diff --git a/packages/domain/src/domain/ml/demand.py b/packages/domain/src/domain/ml/demand.py
index c8b2c1f3..738eca74 100644
--- a/packages/domain/src/domain/ml/demand.py
+++ b/packages/domain/src/domain/ml/demand.py
@@ -99,11 +99,25 @@ def _default_occupants_sap_j(total_floor_area_m2: float) -> float:
 def predicted_hot_water_kwh(
     total_floor_area_m2: Optional[float],
     seasonal_efficiency_water: float,
+    *,
+    cylinder_size: Optional[int] = None,
+    cylinder_insulation_thickness_mm: Optional[int] = None,
+    cylinder_insulation_type: Optional[int] = None,
+    age_band: Optional[str] = None,
+    has_wwhrs: bool = False,
+    has_solar_water_heating: bool = False,
 ) -> float:
-    """Annual delivered hot-water kWh (SAP10.2 Appendix J simplified).
+    """Annual delivered hot-water kWh per SAP10.2 Appendix J (slice 17b).
 
-    Uses default occupancy from TFA, daily volume 25*N+36 litres, delta-T
-    55 - 12 = 43 K, no FGHRS / WWHRS adjustment.
+    Components (all kWh useful, sum then divided by efficiency for delivered):
+      useful_demand = 4.18 * Vd * 43 * 365 / 3600  (Vd in litres/day)
+      distribution_loss = useful_demand * 0.15
+      storage_loss     = volume * insulation_factor * 365 * 0.6
+      primary_loss(age) = 245 (A-J) or 60 (K-M)
+      wwhrs_credit     = useful_demand * 0.12  if has_wwhrs
+      solar_hw_credit  = 250                   if has_solar_water_heating
+
+    Defaults follow RdSAP10 §11 / Table 29 for missing cylinder fields.
     """
     if total_floor_area_m2 is None or total_floor_area_m2 <= 0:
         return 0.0
@@ -111,11 +125,94 @@ def predicted_hot_water_kwh(
         return 0.0
     n = _default_occupants_sap_j(total_floor_area_m2)
     vd_litres = 25.0 * n + 36.0
-    # 4.18 kJ/(kg K) * litres/day * delta-K * days = kJ/yr; /3600 -> kWh/yr.
     useful_kwh = 4.18 * vd_litres * (55.0 - 12.0) * 365.0 / 3600.0
-    # Add ~10% distribution + storage losses (SAP10.2 §L Table 3a typical).
-    useful_with_losses = useful_kwh * 1.10
-    return useful_with_losses / seasonal_efficiency_water
+    distribution_loss = useful_kwh * 0.15
+    storage_loss = _cylinder_storage_loss_kwh(
+        cylinder_size=cylinder_size,
+        cylinder_insulation_thickness_mm=cylinder_insulation_thickness_mm,
+        cylinder_insulation_type=cylinder_insulation_type,
+        age_band=age_band,
+    )
+    primary_loss = _primary_circuit_loss_kwh(age_band)
+    wwhrs_credit = useful_kwh * 0.12 if has_wwhrs else 0.0
+    solar_credit = 250.0 if has_solar_water_heating else 0.0
+    total_useful = max(
+        0.0,
+        useful_kwh + distribution_loss + storage_loss + primary_loss - wwhrs_credit - solar_credit,
+    )
+    return total_useful / seasonal_efficiency_water
+
+
+# SAP10.2 cylinder volume by RdSAP10 size code (Table 28).
+_CYLINDER_VOLUME_L: Final[dict[int, float]] = {1: 110.0, 2: 160.0, 3: 210.0}
+
+# SAP10.2 Table 2 storage loss factor (kWh / litre / day) by insulation
+# thickness in mm.  Lower number = better insulation.
+_STORAGE_LOSS_FACTOR: Final[dict[int, float]] = {
+    0: 0.0203,    # uninsulated -> high loss
+    12: 0.0152,   # 12 mm jacket
+    25: 0.0078,   # 25 mm foam
+    38: 0.0056,
+    50: 0.0043,
+    80: 0.0025,
+    100: 0.0022,
+    150: 0.0014,
+    200: 0.0011,
+}
+
+# RdSAP10 Table 29 cylinder-insulation default by age band when unknown:
+# A-F -> 12 mm jacket, G-H -> 25 mm foam, I-M -> 38 mm foam.
+_AGE_TO_DEFAULT_CYLINDER_INS_MM: Final[dict[str, int]] = {
+    "A": 12, "B": 12, "C": 12, "D": 12, "E": 12, "F": 12,
+    "G": 25, "H": 25,
+    "I": 38, "J": 38, "K": 38, "L": 38, "M": 38,
+}
+
+
+def _cylinder_storage_loss_kwh(
+    cylinder_size: Optional[int],
+    cylinder_insulation_thickness_mm: Optional[int],
+    cylinder_insulation_type: Optional[int],
+    age_band: Optional[str],
+) -> float:
+    """Annual cylinder storage loss (kWh useful, before efficiency division).
+
+    Returns 0 when no cylinder is described AND age_band is unknown (assume
+    instantaneous / combi without storage).  Heated-space modifier 0.6.
+    """
+    if cylinder_size is None and age_band is None:
+        return 0.0
+    volume = _CYLINDER_VOLUME_L.get(cylinder_size or 1, 110.0)
+    thickness = cylinder_insulation_thickness_mm
+    if thickness is None and age_band is not None:
+        thickness = _AGE_TO_DEFAULT_CYLINDER_INS_MM.get(age_band.upper())
+    if thickness is None:
+        thickness = 38
+    factor = _nearest_storage_loss_factor(thickness)
+    heated_space_modifier = 0.6
+    return volume * factor * 365.0 * heated_space_modifier
+
+
+def _nearest_storage_loss_factor(thickness_mm: int) -> float:
+    """Pick the SAP10.2 Table 2 row with thickness closest <= the supplied
+    value.  For thicknesses below 12 mm, uses the uninsulated 0-row."""
+    candidates = sorted(_STORAGE_LOSS_FACTOR.keys())
+    chosen = candidates[0]
+    for t in candidates:
+        if t <= thickness_mm:
+            chosen = t
+    return _STORAGE_LOSS_FACTOR[chosen]
+
+
+def _primary_circuit_loss_kwh(age_band: Optional[str]) -> float:
+    """Annual primary-pipework loss (kWh useful) by age band.
+
+    RdSAP10 Table 29: pre-2007 (A-J) no primary insulation -> 245 kWh/yr;
+    K, L, M -> full insulation -> 60 kWh/yr. Unknown -> 245.
+    """
+    if age_band is None:
+        return 245.0
+    return 60.0 if age_band.upper() in ("K", "L", "M") else 245.0
 
