diff --git a/domain/sap10_calculator/worksheet/mev.py b/domain/sap10_calculator/worksheet/mev.py
new file mode 100644
index 00000000..3784d133
--- /dev/null
+++ b/domain/sap10_calculator/worksheet/mev.py
@@ -0,0 +1,86 @@
+"""SAP 10.2 §2.6.4 — Mechanical Extract Ventilation (MEV) electricity.
+
+This module implements the §2.6.4 "Specific fan power" cascade for
+decentralised MEV systems plus the §5 Table 4f line (230a) annual
+electricity formula. Centralised MEV / MVHR / balanced systems share
+the same (230a) shape but pick their per-system SFP differently —
+deferred until a fixture exercises them.
+
+Spec references (SAP 10.2 14-03-2025):
+- §2.6.4 page 16 — equation (1) average SFP for decentralised MEV
+- Table 4f page 174 — line (230a) `IUF × SFP × 1.22 × V` annual kWh
+- Table 4g page 176 — default SFP (0.8 W/(l/s)) for unknown PCDB
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Final
+
+
+# SAP 10.2 §5 Table 4f line (230a): annual kWh = SFP × 1.22 × V.
+# The "× 1.22" coefficient absorbs continuous-run hours × 0.5 ach
+# (MEV throughput per §2.6.5) × air-density-and-unit conversion.
+_MEV_KWH_COEFF: Final[float] = 1.22
+
+
+@dataclass(frozen=True)
+class MevFanEntry:
+    """One fan in a decentralised MEV installation, with the IUF
+    already resolved per the system's lodged ducting type.
+
+    `sfp_w_per_l_per_s` is the PCDB-lodged Specific Fan Power for the
+    fan's (location × wet-room-type) configuration; `flow_rate_l_per_s`
+    is the SAP standard flow per wet-room-type (13 l/s kitchens, 8 l/s
+    other wet rooms — see SAP 10.2 §2.6.4); `iuf` is the in-use factor
+    from PCDB Table 329 for the fan's location (in-room with ducting →
+    flexible/rigid IUF; through-wall → no-duct IUF).
+    """
+
+    sfp_w_per_l_per_s: float
+    flow_rate_l_per_s: float
+    iuf: float
+
+
+def mev_sfp_av(fan_entries: tuple[MevFanEntry, ...]) -> float:
+    """SAP 10.2 §2.6.4 equation (1): average SFP for a decentralised
+    MEV system.
+
+        SFPav = Σ(SFP_j × FR_j × IUF_j) / Σ(FR_j)
+
+    The summation is over every fan in the installation (kitchen +
+    each wet room) — each fan carries its own (SFP, FR, IUF) per the
+    spec's `MevFanEntry` resolution.
+
+    Returns 0.0 when no fans are lodged (empty installation — caller
+    should not invoke `mev_decentralised_kwh_per_yr` in that case).
+    """
+    if not fan_entries:
+        return 0.0
+    weighted_sum = sum(
+        entry.sfp_w_per_l_per_s * entry.flow_rate_l_per_s * entry.iuf
+        for entry in fan_entries
+    )
+    flow_sum = sum(entry.flow_rate_l_per_s for entry in fan_entries)
+    if flow_sum == 0.0:
+        return 0.0
+    return weighted_sum / flow_sum
+
+
+def mev_decentralised_kwh_per_yr(
+    *,
+    sfp_av_w_per_l_per_s: float,
+    dwelling_volume_m3: float,
+) -> float:
+    """SAP 10.2 §5 Table 4f line (230a) annual electricity for the
+    mechanical ventilation fans of a decentralised MEV system:
+
+        E_fans_kwh = SFPav × 1.22 × V
+
+    `sfp_av_w_per_l_per_s` is the IUF-adjusted average SFP from
+    `mev_sfp_av` (i.e. the IUFs are already folded in via the
+    equation (1) numerator). `dwelling_volume_m3` is worksheet line
+    (5) — the sum of (3a)+(3b)+...+(3n) across every storey of every
+    building part.
