§8f slice 1: fabric_energy_efficiency_kwh_per_m2_yr + 6-fixture conformance + atomic wiring

Spec line 7898: (109) = (98a) ÷ (4) + (108). New `worksheet/fabric_energy_efficiency.py` exposes a free function (no dataclass — single scalar output); `SpaceHeatingResult.space_heating_requirement_kwh_per_yr` (Σ(98a)) added so the spec literal — pre Appendix H solar offset — is the FEE input, not Σ(98c).

cert_to_inputs computes FEE from local SpaceHeatingResult + SpaceCoolingResult and passes via new `CalculatorInputs.fabric_energy_efficiency_kwh_per_m2_yr` (default 0.0 for backwards compat); calculator pass-through to `SapResult.fabric_energy_efficiency_kwh_per_m2_yr`. MonthlyEntry untouched — FEE has no per-month physics, only an annual scalar.

Six Elmhurst fixtures all (98b)=0 + (108)=0 → LINE_109 = LINE_99 exactly; ALL_FIXTURES asserts within 5e-3 tolerance (display-rounding floor inherited from LINE_98C_ANNUAL_KWH pins). Round-trip test asserts SapResult.fee equals space_heating_kwh_per_yr / TFA for the SAP10 minimal cert.

§11 compliance conditions (different ventilation / HW / lighting / gains column) are deferred — the FEE here is computed off rating-conditions inputs as a transparency output. Future §11 slice invokes the same function with §11-conditions upstream values.

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

packages/domain/src/domain/sap/calculator.py

@@ -135,6 +135,11 @@ class CalculatorInputs:
     # compatibility — every cert without `has_fixed_air_conditioning`
     # collapses cooling to zero.
     space_cooling_monthly_kwh: tuple[float, ...] = (0.0,) * 12
+    # SAP10.2 (109) — Fabric Energy Efficiency precomputed by cert_to_inputs
+    # via `fabric_energy_efficiency_kwh_per_m2_yr` from the §8/§8c results.
+    # Default 0.0 for backwards compatibility — synthetic CalculatorInputs
+    # constructions without cert_to_inputs leave it unset.
+    fabric_energy_efficiency_kwh_per_m2_yr: float = 0.0
 
 
 @dataclass(frozen=True)
@@ -167,6 +172,7 @@ class SapResult:
     co2_kg_per_yr: float
     space_heating_kwh_per_yr: float
     space_cooling_kwh_per_yr: float
+    fabric_energy_efficiency_kwh_per_m2_yr: float
     main_heating_fuel_kwh_per_yr: float
     secondary_heating_fuel_kwh_per_yr: float
     hot_water_kwh_per_yr: float
@@ -392,6 +398,7 @@ def calculate_sap_from_inputs(inputs: CalculatorInputs) -> SapResult:
         co2_kg_per_yr=co2,
         space_heating_kwh_per_yr=space_heating_kwh,
         space_cooling_kwh_per_yr=space_cooling_kwh,
+        fabric_energy_efficiency_kwh_per_m2_yr=inputs.fabric_energy_efficiency_kwh_per_m2_yr,
         main_heating_fuel_kwh_per_yr=main_fuel_kwh,
         secondary_heating_fuel_kwh_per_yr=secondary_fuel_kwh,
         hot_water_kwh_per_yr=inputs.hot_water_kwh_per_yr,

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

@@ -72,6 +72,9 @@ from domain.sap.worksheet.mean_internal_temperature import (
     mean_internal_temperature_monthly,
 )
 from domain.sap.worksheet.solar_gains import solar_gains_from_cert
+from domain.sap.worksheet.fabric_energy_efficiency import (
+    fabric_energy_efficiency_kwh_per_m2_yr,
+)
 from domain.sap.worksheet.space_cooling import space_cooling_monthly_kwh
 from domain.sap.worksheet.space_heating import space_heating_monthly_kwh
 from domain.sap.worksheet.ventilation import (
@@ -958,6 +961,16 @@ def cert_to_inputs(
         intermittency_factor=0.25,
     )
 
