fix(tariff): map electric boilers 191/193-196 to their Table 12a Grid 1 rows

SAP 10.2 Table 4a electric boilers (PDF p.170) split across three distinct
Table 12a Grid 1 SH rows (PDF p.191), not one "direct-acting" family as the
stale TODO in `_table_12a_system_for_main` implied:

  - 191 Direct-acting electric boiler   -> "Direct-acting electric boiler (a)"
    row: 7-hour 0.90, 10-hour 0.50 (NOT the 1.00/0.50 "Other direct-acting
    electric heating" room-heater row).
  - 193/194/195/196 Electric dry core / water storage boiler -> "Electric dry
    core or water storage boiler" row: 7-hour 0.00 (charged wholly off-peak =
    100% low rate, identical to the None fallback).
  - 192 Electric CPSU -> Appendix F; left falling through to None (off-peak
    low) until the Appendix-F high-rate cascade is implemented.

The enum + fractions already existed in table_12a.py; only the code->enum
mapping was missing. Resolves the TODO and pins the spec-correct 0.00 for the
storage boilers so 195 can't be mis-"fixed" up to a direct-acting fraction.

Forward guard, 0 corpus impact: storage boilers already billed 100% low via
the None fallback, and all corpus 191 certs are on standard tariff (Table 12a
off-peak split never fires). Corpus gauge unchanged 73.3% / MAE 0.774.

Pin: test_electric_boilers_191_195_map_to_distinct_table_12a_grid1_rows.
pyright strict gate not run locally (pyright not installed in this container).

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

domain/sap10_calculator/rdsap/cert_to_inputs.py

@@ -2520,6 +2520,25 @@ _ELECTRIC_ROOM_HEATER_SAP_CODES: Final[frozenset[int]] = frozenset(
     {691, 692, 693, 694, 699}
 )
 
+# SAP 10.2 Table 4a electric boilers (PDF p.170, codes 191-196) → their
+# Table 12a Grid 1 SH rows (PDF p.191). NOTE the boiler families do NOT all
+# share a row — read the spec exactly:
+#   191  Direct-acting electric boiler        → "Direct-acting electric boiler
+#        (a)" row: 7-hour 0.90, 10-hour 0.50  (NOT the "Other direct-acting
+#        electric heating" 1.00/0.50 room-heater row).
+#   192  Electric CPSU                        → "Electric CPSU" row: Appendix F
+#        (no flat Table 12a fraction — left as a documented gap, see below).
+#   193/194  Electric dry core storage boiler → "Electric dry core or water
+#   195/196  Electric water storage boiler      storage boiler" row: 7-hour
+#        0.00 (charged wholly off-peak — identical to the 100%-low-rate the
+#        None fallback already gave; mapped EXPLICITLY so the spec-correct
+#        0.00 is pinned and can't be "fixed" up to a wrong direct-acting 1.00).
+_DIRECT_ACTING_ELECTRIC_BOILER_CODES: Final[frozenset[int]] = frozenset({191})
+_ELECTRIC_STORAGE_BOILER_CODES: Final[frozenset[int]] = frozenset(
+    {193, 194, 195, 196}
+)
+_ELECTRIC_CPSU_CODES: Final[frozenset[int]] = frozenset({192})
+
 