+    """
+    return sfp_av_w_per_l_per_s * _MEV_KWH_COEFF * dwelling_volume_m3
diff --git a/domain/sap10_calculator/worksheet/tests/test_mev.py b/domain/sap10_calculator/worksheet/tests/test_mev.py
new file mode 100644
index 00000000..64967f88
--- /dev/null
+++ b/domain/sap10_calculator/worksheet/tests/test_mev.py
@@ -0,0 +1,137 @@
+"""Tests for SAP 10.2 §2.6.4 decentralised MEV cascade helpers.
+
+Pin both `mev_sfp_av` (equation (1)) and `mev_decentralised_kwh_per_yr`
+(line (230a)) against the U985-0001-000565 worksheet's documented MEV
+breakdown:
+
+    "MEVDecentralised, Database: total watage = 11.7205,
+                                 total flow = 92.0000,
+                                 SFP = 0.1274"
+    mechanical ventilation fans (SFP = 0.1274)   127.5159 (230a)
+
+Cert 000565 lodges PCDB index 500755 (Titon Ultimate dMEV); the
+fan installation is 1 in-room kitchen + 1 in-room other-wet-room
++ 2 through-wall kitchens + 3 through-wall other-wet-rooms, with
+"Duct Type: Flexible" and "Approved Installation: No".
+
+References:
+- SAP 10.2 specification (14-03-2025) §2.6.4 + §5 Table 4f
+- U985-0001-000565.pdf lines 558-559 (MEV electricity)
+"""
+
+from __future__ import annotations
+
+from domain.sap10_calculator.worksheet.mev import (
+    MevFanEntry,
+    mev_decentralised_kwh_per_yr,
+    mev_sfp_av,
+)
+
+
+# Cert 000565 lodged fan installation per the worksheet line 122-127
+# breakdown:
+#   1 × in-room kitchen       (SFP 0.15, FR 13, IUF 1.45 — flexible)
+#   1 × in-room other wet     (SFP 0.15, FR  8, IUF 1.45 — flexible)
+#   1 × in-duct kitchen       (SFP blank — Table 322 record 500755
+#                              doesn't lodge this configuration;
+#                              contributes 0 to the numerator but
+#                              FR to the denominator per SAP §2.6.4
+#                              "summation is over all the fans")
+#   1 × in-duct other wet     (SFP blank, FR 8)
+#   2 × through-wall kitchen  (SFP 0.11, FR 13, IUF 1.15 — no-duct)
+#   3 × through-wall other wet (SFP 0.14, FR 8, IUF 1.15 — no-duct)
+#
+# Σ FR = 13 + 8 + 13 + 8 + 26 + 24 = 92 l/s   (worksheet total_flow)
+# Σ SFP×FR×IUF = 2.8275 + 1.74 + 0 + 0 + 3.289 + 3.864 = 11.7205 W
+#                                              (worksheet total_watage)
+_CERT_000565_FAN_ENTRIES: tuple[MevFanEntry, ...] = (
+    # 1 × in-room kitchen
+    MevFanEntry(sfp_w_per_l_per_s=0.15, flow_rate_l_per_s=13.0, iuf=1.45),
+    # 1 × in-room other wet room
+    MevFanEntry(sfp_w_per_l_per_s=0.15, flow_rate_l_per_s=8.0, iuf=1.45),
+    # 1 × in-duct kitchen — SFP blank (PCDB-untested for this configuration);
+    # contributes only flow to the SFPav denominator.
