diff --git a/packages/domain/src/domain/sap/worksheet/energy_requirements.py b/packages/domain/src/domain/sap/worksheet/energy_requirements.py
new file mode 100644
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+++ b/packages/domain/src/domain/sap/worksheet/energy_requirements.py
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+"""SAP 10.2 §9a Energy requirements — individual heating systems.
+
+Spec lines 7909-7953 (worksheet block §9a). Composes the per-system fuel
+kWh from the §8 space-heating tuple (98c)m, the Table 11 secondary
+fraction (201), and the per-system efficiencies (206)/(207)/(208). The
+formula for the main system 1 line ref (211)m is:
+
+    (211)m = (98c)m × (204) × 100 / (206)
+
+where (204) = (202) × (1 − (203)) and (202) = 1 − (201). Single-main
+case ((203) = 0) collapses (204) to (202), so (211)m = (98c)m × (202) ×
+100 / (206). Same shape for secondary (215)m and main 2 (213)m.
+
+Two-main split ((203) > 0) and cooling-fuel (209)/(221) are zero-branch
+placeholders in scope A — populated once first cert exercises them.
+
+Reference: SAP 10.2 specification (14-03-2025) §9a (lines 7909-7953).
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True)
+class EnergyRequirementsResult:
+    """SAP 10.2 §9a worksheet line refs (201)..(221).
+
+    Scope-A populated lines: (201), (202), (204), (206), (208), (211)m,
+    (211), (215)m, (215). Two-main and cooling-fuel line refs ((203),
+    (205), (207), (209), (213)m, (213), (221)) are zero-branch
+    placeholders until the first multi-main / fixed-AC cert lands.
+    """
+
+    # Fractions (Table 11)
+    secondary_heating_fraction: float                       # (201)
+    main_heating_total_fraction: float                      # (202) = 1 - (201)
+    main_2_of_main_fraction: float                          # (203)
+    main_1_of_total_fraction: float                         # (204) = (202) × (1 - (203))
+    main_2_of_total_fraction: float                         # (205) = (202) × (203)
+    # Efficiencies (%)
+    main_1_efficiency_pct: float                            # (206)
+    main_2_efficiency_pct: float                            # (207)
+    secondary_efficiency_pct: float                         # (208)
+    cooling_seer: float                                     # (209)
+    # Per-month fuel (kWh)
+    main_1_fuel_monthly_kwh: tuple[float, ...]              # (211)m
+    main_2_fuel_monthly_kwh: tuple[float, ...]              # (213)m
+    secondary_fuel_monthly_kwh: tuple[float, ...]           # (215)m
+    # Annual totals (kWh)
+    main_1_fuel_kwh_per_yr: float                           # (211)
+    main_2_fuel_kwh_per_yr: float                           # (213)
+    secondary_fuel_kwh_per_yr: float                        # (215)
+    cooling_fuel_kwh_per_yr: float                          # (221)
+
+
+def space_heating_fuel_monthly_kwh(
+    *,
+    space_heating_monthly_kwh: tuple[float, ...],
+    secondary_heating_fraction: float,
+    main_heating_efficiency_pct: float,
+    secondary_heating_efficiency_pct: float,
+) -> EnergyRequirementsResult:
+    """SAP 10.2 §9a orchestrator — produce (201)..(221) line refs.
+
+    Scope A: single-main + secondary only. Two-main ((203) > 0) and
+    cooling-fuel (Table 10c SEER) populate the zero-branch placeholder
+    fields with computed values when their respective slices land.
+    """
+    fraction_201 = secondary_heating_fraction
+    fraction_202 = 1.0 - fraction_201
+    fraction_203 = 0.0  # scope A: no main 2
+    fraction_204 = fraction_202 * (1.0 - fraction_203)
+    fraction_205 = fraction_202 * fraction_203
+
+    main_1_eff = main_heating_efficiency_pct
+    secondary_eff = secondary_heating_efficiency_pct
+
+    main_1_fuel_monthly = tuple(
+        q * fraction_204 * 100.0 / main_1_eff if main_1_eff > 0 else 0.0
+        for q in space_heating_monthly_kwh
+    )
+    secondary_fuel_monthly = tuple(
+        q * fraction_201 * 100.0 / secondary_eff if secondary_eff > 0 else 0.0
+        for q in space_heating_monthly_kwh
+    )
+
+    return EnergyRequirementsResult(
+        secondary_heating_fraction=fraction_201,
+        main_heating_total_fraction=fraction_202,
+        main_2_of_main_fraction=fraction_203,
+        main_1_of_total_fraction=fraction_204,
+        main_2_of_total_fraction=fraction_205,
+        main_1_efficiency_pct=main_1_eff,
+        main_2_efficiency_pct=0.0,
+        secondary_efficiency_pct=secondary_eff,
+        cooling_seer=0.0,
+        main_1_fuel_monthly_kwh=main_1_fuel_monthly,
+        main_2_fuel_monthly_kwh=(0.0,) * 12,
+        secondary_fuel_monthly_kwh=secondary_fuel_monthly,
+        main_1_fuel_kwh_per_yr=sum(main_1_fuel_monthly),
+        main_2_fuel_kwh_per_yr=0.0,
+        secondary_fuel_kwh_per_yr=sum(secondary_fuel_monthly),
+        cooling_fuel_kwh_per_yr=0.0,
+    )
diff --git a/packages/domain/src/domain/sap/worksheet/tests/test_energy_requirements.py b/packages/domain/src/domain/sap/worksheet/tests/test_energy_requirements.py
new file mode 100644
index 00000000..b765adcf
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+++ b/packages/domain/src/domain/sap/worksheet/tests/test_energy_requirements.py
@@ -0,0 +1,114 @@
+"""Tests for SAP 10.2 §9a Energy requirements (individual heating systems).