 def _table_12a_system_for_main(
     main: Optional[MainHeatingDetail],
@@ -2539,8 +2558,9 @@ def _table_12a_system_for_main(
     - 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
+    - Direct-acting electric boiler (191) → 0.90/0.50; electric storage
+      boilers (193/194/195/196) → 0.00 — wired
+    - Electric CPSU (192) — Appendix F high-rate cascade still TODO
     """
     if main is None:
         return None
@@ -2601,6 +2621,19 @@ def _table_12a_system_for_main(
         if code == 408:
             return Table12aSystem.INTEGRATED_STORAGE_DIRECT
         return Table12aSystem.OTHER_STORAGE_HEATERS
+    # Electric boilers (Table 4a codes 191-196) — resolve the misleading TODO
+    # that lumped them as one "direct-acting" family. They split across THREE
+    # distinct Table 12a Grid 1 rows (see `_DIRECT_ACTING_ELECTRIC_BOILER_CODES`
+    # et al). 191 alone is direct-acting (0.90/0.50); 193-196 are storage
+    # boilers (0.00 = 100% low, the spec-correct value the None fallback gave —
+    # so this is a forward guard, not a corpus mover); 192 CPSU needs Appendix F
+    # and is left to fall through to None (the off-peak-low fallback) until the
+    # Appendix-F high-rate cascade is implemented.
+    if code is not None and _is_electric_main(main):
+        if code in _DIRECT_ACTING_ELECTRIC_BOILER_CODES:
+            return Table12aSystem.DIRECT_ACTING_ELECTRIC_BOILER
+        if code in _ELECTRIC_STORAGE_BOILER_CODES:
+            return Table12aSystem.ELECTRIC_DRY_CORE_OR_WATER_STORAGE
     return None
 
 

tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py

@@ -65,6 +65,7 @@ from domain.sap10_calculator.rdsap.cert_to_inputs import (
     _TABLE_4G_DEFAULT_MEV_SFP_W_PER_L_PER_S,  # pyright: ignore[reportPrivateUsage]
     dimensions_from_cert,
     _table_12_factor_fuel_code,  # pyright: ignore[reportPrivateUsage]
+    _table_12a_system_for_main,  # pyright: ignore[reportPrivateUsage]
     _is_electric_main,  # pyright: ignore[reportPrivateUsage]
     _is_heat_network_electric_main,  # pyright: ignore[reportPrivateUsage]
     _is_electric_water,  # pyright: ignore[reportPrivateUsage]
@@ -911,6 +912,49 @@ def test_no_system_electric_heaters_assumed_code_699_bills_direct_acting_split()
     assert abs(inputs.space_heating_fuel_cost_gbp_per_kwh - 0.1109) <= 1e-9
 
 
+def test_electric_boilers_191_195_map_to_distinct_table_12a_grid1_rows() -> None:
+    # Arrange — SAP 10.2 Table 4a electric boilers (PDF p.170) do NOT share
+    # one Table 12a Grid 1 row (PDF p.191). Read exactly: code 191
+    # "Direct-acting electric boiler (a)" → 7-hour 0.90 / 10-hour 0.50 (its
+    # OWN row, NOT the 1.00/0.50 "Other direct-acting electric heating"
+    # room-heater row); code 195 "Electric water storage boiler" → the
+    # "Electric dry core or water storage boiler" row → 7-hour 0.00 (charged
+    # wholly off-peak = 100% low rate, identical to the None fallback). This
+    # pins the spec-correct 0.00 so 195 can't be mis-"fixed" up to a wrong
+    # direct-acting fraction.
+    from domain.sap10_calculator.tables.table_12a import (
+        Table12aSystem,
+        Tariff,
+        space_heating_high_rate_fraction,
+    )
+
+    def _electric_boiler_main(code: int) -> MainHeatingDetail:
+        return MainHeatingDetail(
+            has_fghrs=False,
+            main_fuel_type=29,  # electricity
+            heat_emitter_type=0,
+            emitter_temperature=1,
+            main_heating_control=2106,
+            main_heating_category=2,  # boiler
+            sap_main_heating_code=code,
+        )
+
+    # Act
+    direct_acting = _table_12a_system_for_main(_electric_boiler_main(191))
+    storage = _table_12a_system_for_main(_electric_boiler_main(195))
+
+    # Assert — distinct rows with their published fractions.
+    assert direct_acting is Table12aSystem.DIRECT_ACTING_ELECTRIC_BOILER
+    assert storage is Table12aSystem.ELECTRIC_DRY_CORE_OR_WATER_STORAGE
+    assert (
+        space_heating_high_rate_fraction(direct_acting, Tariff.SEVEN_HOUR) == 0.90
+    )
+    assert (
+        space_heating_high_rate_fraction(direct_acting, Tariff.TEN_HOUR) == 0.50
+    )
+    assert space_heating_high_rate_fraction(storage, Tariff.SEVEN_HOUR) == 0.00
+
+
 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