inputs(epc)
+
+    # Assert — 100% low rate, 5.50 p/kWh.
+    assert abs(inputs.space_heating_fuel_cost_gbp_per_kwh - 0.0550) <= 1e-9
+
+
 def test_heat_network_distribution_electricity_per_sap_10_2_appendix_c_3_2() -> None:
     # Arrange — heat-network main (Table 4a code 301 = community heating,
     # category 6). SAP 10.2 Appendix C §C3.2 (PDF p.51): distribution

tests/infrastructure/epc_client/test_sap_accuracy_corpus.py

@@ -41,10 +41,12 @@ _CORPUS = Path(
 )
 
 # Measured floors/ceilings over the fixed corpus at HEAD (1000 certs, 0 skips).
-# Current: SAP within-0.5 = 65.9%, SAP MAE = 1.160 (heat-network Table 4c(3)
-# flat-rate charging factor, this slice: community cluster 38% -> 62% within-0.5).
+# Current: SAP within-0.5 = 66.1%, SAP MAE = 1.128 (Table 12a Grid 1
+# integrated-storage code-408 0.20 high-rate fraction, this slice: sap408
+# over-rate +14.6/+12.9/+12.7 -> +7.1/+5.1/+3.4; prior slice was the
+# heat-network Table 4c(3) flat-rate charging factor).
 # CO2 MAE = 0.28 t/yr (signed +0.17 — a systematic over-estimate, see below).
-# PE  MAE = 14.7 kWh/m2/yr (signed +9.2).
+# PE  MAE = 14.7 kWh/m2/yr (signed +9.1).
 #
 # The SAP (cost) gauge is the optimised target — its floor/ceiling are TIGHT.
 # CO2 and PE are reported + LOOSELY guarded: both run ~+5% high vs the lodged
@@ -63,8 +65,8 @@ _CORPUS = Path(
 #      energy were 5% high; actual SAP bias is +0.145).
 # So closing demand over-estimates lifts BOTH the SAP gauge and PE/CO2; there is
 # no one-slice factor fix. RATCHET any ceiling up when a slice tightens it.
-_MIN_WITHIN_HALF_SAP = 0.63
-_MAX_SAP_MAE = 1.22
+_MIN_WITHIN_HALF_SAP = 0.64
+_MAX_SAP_MAE = 1.18
 _MAX_CO2_MAE_TONNES = 0.35      # t CO2 / yr vs co2_emissions_current
 _MAX_PE_PER_M2_MAE = 16.0       # kWh / m2 / yr vs energy_consumption_current