diff --git a/domain/sap10_calculator/rdsap/cert_to_inputs.py b/domain/sap10_calculator/rdsap/cert_to_inputs.py
index 71367992..7b77c669 100644
--- a/domain/sap10_calculator/rdsap/cert_to_inputs.py
+++ b/domain/sap10_calculator/rdsap/cert_to_inputs.py
@@ -105,6 +105,9 @@ from domain.sap10_calculator.tables.table_12a import (
     tariff_from_meter_type,
     water_heating_high_rate_fraction,
 )
+from domain.sap10_calculator.tables.table_13 import (
+    electric_dhw_high_rate_fraction,
+)
 from domain.sap10_calculator.tables.table_32 import (
     additional_standing_charges_gbp,
     is_electric_fuel_code,
@@ -2265,6 +2268,9 @@ def _hot_water_fuel_cost_gbp_per_kwh(
     *,
     water_heating_code: Optional[int] = None,
     inherit_main_for_community_heating: bool = False,
+    cylinder_volume_l: Optional[float] = None,
+    occupancy_n: Optional[float] = None,
+    immersion_single: Optional[bool] = None,
 ) -> float:
     """Hot water bills at the *water-heating* fuel's rate. When the
     water-heating fuel is electric AND tariff is off-peak, bill at the
@@ -2278,10 +2284,18 @@ def _hot_water_fuel_cost_gbp_per_kwh(
     ∈ {901, 902, 914}) and that main is a PCDB Table 362 heat pump, the
     HW bills per SAP 10.2 Table 12a Grid 1 WH column (PDF p.191) — the
     ASHP/GSHP-from-database row carries a 0.70 high-rate fraction at
-    7-hour and 10-hour, NOT 100% off-peak low rate. Electric IMMERSION
-    (WHC 903) is a different Table 12a row (off-peak immersion 0.17 /
-    Table 13) and stays on the 100%-low-rate fallback until that slice
-    lands.
+    7-hour and 10-hour, NOT 100% off-peak low rate.
+
+    Electric IMMERSION exception (WHC 903): Table 12a's "Immersion water
+    heater" row (PDF p.191) routes the WH column to Table 13 (PDF p.197).
+    The Table 13 high-rate fraction — a function of cylinder volume,
+    assumed occupancy and single-/dual-immersion — gives the proportion
+    billed at the high rate, the remainder at the low rate. Without it
+    the immersion HW billed 100% at the off-peak low rate, under-costing
+    the dwelling and over-rating it (median +0.98 SAP across the off-peak
+    WHC-903 API cohort). Needs `cylinder_volume_l` + `occupancy_n` +
+    `immersion_single`; absent any of them (no cylinder / volume not
+    resolvable) it falls back to the 100%-low-rate scalar.
 
     `inherit_main_for_community_heating`: per S0380.173, when WHC
     ∈ {901, 902, 914} AND main is a heat network, ignore the cert-
@@ -2306,6 +2320,21 @@ def _hot_water_fuel_cost_gbp_per_kwh(
             )
             blended = high_frac * high_rate + (1.0 - high_frac) * low_rate
             return blended * _PENCE_TO_GBP
+        if (
+            water_heating_code == _WHC_ELECTRIC_IMMERSION
+            and cylinder_volume_l is not None
+            and occupancy_n is not None
+            and immersion_single is not None
+        ):
+            high_rate, low_rate = _tariff_high_low_rates_p_per_kwh(tariff)
+            high_frac = electric_dhw_high_rate_fraction(
+                cylinder_volume_l=cylinder_volume_l,
+                occupancy_n=occupancy_n,
+                single_immersion=immersion_single,
+                tariff=tariff,
+            )
+            blended = high_frac * high_rate + (1.0 - high_frac) * low_rate
+            return blended * _PENCE_TO_GBP
         return _off_peak_low_rate_gbp_per_kwh(tariff)
     if water_heating_fuel is not None:
         return prices.unit_price_p_per_kwh(water_heating_fuel) * _PENCE_TO_GBP
@@ -4851,6 +4880,17 @@ _CYLINDER_SIZE_CODE_TO_LITRES: Final[dict[int, float]] = {
     2: 110.0, 3: 160.0, 4: 210.0
 }
 
+# RdSAP `immersion_heating_type` lodgement codes. Code 1 = DUAL immersion,
+# code 2 = SINGLE. Confirmed against RdSAP 10 §10.5 (PDF p.54 — an
+# immersion is "assumed dual" on a dual/off-peak meter) cross-checked
+# with the API cohort: code 1 sits 3.6:1 on dual meters (40 vs 11 single)
+# while code 2 sits on single meters (22 single vs 16 dual). This INVERTS
+# the unverified "1=single, 2=dual" annotation in an earlier handover —
+# the dual code (1) carries Table 13's small high-rate fraction, matching
+# the cohort's over-rating direction; treating code 1 as single overshot.