+    # SAP10.2 (109) — Fabric Energy Efficiency. Spec literal: (98a) / (4) +
+    # (108). For corpora without Appendix H solar space heating, (98a) == (98c).
+    # §11 compliance would re-run with different conditions; transparency-only
+    # for ratings.
+    fee_kwh_per_m2 = fabric_energy_efficiency_kwh_per_m2_yr(
+        space_heating_kwh_per_yr=space_heating_result.space_heating_requirement_kwh_per_yr,
+        total_floor_area_m2=dim.total_floor_area_m2,
+        space_cooling_per_m2_kwh=space_cooling_result.space_cooling_per_m2_kwh,
+    )
+
     return CalculatorInputs(
         dimensions=dim,
         heat_transmission=ht,
@@ -983,6 +996,8 @@ def cert_to_inputs(
         # SAP10.2 (107)m — space cooling kWh/month from §8c orchestrator
         # above (includes Jun-Aug inclusion mask + 1-kWh clamp).
         space_cooling_monthly_kwh=space_cooling_result.space_cooling_monthly_kwh,
+        # SAP10.2 (109) — Fabric Energy Efficiency precomputed above.
+        fabric_energy_efficiency_kwh_per_m2_yr=fee_kwh_per_m2,
         region=_region_index(epc.region_code),
         control_type=control_type_value,
         responsiveness=responsiveness_value,

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

@@ -313,6 +313,28 @@ def test_no_ac_cert_round_trips_to_zero_space_cooling_on_sap_result() -> None:
     assert all(entry.space_cool_requirement_kwh == 0.0 for entry in result.monthly)
 
 
+def test_no_ac_cert_round_trips_fee_equals_space_heating_per_m2() -> None:
+    """For an RdSAP cert without fixed AC, (108) = 0, so SAP 10.2 (109) Fabric
+    Energy Efficiency = (Σ(98a) / TFA) + 0 = annual space heating per m². No
+    Appendix H solar space heating means Σ(98a) == Σ(98c), so the FEE matches
+    `space_heating_kwh_per_yr / TFA` to float-equality."""
+    # Arrange
+    epc = _typical_semi_detached_epc()
+    assert epc.sap_heating.has_fixed_air_conditioning is False
+
+    # Act
+    result = Sap10Calculator().calculate(epc)
+
+    # Assert
+    expected_fee = (
+        result.space_heating_kwh_per_yr / result.intermediate["tfa_m2"]
+    )
+    assert result.fabric_energy_efficiency_kwh_per_m2_yr == pytest.approx(
+        expected_fee, abs=1e-9
+    )
+    assert result.space_cooling_kwh_per_yr == 0.0
+
+
 def test_calculator_always_uses_uk_average_weather_for_rating() -> None:
     # Arrange — SAP 10.2 Appendix U explicitly states: "Calculations for
     # ratings (SAP rating and environmental impact rating) are done with

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

@@ -0,0 +1,30 @@
+"""SAP 10.2 §8f Fabric Energy Efficiency — line ref (109).
+
+Spec line 7898: (109) = (98a) ÷ (4) + (108). The FEE is the dwelling's
+intrinsic fabric demand in kWh/m²/yr — space heating per m² (pre Appendix
+H solar offset) plus space cooling per m². §11 of the spec says FEE is
+"calculated only under special conditions" — new-build compliance — and
+under those conditions the whole worksheet is re-run with different
+ventilation / hot water / lighting / gains-column assumptions. For
+existing-dwelling ratings we expose FEE as a transparency output computed
+off rating-conditions (98a) and (108); a future §11 compliance slice
+would invoke this function with §11-conditions upstream values.
+
+Reference: SAP 10.2 specification (14-03-2025) worksheet block §8f
+(line 7898) and §11 (lines 2151-2164).
+"""
+
+from __future__ import annotations
+
+
+def fabric_energy_efficiency_kwh_per_m2_yr(
+    *,
+    space_heating_kwh_per_yr: float,
+    total_floor_area_m2: float,
+    space_cooling_per_m2_kwh: float,
+) -> float:
+    """SAP 10.2 (109) = (98a) ÷ (4) + (108). Returns 0.0 if TFA ≤ 0 (matches
+    the §8 (99) / §8c (108) edge-case convention)."""
+    if total_floor_area_m2 <= 0:
+        return 0.0
+    return space_heating_kwh_per_yr / total_floor_area_m2 + space_cooling_per_m2_kwh

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

@@ -66,6 +66,7 @@ class SpaceHeatingResult:
     space_heating_requirement_monthly_kwh: tuple[float, ...]    # (98a)
     solar_space_heating_monthly_kwh: tuple[float, ...]          # (98b)
     total_space_heating_monthly_kwh: tuple[float, ...]          # (98c)
+    space_heating_requirement_kwh_per_yr: float                 # Σ(98a) — FEE input
     total_space_heating_kwh_per_yr: float                       # Σ(98c)
     space_heating_per_m2_kwh: float                             # (99)
 
@@ -123,6 +124,7 @@ def space_heating_monthly_kwh(
         q_solar_98b.append(0.0)
         q_total_98c.append(q98a)
 
+    annual_98a = sum(q_heat_98a)
     annual_98c = sum(q_total_98c)
     per_m2_99 = annual_98c / total_floor_area_m2 if total_floor_area_m2 > 0 else 0.0
 
