§5 slice 4: (72) water_heating_gains_monthly_w — bridge from §4 (65)m

Pure unit conversion: G_WH,m = 1000 × (65)m / (n_m × 24). The §4
heat_gains_from_water_heating_monthly_kwh output already encodes the
25%/80% spec-recovery factors for delivered-heat vs pipe-side losses;
this bridge just lands the kWh/month into watts for the §5 sum.

Verified against Elmhurst U985-0001-000490 (72)m row — exact to 4 d.p.

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

packages/domain/src/domain/sap/worksheet/internal_gains.py

@@ -32,6 +32,9 @@ from typing import Final, Optional
 
 
 _MONTHS_IN_YEAR: Final[int] = 12
+_DAYS_PER_MONTH: Final[tuple[int, ...]] = (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31)
+_HOURS_PER_DAY: Final[float] = 24.0
+_KWH_TO_WH: Final[float] = 1000.0
 _METABOLIC_GAIN_W_PER_OCCUPANT_COL_A: Final[float] = 60.0
 _LOSSES_W_PER_OCCUPANT_COL_A: Final[float] = -40.0
 _COOKING_W_BASE_COL_A: Final[float] = 35.0
@@ -70,6 +73,23 @@ def cooking_monthly_w(*, n_occupants: float) -> tuple[float, ...]:
     return tuple(gain for _ in range(_MONTHS_IN_YEAR))
 
 
+def water_heating_gains_monthly_w(
+    *,
+    heat_gains_from_water_heating_monthly_kwh: tuple[float, ...],
+) -> tuple[float, ...]:
+    """SAP 10.2 §5 line (72) — water-heating contribution to internal gains.
+
+    Table 5 row "Water heating": G_WH,m = 1000 × (65)m / (n_m × 24). Pure
+    unit conversion from §4 line (65)m (kWh/month) to watts. (65)m itself
+    already encodes the 25%/80% spec-recovery factors for delivered-heat
+    vs pipe-side losses (see §4 heat_gains_from_water_heating_monthly_kwh).
+    """
+    return tuple(
+        kwh * _KWH_TO_WH / (days * _HOURS_PER_DAY)
+        for kwh, days in zip(heat_gains_from_water_heating_monthly_kwh, _DAYS_PER_MONTH)
+    )
+
+
 _METABOLIC_W_PER_OCCUPANT: Final[float] = 60.0
 
 

packages/domain/src/domain/sap/worksheet/tests/test_internal_gains.py

@@ -16,6 +16,7 @@ from domain.sap.worksheet.internal_gains import (
     cooking_monthly_w,
     losses_monthly_w,
     metabolic_monthly_w,
+    water_heating_gains_monthly_w,
 )
 
 
@@ -87,3 +88,36 @@ def test_cooking_gains_match_table_5_col_a_formula() -> None:
     assert len(monthly) == 12
     for m, value in enumerate(monthly):
         assert value == pytest.approx(35.0 + 7.0 * n, abs=1e-9), f"month {m+1}"
+
+
+def test_water_heating_gains_bridge_converts_kwh_per_month_to_watts() -> None:
+    """SAP 10.2 §5 line (72) — water-heating contribution to internal gains.
+
+    Table 5 row "Water heating": G_WH,m = 1000 × (65)m / (n_m × 24) where
+    (65)m is the §4 line "Heat gains from water heating" in kWh/month
+    and n_m is days in month m. Pure unit conversion: kWh/month → W.
+
+    Verified against the Elmhurst U985-0001-000490 worksheet:
+        (65) Jan = 75.2034 kWh/month
+        (72) Jan = 75.2034 × 1000 / (24 × 31) = 101.0798 W
+    matches the worksheet (72)m row to 4 d.p.
+    """
+    # Arrange — 000490's full LINE_65_M tuple feeds the bridge.
+    heat_gains_kwh = (
+        75.2034, 66.4381, 70.4305, 61.5896, 59.4711, 53.3391,
+        52.4705, 54.6991, 55.4582, 62.1383, 66.4342, 74.3403,
+    )
+    expected_w = (
+        101.0798, 98.8663, 94.6647, 85.5412, 79.9343, 74.0821,
+        70.5249, 73.5203, 77.0253, 83.5192, 92.2698, 99.9197,
+    )
+
+    # Act
+    monthly = water_heating_gains_monthly_w(
+        heat_gains_from_water_heating_monthly_kwh=heat_gains_kwh,
+    )
+
+    # Assert
+    assert len(monthly) == 12
+    for m, (actual, exp) in enumerate(zip(monthly, expected_w)):
+        assert actual == pytest.approx(exp, abs=1e-3), f"month {m+1}"