+_IMMERSION_TYPE_DUAL: Final[int] = 1
+_IMMERSION_TYPE_SINGLE: Final[int] = 2
+
 # RdSAP 10 §10.5 code 7-11: cylinder insulation type. Empirical mapping
 # from the ASHP cohort (all 7 certs lodge code 1, worksheet shows
 # "Foam" → factory-applied per SAP 10.2 Table 2 Note 2).
@@ -5673,6 +5713,21 @@ def _hot_water_cylinder_volume_l(epc: EpcPropertyData) -> Optional[float]:
     return _CYLINDER_SIZE_CODE_TO_LITRES.get(size_code)
 
 
+def _immersion_is_single(epc: EpcPropertyData) -> Optional[bool]:
+    """True for a single immersion, False for a dual immersion, None when
+    the cert lodges no recognised `immersion_heating_type`. Maps the
+    RdSAP code (1 = dual, 2 = single — see `_IMMERSION_TYPE_DUAL`).
+    None makes the Table 13 high-rate-fraction caller fall back to the
+    100%-low-rate scalar rather than guess the immersion configuration.
+    """
+    code = _int_or_none(epc.sap_heating.immersion_heating_type)
+    if code == _IMMERSION_TYPE_DUAL:
+        return False
+    if code == _IMMERSION_TYPE_SINGLE:
+        return True
+    return None
+
+
 def _table_3a_combi_loss_default_applies(main: Optional[MainHeatingDetail]) -> bool:
     """Gate for the Table 3a keep-hot 600 kWh/yr fall-through per SAP 10.2
     §4 line 7702. Returns True only when the main heating system is in the
@@ -7058,6 +7113,9 @@ def cert_to_inputs(
             prices,
             water_heating_code=epc.sap_heating.water_heating_code,
             inherit_main_for_community_heating=_community_hw_inherit,
+            cylinder_volume_l=_hot_water_cylinder_volume_l(epc),
+            occupancy_n=wh_result.occupancy if wh_result is not None else None,
+            immersion_single=_immersion_is_single(epc),
         )
         hw_co2_factor = _hot_water_co2_factor_kg_per_kwh(
             epc, hw_monthly_kwh_for_factors, _rdsap_tariff(epc),
diff --git a/domain/sap10_calculator/tables/table_13.py b/domain/sap10_calculator/tables/table_13.py
new file mode 100644
index 00000000..f96bd71d
--- /dev/null
+++ b/domain/sap10_calculator/tables/table_13.py
@@ -0,0 +1,70 @@
+"""SAP 10.2 Table 13 — high-rate fraction for electric DHW heating.
+
+Sourced verbatim from `domain/sap10_calculator/docs/specs/sap-10-2-full-
+specification-2025-03-14.pdf`, page 197 (Table 13). RdSAP10 §10.5 (PDF
+p.54) routes electric immersion water heating here via Table 12a's
+"Immersion water heater" row, whose Water-heating column reads "Fraction
+from Table 13".
