Appendix L slice 1: annual_lighting_kwh extraction

Surfaces the SAP10.2 Appendix L L1-L12 annual lighting kWh as a public
free fn alongside lighting_monthly_w. Refactors lighting_monthly_w to
compose it. One source of truth shared by the §5 gains side and the
forthcoming cost side (inputs.lighting_kwh_per_yr) — slice 2 wires
internal_gains_from_cert + cert_to_inputs.

Synthetic L1-L12 test pins a hand-computed dwelling
(TFA=100, N=2.0, C_L=10000, ε=100, D=1.0) at 189.152079 kWh, abs=1e-3.

6-fixture LINE_67 conformance tests (Elmhurst 000474..000516) act as a
regression check on the monthly cosine + 0.85 internal-fraction
composition — all green.

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

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

@@ -205,6 +205,47 @@ def appliances_monthly_w(
     return tuple(monthly)
 
 
+def annual_lighting_kwh(
+    *,
+    total_floor_area_m2: float,
+    n_occupants: float,
+    fixed_lighting_capacity_lm: float,
+    fixed_lighting_efficacy_lm_per_w: float,
+    daylight_factor: float,
+) -> float:
+    """SAP 10.2 Appendix L L1-L12 — annual lighting energy in kWh/yr.
+
+    The scalar leaf shared by the §5 gains side (composed into
+    `lighting_monthly_w` via the seasonal cosine modulation) and the
+    cost side (`inputs.lighting_kwh_per_yr`). Surfacing it via this
+    public free fn lets the cert→inputs precompute reuse the same
+    derivation that drives (67)m — one source of truth.
+
+    See `lighting_monthly_w` for the per-kwarg semantics + RdSAP §12-1
+    lamp-type / L5b / L8c / L2a/L2b fallback rules.
+    """
+    lambda_b = (
+        _LIGHTING_LAMBDA_B_COEFF
+        * (total_floor_area_m2 * n_occupants) ** _LIGHTING_LAMBDA_B_EXPONENT
+    )
+    lambda_req = (2.0 / 3.0) * lambda_b * daylight_factor
+    c_l_ref = _LIGHTING_C_L_REF_PER_M2 * total_floor_area_m2
+    lambda_prov = (
+        lambda_req * fixed_lighting_capacity_lm / c_l_ref if c_l_ref > 0 else 0.0
+    )
+    lambda_topup = max(0.0, lambda_req / 3.0 - lambda_prov)
+    e_l_fixed = (
+        max(lambda_req, lambda_prov) / fixed_lighting_efficacy_lm_per_w
+        if fixed_lighting_efficacy_lm_per_w > 0
+        else 0.0
+    )
+    e_l_topup = lambda_topup / _LIGHTING_TOPUP_EFFICACY_LM_PER_W
+    e_l_portable = (
+        (1.0 / 3.0) * lambda_b * daylight_factor / _LIGHTING_PORTABLE_EFFICACY_LM_PER_W
+    )
+    return e_l_fixed + e_l_topup + e_l_portable
+
+
 def lighting_monthly_w(
     *,
     total_floor_area_m2: float,
@@ -231,26 +272,13 @@ def lighting_monthly_w(
 
     L12 reduced-gain branch (L12a, used for new-build DPER/TPER) is deferred.
     """
-    lambda_b = (
-        _LIGHTING_LAMBDA_B_COEFF
-        * (total_floor_area_m2 * n_occupants) ** _LIGHTING_LAMBDA_B_EXPONENT
+    e_l_annual_kwh = annual_lighting_kwh(
+        total_floor_area_m2=total_floor_area_m2,
+        n_occupants=n_occupants,
+        fixed_lighting_capacity_lm=fixed_lighting_capacity_lm,
+        fixed_lighting_efficacy_lm_per_w=fixed_lighting_efficacy_lm_per_w,
+        daylight_factor=daylight_factor,
     )
-    lambda_req = (2.0 / 3.0) * lambda_b * daylight_factor
-    c_l_ref = _LIGHTING_C_L_REF_PER_M2 * total_floor_area_m2
-    lambda_prov = (
-        lambda_req * fixed_lighting_capacity_lm / c_l_ref if c_l_ref > 0 else 0.0
-    )
-    lambda_topup = max(0.0, lambda_req / 3.0 - lambda_prov)
-    e_l_fixed = (
-        max(lambda_req, lambda_prov) / fixed_lighting_efficacy_lm_per_w
-        if fixed_lighting_efficacy_lm_per_w > 0
-        else 0.0
-    )
-    e_l_topup = lambda_topup / _LIGHTING_TOPUP_EFFICACY_LM_PER_W
-    e_l_portable = (
-        (1.0 / 3.0) * lambda_b * daylight_factor / _LIGHTING_PORTABLE_EFFICACY_LM_PER_W
-    )
-    e_l_annual_kwh = e_l_fixed + e_l_topup + e_l_portable
     monthly: list[float] = []
     for m_idx, days in enumerate(_DAYS_PER_MONTH):
         m = m_idx + 1

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

@@ -16,6 +16,7 @@ from domain.sap.worksheet.internal_gains import (
     InternalGainsResult,
     OvershadingCategory,
     PumpDateCategory,
+    annual_lighting_kwh,
     appliances_monthly_w,
     balanced_mv_no_hr_fan_w,
     central_heating_pump_w,
@@ -231,6 +232,43 @@ def test_lighting_gains_match_appendix_l1_l12_for_000490() -> None:
         assert actual == pytest.approx(exp, abs=5e-3), f"month {m+1}"
 
 
+def test_annual_lighting_kwh_matches_hand_computed_appendix_l_cascade() -> None:
+    """Synthetic L1-L12 cascade on a clean dwelling. Hand-derived via the
+    SAP 10.2 Appendix L formulas:
+
+      TFA=100 m², N=2.0, C_L,fixed=10000 lm, ε_fixed=100 lm/W, D=1.0
+
+      λ_b      = 11.2 × 59.73 × (200)^0.4714           = 8130.477969
+      λ_req    = (2/3) × λ_b × D                       = 5420.318646
+      C_L_ref  = 330 × TFA                             = 33000.0
+      λ_prov   = λ_req × C_L_fixed / C_L_ref           = 1642.520802
+      λ_topup  = max(0, λ_req/3 - λ_prov)              =  164.252080
+      E_L_fixed    = max(λ_req, λ_prov) / ε_fixed      =   54.203186
+      E_L_topup    = λ_topup / 21.3                    =    7.711365
+      E_L_portable = (1/3) × λ_b × D / 21.3            =  127.237527
+
+      e_l_annual_kwh = 189.152079
+
+    Pins the new public leaf `annual_lighting_kwh` directly so the cost
+    side (`inputs.lighting_kwh_per_yr`) and the gains side (§5 (67) via
+    `lighting_monthly_w`) share one source of truth.
+    """
+    # Arrange
+    expected_kwh = 189.152079
+
+    # Act
+    actual_kwh = annual_lighting_kwh(
+        total_floor_area_m2=100.0,
+        n_occupants=2.0,
+        fixed_lighting_capacity_lm=10000.0,
+        fixed_lighting_efficacy_lm_per_w=100.0,
+        daylight_factor=1.0,
+    )
+
+    # Assert
+    assert actual_kwh == pytest.approx(expected_kwh, abs=1e-3)
+
+
 def test_pumps_fans_seasonal_mask_zeroes_summer_months_jun_to_sep() -> None:
     """SAP10.2 Table 5a footnote a/b: pumps and warm-air fans are "Set to
     zero in summer months". Year-round contributions (PIV, balanced MV