+
+Reference: SAP 10.2 specification (14-03-2025) worksheet block §9a
+(lines 7909-7953).
+"""
+
+from __future__ import annotations
+
+from domain.sap.worksheet.energy_requirements import (
+    space_heating_fuel_monthly_kwh,
+)
+
+
+def test_single_main_no_secondary_converts_q_heat_by_efficiency() -> None:
+    """Spec lines 7940-7941: (211)m = (98c)m × (204) × 100 / (206). With no
+    secondary system (201)=0, (204) collapses to 1.0 so (211)m = (98c)m /
+    (206)/100. With Σ(98c) = 4400 kWh at 80% efficiency, Σ(211) = 4400 ×
+    100/80 = 5500 kWh."""
+    # Arrange
+    monthly_space_heating = (
+        1000.0, 800.0, 600.0, 400.0, 200.0, 0.0,
+        0.0, 0.0, 0.0, 200.0, 400.0, 800.0,
+    )
+
+    # Act
+    result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=monthly_space_heating,
+        secondary_heating_fraction=0.0,
+        main_heating_efficiency_pct=80.0,
+        secondary_heating_efficiency_pct=100.0,
+    )
+
+    # Assert
+    assert result.main_1_fuel_kwh_per_yr == 5500.0
+
+
+def test_table_11_secondary_fraction_splits_q_heat_between_main_and_secondary() -> None:
+    """Spec lines 7912-7914 + 7940-7951: (201) Table 11 secondary fraction
+    routes (98c)m × (201) to the secondary system, (98c)m × (202) to the
+    main. With (201)=0.1, both efficiencies 80%, Σ(98c)=4400 → Σ(211) =
+    4400 × 0.9 × 100/80 = 4950 kWh; Σ(215) = 4400 × 0.1 × 100/80 = 550 kWh."""
+    # Arrange
+    monthly_space_heating = (
+        1000.0, 800.0, 600.0, 400.0, 200.0, 0.0,
+        0.0, 0.0, 0.0, 200.0, 400.0, 800.0,
+    )
+
+    # Act
+    result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=monthly_space_heating,
+        secondary_heating_fraction=0.1,
+        main_heating_efficiency_pct=80.0,
+        secondary_heating_efficiency_pct=80.0,
+    )
+
+    # Assert
+    assert result.secondary_heating_fraction == 0.1
+    assert result.main_heating_total_fraction == 0.9
+    assert result.main_1_fuel_kwh_per_yr == 4950.0
+    assert result.secondary_fuel_kwh_per_yr == 550.0
+
+
+def test_per_month_fuel_preserves_summer_clamp_zeros_from_98c() -> None:
+    """The §8 Table 9c summer clamp zeros (98c)m for Jun..Sep. §9a's per-
+    month (211)m / (215)m tuples are linear in (98c)m so they inherit the
+    summer zeros — important downstream because §10a fuel-cost cascade
+    indexes the per-month tuples directly."""
+    # Arrange
+    monthly_space_heating = (
+        1000.0, 800.0, 600.0, 400.0, 200.0, 0.0,
+        0.0, 0.0, 0.0, 200.0, 400.0, 800.0,
+    )
+
+    # Act
+    result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=monthly_space_heating,
+        secondary_heating_fraction=0.1,
+        main_heating_efficiency_pct=80.0,
+        secondary_heating_efficiency_pct=80.0,
+    )
+
+    # Assert
+    assert result.main_1_fuel_monthly_kwh[5] == 0.0  # Jun
+    assert result.main_1_fuel_monthly_kwh[8] == 0.0  # Sep
+    assert result.secondary_fuel_monthly_kwh[5] == 0.0
+    assert result.secondary_fuel_monthly_kwh[8] == 0.0
+    assert result.main_1_fuel_monthly_kwh[0] == 1125.0  # Jan: 1000 × 0.9 × 100/80
+    assert result.secondary_fuel_monthly_kwh[0] == 125.0  # Jan: 1000 × 0.1 × 100/80
+
+
+def test_scope_a_two_main_and_cooling_fuel_remain_zero_placeholders() -> None:
+    """Scope A defers two-main system ((203)/(205)/(207)/(213)) and cooling
+    SEER ((209)/(221)). EnergyRequirementsResult mirrors the worksheet's
+    full §9a shape but those fields stay zero regardless of inputs — first
+    multi-main / fixed-AC cert triggers the slice that populates them."""
+    # Arrange
+    monthly_space_heating = (1000.0,) * 12
+
+    # Act
+    result = space_heating_fuel_monthly_kwh(
+        space_heating_monthly_kwh=monthly_space_heating,
+        secondary_heating_fraction=0.1,
+        main_heating_efficiency_pct=80.0,
+        secondary_heating_efficiency_pct=80.0,
+    )
+
+    # Assert
+    assert result.main_2_of_main_fraction == 0.0
+    assert result.main_2_of_total_fraction == 0.0
+    assert result.main_2_efficiency_pct == 0.0
+    assert result.main_2_fuel_monthly_kwh == (0.0,) * 12
+    assert result.main_2_fuel_kwh_per_yr == 0.0
+    assert result.cooling_seer == 0.0
+    assert result.cooling_fuel_kwh_per_yr == 0.0