+
+The table gives the fraction of DHW electricity consumed at the HIGH
+rate for a cylinder with a single or dual immersion heater on an
+off-peak tariff; the remainder is at the low rate. Note 2 of the table
+supplies exact equations equivalent to the tabulated (rounded) grid —
+this module evaluates those equations, so no floor-area interpolation
+is needed:
+
+    7-hour tariff (>= 7 hours/day at the low rate)
+      Dual:   [(6.8 - 0.024 V) N + 14 - 0.07 V] / 100
+      Single: [(14530 - 762 N) / V - 80 + 10 N] / 100
+
+    10-hour tariff (>= 10 hours/day at the low rate)
+      Dual:   [(6.8 - 0.036 V) N + 14 - 0.105 V] / 100
+      Single: [(14530 - 762 N) / (1.5 V) - 80 + 10 N] / 100
+
+where V is the cylinder volume (litres) and N is the assumed occupancy
+(Appendix J Table 1b). Per Note 2 the result is clamped to [0, 1]. Per
+Note 1 the 10-hour equations apply to any tariff providing at least 10
+hours/day at the low rate (so 18-hour and 24-hour use the 10-hour
+column). Heat pumps providing water heating only are treated as dual
+immersion (Note 1) — out of scope of this helper (callers route those
+via Table 12a).
+"""
+
+from __future__ import annotations
+
+from domain.sap10_calculator.tables.table_12a import Tariff
+
+
+def electric_dhw_high_rate_fraction(
+    *,
+    cylinder_volume_l: float,
+    occupancy_n: float,
+    single_immersion: bool,
+    tariff: Tariff,
+) -> float:
+    """SAP 10.2 Table 13 (PDF p.197) high-rate fraction for an electric
+    immersion DHW cylinder on an off-peak tariff.
+
+    `single_immersion` selects the single- vs dual-immersion equation
+    (RdSAP10 §10.5 p.54: an immersion is assumed dual on a dual meter).
+    The 7-hour tariff uses the 7-hour equations; every other off-peak
+    tariff (10/18/24-hour, all >= 10 hours low-rate per Note 1) uses the
+    10-hour equations. STANDARD has no off-peak split and is rejected —
+    callers must early-return before this fires.
+    """
+    if tariff is Tariff.STANDARD:
+        raise ValueError("Table 13 high-rate fraction is undefined for STANDARD")
+    v = cylinder_volume_l
+    n = occupancy_n
+    if tariff is Tariff.SEVEN_HOUR:
+        if single_immersion:
+            fraction = ((14530 - 762 * n) / v - 80 + 10 * n) / 100
+        else:
+            fraction = ((6.8 - 0.024 * v) * n + 14 - 0.07 * v) / 100
+    else:
+        # >= 10 hours/day at the low rate (10/18/24-hour) — Note 1.
+        if single_immersion:
+            fraction = ((14530 - 762 * n) / (1.5 * v) - 80 + 10 * n) / 100
+        else:
+            fraction = ((6.8 - 0.036 * v) * n + 14 - 0.105 * v) / 100
+    return max(0.0, min(1.0, fraction))
diff --git a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
index d0f12b4d..83a65467 100644
--- a/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
+++ b/tests/domain/sap10_calculator/rdsap/test_cert_to_inputs.py
@@ -3271,8 +3271,9 @@ def test_hot_water_from_pcdb_heat_pump_bills_at_app_n_wh_high_rate() -> None:
     # 10-hour. `_hot_water_fuel_cost_gbp_per_kwh` previously billed any
     # electric off-peak HW at 100% low rate (its TODO), over-crediting the
     # HP-DHW cat-4 cluster. Electric IMMERSION (WHC 903) is a different
-    # Table 12a row (off-peak immersion 0.17 / Table 13) and must stay on
-    # the 100%-low-rate fallback here.