 
 def predicted_lighting_kwh(
diff --git a/packages/domain/src/domain/ml/tests/test_demand.py b/packages/domain/src/domain/ml/tests/test_demand.py
index 10df0804..2e6482d0 100644
--- a/packages/domain/src/domain/ml/tests/test_demand.py
+++ b/packages/domain/src/domain/ml/tests/test_demand.py
@@ -62,6 +62,99 @@ def test_predicted_hot_water_returns_zero_for_unspecified_floor_area() -> None:
     assert predicted_hot_water_kwh(total_floor_area_m2=None, seasonal_efficiency_water=0.84) == 0.0
 
 
+def test_predicted_hot_water_kwh_adds_storage_loss_when_cylinder_described() -> None:
+    # Arrange — SAP10.2 Appendix J / Table 2: cylinder storage loss adds to
+    # the delivered DHW load.  For a 110L cylinder with 38 mm foam (typical
+    # post-1992) the loss factor is 0.0056 kWh/L/day; annual loss in heated
+    # space = 110 * 0.0056 * 365 * 0.6 = 135 kWh useful -> delivered loss
+    # /efficiency.  Same home without cylinder description gets the simple
+    # formula (no storage term).
+
+    # Act
+    with_cylinder = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0,
+        seasonal_efficiency_water=0.84,
+        cylinder_size=1,
+        cylinder_insulation_thickness_mm=38,
+        cylinder_insulation_type=2,  # foam
+    )
+    without_cylinder = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0,
+        seasonal_efficiency_water=0.84,
+    )
+
+    # Assert
+    assert with_cylinder > without_cylinder
+    # storage_loss = 110 * 0.0056 * 365 * 0.6 / 0.84 ≈ 161 kWh delivered.
+    assert (with_cylinder - without_cylinder) == pytest.approx(161.0, abs=15.0)
+
+
+def test_predicted_hot_water_kwh_lower_storage_loss_for_thicker_insulation() -> None:
+    # Arrange — same cylinder size, 12mm jacket vs 100mm foam.
+
+    # Act
+    jacket = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84,
+        cylinder_size=1, cylinder_insulation_thickness_mm=12, cylinder_insulation_type=1,
+    )
+    foam_100mm = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84,
+        cylinder_size=1, cylinder_insulation_thickness_mm=100, cylinder_insulation_type=2,
+    )
+
+    # Assert
+    assert jacket > foam_100mm
+
+
+def test_predicted_hot_water_kwh_drops_with_wwhrs() -> None:
+    # Arrange — WWHRS recovers ~15% of bath energy.
+
+    # Act
+    no_wwhrs = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84, has_wwhrs=False,
+    )
+    with_wwhrs = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84, has_wwhrs=True,
+    )
+
+    # Assert
+    assert with_wwhrs < no_wwhrs
+
+
+def test_predicted_hot_water_kwh_drops_with_solar_water_heating() -> None:
+    # Arrange — solar HW saves ~250 kWh/yr (SAP10.2 Appendix G simplified).
+
+    # Act
+    no_solar = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84, has_solar_water_heating=False,
+    )
+    with_solar = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84, has_solar_water_heating=True,
+    )
+
+    # Assert
+    assert with_solar < no_solar
+
+
+def test_predicted_hot_water_kwh_uses_age_band_default_when_insulation_unspecified() -> None:
+    # Arrange — RdSAP10 Table 29: A-F -> 12mm jacket; G-H -> 25mm foam; I-M -> 38mm foam.
+    # Age G cylinder with no explicit insulation should default to 25mm foam,
+    # giving a lower loss than age A (12mm jacket).
+
+    # Act
+    age_a = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84,
+        cylinder_size=1, age_band="A",
+    )
+    age_g = predicted_hot_water_kwh(
+        total_floor_area_m2=80.0, seasonal_efficiency_water=0.84,
+        cylinder_size=1, age_band="G",
+    )
+
+    # Assert
+    assert age_g < age_a
+
+
 def test_predicted_hot_water_typical_uk_home_falls_in_sensible_range() -> None:
     # Arrange — 80 m^2 home, gas-combi efficiency.
 