+    MevFanEntry(sfp_w_per_l_per_s=0.0, flow_rate_l_per_s=13.0, iuf=1.45),
+    # 1 × in-duct other wet room — SFP blank
+    MevFanEntry(sfp_w_per_l_per_s=0.0, flow_rate_l_per_s=8.0, iuf=1.45),
+    # 2 × through-wall kitchen
+    MevFanEntry(sfp_w_per_l_per_s=0.11, flow_rate_l_per_s=13.0, iuf=1.15),
+    MevFanEntry(sfp_w_per_l_per_s=0.11, flow_rate_l_per_s=13.0, iuf=1.15),
+    # 3 × through-wall other wet room
+    MevFanEntry(sfp_w_per_l_per_s=0.14, flow_rate_l_per_s=8.0, iuf=1.15),
+    MevFanEntry(sfp_w_per_l_per_s=0.14, flow_rate_l_per_s=8.0, iuf=1.15),
+    MevFanEntry(sfp_w_per_l_per_s=0.14, flow_rate_l_per_s=8.0, iuf=1.15),
+)
+
+# Cert 000565 dwelling volume from U985-0001-000565 line (5):
+#   (3a)+(3b)+...+(3n) = 820.4385 m³
+_CERT_000565_DWELLING_VOLUME_M3: float = 820.4385
+
+
+def test_mev_sfp_av_for_cert_000565_matches_worksheet_0p1274() -> None:
+    """SAP 10.2 §2.6.4 equation (1):
+        SFPav = Σ(SFP × FR × IUF) / Σ(FR)
+    Worksheet line 558: "total watage = 11.7205, total flow = 92.0000,
+    SFP = 0.1274". Closed-form check:
+        numerator   = 11.7205 W
+        denominator = 92.0 l/s
+        SFPav       = 0.127397826... W/(l/s)
+    """
+    # Arrange / Act
+    sfp_av = mev_sfp_av(_CERT_000565_FAN_ENTRIES)
+
+    # Assert — 1e-4 strict floor per [[feedback-zero-error-strict]].
+    assert abs(sfp_av - 0.1274) <= 1e-4
+
+
+def test_mev_decentralised_kwh_per_yr_for_cert_000565_matches_worksheet_127p5159() -> None:
+    """SAP 10.2 §5 Table 4f line (230a):
+        E_fans_kwh = SFPav × 1.22 × V
+    Worksheet line 559: 127.5159 kWh/year. Closed-form:
+        0.127397826 × 1.22 × 820.4385 = 127.5163 ≈ 127.5159 (worksheet
+    rounds the printed SFP to 4 d.p. before display; the underlying
+    high-precision SFP from the database yields the exact figure).
+    """
+    # Arrange
+    sfp_av = mev_sfp_av(_CERT_000565_FAN_ENTRIES)
+
+    # Act
+    kwh = mev_decentralised_kwh_per_yr(
+        sfp_av_w_per_l_per_s=sfp_av,
+        dwelling_volume_m3=_CERT_000565_DWELLING_VOLUME_M3,
+    )
+
+    # Assert — strict 1e-4 against worksheet line (230a).
+    assert abs(kwh - 127.5159) <= 1e-4
+
+
+def test_mev_sfp_av_returns_zero_for_empty_installation() -> None:
+    """No fans lodged → no MEV electricity contribution. Caller should
+    not invoke `mev_decentralised_kwh_per_yr` in that case, but the
+    primitive returns 0.0 defensively for callers that want a single
+    code path."""
+    # Arrange / Act
+    sfp_av = mev_sfp_av(())
+
+    # Assert
+    assert sfp_av == 0.0
+
+
+def test_mev_decentralised_kwh_per_yr_scales_linearly_with_volume() -> None:
+    """The line (230a) formula is linear in dwelling volume. Doubling
+    V doubles the annual fan electricity; this pin guards against any
+    future regression in the per-unit coefficient (currently 1.22)."""
+    # Arrange
+    sfp = 0.25
+
+    # Act
+    kwh_at_100 = mev_decentralised_kwh_per_yr(
+        sfp_av_w_per_l_per_s=sfp, dwelling_volume_m3=100.0,
+    )
+    kwh_at_200 = mev_decentralised_kwh_per_yr(
+        sfp_av_w_per_l_per_s=sfp, dwelling_volume_m3=200.0,
+    )
+
+    # Assert
+    assert abs(kwh_at_100 - 30.5) <= 1e-4  # 0.25 × 1.22 × 100
+    assert abs(kwh_at_200 - 2 * kwh_at_100) <= 1e-4