+    # Table 12a row (Table 13) — without the cylinder volume / occupancy /
+    # immersion-type inputs (not passed here) it falls back to the
+    # 100%-low-rate scalar; the Table 13 blend is locked separately below.
     from domain.sap10_calculator.tables.table_12a import Tariff
     from domain.sap10_calculator.rdsap.cert_to_inputs import (
         _hot_water_fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
@@ -3305,6 +3306,56 @@ def test_hot_water_from_pcdb_heat_pump_bills_at_app_n_wh_high_rate() -> None:
     assert abs(rate_immersion - 0.0750) <= 1e-6
 
 
+def test_hot_water_immersion_off_peak_bills_at_table_13_blend() -> None:
+    # Arrange — SAP 10.2 Table 12a (PDF p.191) "Immersion water heater"
+    # row routes the WH column to Table 13 (PDF p.197). For an electric
+    # immersion (WHC 903) on an off-peak tariff with a known cylinder
+    # volume + occupancy + immersion type, the HW bills at the Table 13
+    # high-rate fraction blend, NOT 100% at the off-peak low rate. A dual
+    # immersion (small fraction) bills only a little above the low rate; a
+    # single immersion (large fraction) bills much closer to the high rate.
+    # Pre-slice both billed 100% at the 7-hour low rate 5.50 p (£0.0550),
+    # under-costing the dwelling and over-rating it (median +0.98 SAP
+    # across the off-peak WHC-903 API cohort).
+    from domain.sap10_calculator.tables.table_12a import Tariff
+    from domain.sap10_calculator.rdsap.cert_to_inputs import (
+        _hot_water_fuel_cost_gbp_per_kwh,  # pyright: ignore[reportPrivateUsage]
+    )
+    from domain.sap10_calculator.tables.table_13 import (
+        electric_dhw_high_rate_fraction,
+    )
+    high_p, low_p = 15.29, 5.50  # Table 32 codes 32 / 31 (7-hour)
+    n_occupants = 2.7395  # Appendix J Table 1b N at 100 m²
+
+    # Act — 110 L cylinder, occupancy N(100), dual (False) vs single (True).
+    rate_dual = _hot_water_fuel_cost_gbp_per_kwh(
+        29, None, Tariff.SEVEN_HOUR, SAP_10_2_SPEC_PRICES,
+        water_heating_code=903, cylinder_volume_l=110.0,
+        occupancy_n=n_occupants, immersion_single=False,
+    )
+    rate_single = _hot_water_fuel_cost_gbp_per_kwh(
+        29, None, Tariff.SEVEN_HOUR, SAP_10_2_SPEC_PRICES,
+        water_heating_code=903, cylinder_volume_l=110.0,
+        occupancy_n=n_occupants, immersion_single=True,
+    )
+
+    # Assert — each rate equals its Table 13 blend; single > dual; both
+    # strictly above the 100%-low fallback (5.50 p) it replaces.
+    frac_dual = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=n_occupants,
+        single_immersion=False, tariff=Tariff.SEVEN_HOUR,
+    )
+    frac_single = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=n_occupants,
+        single_immersion=True, tariff=Tariff.SEVEN_HOUR,
+    )
+    expected_dual = (frac_dual * high_p + (1 - frac_dual) * low_p) / 100
+    expected_single = (frac_single * high_p + (1 - frac_single) * low_p) / 100
+    assert abs(rate_dual - expected_dual) <= 1e-9
+    assert abs(rate_single - expected_single) <= 1e-9
+    assert rate_single > rate_dual > 0.0550
+
+
 def test_space_heating_pcdb_heat_pump_without_sap_code_bills_at_app_n_high_rate() -> None:
     # Arrange — an API-path heat pump resolves via its PCDB Table 362
     # index alone (data_source=1, no Table-4a SAP code lodged), so
diff --git a/tests/domain/sap10_calculator/test_table_13.py b/tests/domain/sap10_calculator/test_table_13.py
new file mode 100644
index 00000000..d6857cc2
--- /dev/null
+++ b/tests/domain/sap10_calculator/test_table_13.py
@@ -0,0 +1,103 @@
+"""SAP 10.2 Table 13 — high-rate fraction for electric DHW heating.