@@ -132,6 +134,7 @@ def space_heating_monthly_kwh(
         space_heating_requirement_monthly_kwh=tuple(q_heat_98a),
         solar_space_heating_monthly_kwh=tuple(q_solar_98b),
         total_space_heating_monthly_kwh=tuple(q_total_98c),
+        space_heating_requirement_kwh_per_yr=annual_98a,
         total_space_heating_kwh_per_yr=annual_98c,
         space_heating_per_m2_kwh=per_m2_99,
     )

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000474.py

@@ -388,3 +388,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 186.879

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000477.py

@@ -323,3 +323,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 130.3326

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000480.py

@@ -352,3 +352,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 146.8852

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000487.py

@@ -369,3 +369,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 132.828

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000490.py

@@ -364,3 +364,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 169.2897

packages/domain/src/domain/sap/worksheet/tests/_elmhurst_worksheet_000516.py

@@ -336,3 +336,11 @@ LINE_106_M_INTERMITTENCY_FACTOR = SECTION_8C_INTERMITTENCY_MONTHLY
 LINE_107_M_SPACE_COOLING_KWH = SECTION_8C_ALL_ZERO_MONTHLY
 LINE_107_ANNUAL_KWH: float = 0.0
 LINE_108_PER_M2_KWH: float = 0.0
+
+# ============================================================================
+# §8f Fabric Energy Efficiency (line ref (109))
+# ============================================================================
+# Spec line 7898: (109) = (98a) ÷ (4) + (108). For this fixture (98b) Appendix H
+# solar space heating = 0 → Σ(98a) = Σ(98c) → (98a)/TFA = LINE_99_PER_M2_KWH;
+# (108) = LINE_108_PER_M2_KWH = 0 (no AC). So LINE_109 = LINE_99 exactly.
+LINE_109_FEE_KWH_PER_M2: float = 137.0700

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

@@ -0,0 +1,56 @@
+"""Tests for SAP 10.2 §8f Fabric Energy Efficiency — line ref (109).
+
+Reference: SAP 10.2 specification (14-03-2025) worksheet block §8f
+(line 7898): (109) = (98a) ÷ (4) + (108).
+"""
+
+from __future__ import annotations
+
+from types import ModuleType
+
+import pytest
+
+from domain.sap.worksheet.fabric_energy_efficiency import (
+    fabric_energy_efficiency_kwh_per_m2_yr,
+)
+from domain.sap.worksheet.tests._elmhurst_fixtures import ALL_FIXTURES, fixture_id
+
+
+def test_fabric_energy_efficiency_sums_heating_per_m2_and_cooling_per_m2() -> None:
+    """Spec line 7898: (109) = (98a) ÷ (4) + (108). With Σ(98a) = 1000 kWh
+    and TFA = 100 m² → heating per m² = 10 kWh/m²; adding (108) = 5 kWh/m²
+    cooling gives FEE = 15 kWh/m²/yr."""
+    # Arrange
+    space_heating_kwh_per_yr = 1000.0
+    total_floor_area_m2 = 100.0
+    space_cooling_per_m2_kwh = 5.0
+
+    # Act
+    fee = fabric_energy_efficiency_kwh_per_m2_yr(
+        space_heating_kwh_per_yr=space_heating_kwh_per_yr,
+        total_floor_area_m2=total_floor_area_m2,
+        space_cooling_per_m2_kwh=space_cooling_per_m2_kwh,
+    )
+
+    # Assert
+    assert fee == 15.0
+
+
+@pytest.mark.parametrize("fixture", ALL_FIXTURES, ids=[fixture_id(f) for f in ALL_FIXTURES])
+def test_fabric_energy_efficiency_matches_elmhurst_worksheet_all_fixtures(
+    fixture: ModuleType,
+) -> None:
+    """For Elmhurst fixtures (no AC, no Appendix H solar space heating),
+    (109) = LINE_99 + 0 = LINE_99 exactly. Σ(98a) = Σ(98c) per the (98b)=0
+    invariant, so feeding LINE_98C_ANNUAL_KWH satisfies the spec's "(98a)
+    ÷ (4)" term."""
+    # Arrange
+    # Act
+    fee = fabric_energy_efficiency_kwh_per_m2_yr(
+        space_heating_kwh_per_yr=fixture.LINE_98C_ANNUAL_KWH,
+        total_floor_area_m2=fixture.LINE_4_TFA_M2,
+        space_cooling_per_m2_kwh=fixture.LINE_108_PER_M2_KWH,
+    )
+
+    # Assert
+    assert fee == pytest.approx(fixture.LINE_109_FEE_KWH_PER_M2, abs=5e-3)