§5 slice 2: (71) losses_monthly_w — -40×N const-tuple

SAP10.2 Table 5 "Losses" row: -40 × N watts year-round. Captures
cold-water inflow + evaporation heat sinks. Verified against the
Elmhurst U985-0001-000490 worksheet (71)m row.

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

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

@@ -33,6 +33,7 @@ from typing import Final, Optional
 
 _MONTHS_IN_YEAR: Final[int] = 12
 _METABOLIC_GAIN_W_PER_OCCUPANT_COL_A: Final[float] = 60.0
+_LOSSES_W_PER_OCCUPANT_COL_A: Final[float] = -40.0
 
 
 def metabolic_monthly_w(*, n_occupants: float) -> tuple[float, ...]:
@@ -45,6 +46,17 @@ def metabolic_monthly_w(*, n_occupants: float) -> tuple[float, ...]:
                  for _ in range(_MONTHS_IN_YEAR))
 
 
+def losses_monthly_w(*, n_occupants: float) -> tuple[float, ...]:
+    """SAP 10.2 §5 line (71) — internal-gain losses in watts per month.
+
+    Table 5: losses = -40 × N watts (constant year-round). Comprises heat
+    sunk to incoming cold water + evaporation from washing/showering. Sign
+    is negative — these reduce the dwelling's net internal gains.
+    """
+    return tuple(_LOSSES_W_PER_OCCUPANT_COL_A * n_occupants
+                 for _ in range(_MONTHS_IN_YEAR))
+
+
 _METABOLIC_W_PER_OCCUPANT: Final[float] = 60.0
 
 

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

@@ -12,7 +12,10 @@ Table 5a + Appendix L (lighting/appliances/cooking) + Appendix J Table 1b
 
 import pytest
 
-from domain.sap.worksheet.internal_gains import metabolic_monthly_w
+from domain.sap.worksheet.internal_gains import (
+    losses_monthly_w,
+    metabolic_monthly_w,
+)
 
 
 def test_metabolic_gains_are_60w_per_occupant_constant_across_months() -> None:
@@ -34,3 +37,28 @@ def test_metabolic_gains_are_60w_per_occupant_constant_across_months() -> None:
     assert len(monthly) == 12
     for m, value in enumerate(monthly):
         assert value == pytest.approx(60.0 * n, abs=1e-9), f"month {m+1}"
+
+
+def test_losses_are_negative_40n_constant_across_months() -> None:
+    """SAP 10.2 Table 5 row "Losses": -40 × N watts year-round.
+
+    The losses comprise heat lost to incoming cold water + evaporation
+    (washing, showering, etc.). The sign is negative — internal gains
+    are reduced.
+
+    Verified against the Elmhurst U985-0001-000490 worksheet (71)m row,
+    which shows -85.8725 W flat across all months. For 000490's N=2.1468:
+        -40 × 2.1468 = -85.872 W
+    matches the worksheet to within 4-d.p. display rounding.
+    """
+    # Arrange
+    n = 2.1468
+
+    # Act
+    monthly = losses_monthly_w(n_occupants=n)
+
+    # Assert
+    assert len(monthly) == 12
+    for m, value in enumerate(monthly):
+        assert value == pytest.approx(-40.0 * n, abs=1e-9), f"month {m+1}"
+        assert value < 0, "Losses must be negative (heat-loss convention)"