slice S-B8: extend E7 off-peak rate to HW for E7-tariff dwellings

When the main heating is electric storage / direct-electric (codes
191-196, 401-409, 421-425), the cert almost always carries an
Economy-7 tariff and the immersion HW cylinder runs on the off-peak
timer. Bill HW at the 7h-low rate (5.5 p/kWh) in that case, falling
back to the lower of {7h-low, water_heating_fuel rate} so we never
over-charge an HW fuel that's already cheaper than off-peak.

100-cert parity probe:
  MAE 4.90 → 4.66   (-0.24)
  bias -1.44 → -0.70 (over-correction halved)
  within ±3: 46% → 48%
  within ±5: 67% → 68%
  within ±10: 93% → 94%

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

packages/domain/src/domain/sap/rdsap/cert_to_inputs.py

@@ -383,10 +383,15 @@ def _space_heating_fuel_cost_gbp_per_kwh(main: Optional[MainHeatingDetail]) -> f
 def _hot_water_fuel_cost_gbp_per_kwh(
     main: Optional[MainHeatingDetail], water_heating_fuel: Optional[int]
 ) -> float:
-    """Hot water bills at the *water-heating* fuel's rate — distinct from
-    the main heating fuel for gas-heated dwellings whose DHW runs off an
-    electric immersion. Falls back to the main fuel when the cert
-    doesn't lodge a separate water fuel."""
+    """Hot water bills at the *water-heating* fuel's rate. Special case:
+    an E7-tariff dwelling (electric storage / direct-electric main
+    heating) running an electric immersion HW cylinder bills HW at the
+    7h-low rate too, since these households typically run the immersion
+    on the off-peak timer — RdSAP convention. Falls back to the main
+    fuel when the cert doesn't lodge a separate water fuel."""
+    is_e7 = _is_electric_storage_or_direct(main)
+    if is_e7 and (water_heating_fuel is None or fuel_unit_price_p_per_kwh(water_heating_fuel) > _E7_LOW_RATE_P_PER_KWH):
+        return _E7_LOW_RATE_P_PER_KWH * _PENCE_TO_GBP
     if water_heating_fuel is not None:
         return fuel_unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP
     return _fuel_cost_gbp_per_kwh(main)

packages/domain/src/domain/sap/rdsap/tests/test_cert_to_inputs.py

@@ -335,9 +335,11 @@ def test_electric_storage_heater_space_heating_at_off_peak_rate() -> None:
     # Act
     inputs = cert_to_inputs(epc)
 
-    # Assert
+    # Assert — RdSAP convention: when an E7 dwelling's HW runs on
+    # electric immersion, the immersion is presumed to be on the
+    # off-peak timer, so HW bills at the 7h-low rate too.
     assert inputs.space_heating_fuel_cost_gbp_per_kwh == 0.055
-    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.1319
+    assert inputs.hot_water_fuel_cost_gbp_per_kwh == 0.055
     assert inputs.other_fuel_cost_gbp_per_kwh == 0.1319