diff --git a/packages/domain/src/domain/ml/tests/test_transform.py b/packages/domain/src/domain/ml/tests/test_transform.py
index acd635e7..20734af8 100644
--- a/packages/domain/src/domain/ml/tests/test_transform.py
+++ b/packages/domain/src/domain/ml/tests/test_transform.py
@@ -36,7 +36,7 @@ def test_transform_advertises_version_and_target_columns() -> None:
 
     # Assert
     assert isinstance(schema, TransformSchema)
-    assert schema.transform_version == "2.1.0"
+    assert schema.transform_version == "2.2.0"
     assert schema.transform_version == EpcMlTransform.VERSION
     assert set(schema.target_columns.keys()) == set(_EXPECTED_TARGET_DTYPES.keys())
     for target_name, expected_dtype in _EXPECTED_TARGET_DTYPES.items():
diff --git a/packages/domain/src/domain/ml/transform.py b/packages/domain/src/domain/ml/transform.py
index ec08a575..a3a72821 100644
--- a/packages/domain/src/domain/ml/transform.py
+++ b/packages/domain/src/domain/ml/transform.py
@@ -901,7 +901,7 @@ class EpcMlTransform:
     Version 0.1.0 — schema contract only; feature columns added in subsequent slices.
     """
 
-    VERSION: str = "2.1.0"
+    VERSION: str = "2.2.0"
 
     def schema(self) -> TransformSchema:
         """The cross-repo ML data contract.
@@ -969,9 +969,19 @@ class EpcMlTransform:
             region_code=epc.region_code,
             seasonal_efficiency_main=space_eff,
         )
+        cylinder_size_val = heating_aggregates.get("cylinder_size")
+        cylinder_ins_thk = heating_aggregates.get("cylinder_insulation_thickness_mm")
+        cylinder_ins_type = heating_aggregates.get("cylinder_insulation_type")
+        main_age = building_part_aggregates.get("main_dwelling_construction_age_band")
         pred_hw_kwh = predicted_hot_water_kwh(
             total_floor_area_m2=epc.total_floor_area_m2,
             seasonal_efficiency_water=water_eff,
+            cylinder_size=cylinder_size_val if isinstance(cylinder_size_val, int) else None,
+            cylinder_insulation_thickness_mm=cylinder_ins_thk if isinstance(cylinder_ins_thk, int) else None,
+            cylinder_insulation_type=cylinder_ins_type if isinstance(cylinder_ins_type, int) else None,
+            age_band=main_age if isinstance(main_age, str) else None,
+            has_wwhrs=bool(epc.sap_heating.number_baths_wwhrs and epc.sap_heating.number_baths_wwhrs > 0),
+            has_solar_water_heating=epc.solar_water_heating,
         )
         pred_light_kwh = predicted_lighting_kwh(
             total_floor_area_m2=epc.total_floor_area_m2,