+
+Locks `electric_dhw_high_rate_fraction` against the published table grid
+and the Note-2 clamp at
+`domain/sap10_calculator/docs/specs/sap-10-2-full-specification-2025-03-14.pdf`,
+page 197. Table 12a's "Immersion water heater" row (PDF p.191) routes
+electric immersion DHW here.
+
+The helper evaluates the Note-2 equations, which the spec offers as an
+exact alternative to the rounded grid — the pins below check that the
+equations reproduce the published 2-dp cells.
+"""
+from __future__ import annotations
+
+from domain.sap10_calculator.tables.table_12a import Tariff
+from domain.sap10_calculator.tables.table_13 import (
+    electric_dhw_high_rate_fraction,
+)
+
+# Appendix J Table 1b occupancy N at a few total floor areas (m²) — the
+# anchor for the V/N grid cells below. Computed from the same piecewise
+# formula the §4 worksheet uses (water_heating.assumed_occupancy).
+_N_AT_TFA_100 = 2.7395  # N(100 m²)
+_N_AT_TFA_60 = 1.9816   # N(60 m²)
+
+# Table 13 high-rate-fraction grid cells (PDF p.197), keyed by (floor
+# area row, cylinder litres column, tariff, single?) → published value.
+_GRID_TOL = 0.005  # the published grid is rounded to 2 dp
+
+
+def test_table_13_dual_immersion_matches_published_grid() -> None:
+    # Arrange — SAP 10.2 Table 13 (PDF p.197), 110 L cylinder, dual
+    # immersion. Floor area 100 m² row: 7-hour = 0.18, 10-hour = 0.10.
+
+    # Act
+    seven = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=False, tariff=Tariff.SEVEN_HOUR,
+    )
+    ten = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=False, tariff=Tariff.TEN_HOUR,
+    )
+
+    # Assert
+    assert abs(seven - 0.18) <= _GRID_TOL
+    assert abs(ten - 0.10) <= _GRID_TOL
+
+
+def test_table_13_single_immersion_matches_published_grid() -> None:
+    # Arrange — SAP 10.2 Table 13 (PDF p.197), 110 L cylinder, single
+    # immersion. Floor area 100 m² row: 7-hour = 0.61, 10-hour = 0.23.
+    # Single immersion carries a much larger high-rate fraction than dual.
+
+    # Act
+    seven = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=True, tariff=Tariff.SEVEN_HOUR,
+    )
+    ten = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=True, tariff=Tariff.TEN_HOUR,
+    )
+
+    # Assert
+    assert abs(seven - 0.61) <= _GRID_TOL
+    assert abs(ten - 0.23) <= _GRID_TOL
+
+
+def test_table_13_large_cylinder_single_immersion_clamps_to_zero() -> None:
+    # Arrange — SAP 10.2 Table 13 Note 2 (PDF p.197): "If these formulae
+    # give a value less than zero, set the high-rate fraction to zero." A
+    # 210 L cylinder with single immersion on a 10-hour tariff falls below
+    # zero (the published 210 L 10-hour column is 0), so the helper clamps.
+
+    # Act
+    fraction = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=210.0, occupancy_n=_N_AT_TFA_60,
+        single_immersion=True, tariff=Tariff.TEN_HOUR,
+    )
+
+    # Assert
+    assert fraction == 0.0
+
+
+def test_table_13_eighteen_hour_uses_ten_hour_column() -> None:
+    # Arrange — SAP 10.2 Table 13 Note 1 (PDF p.197): the table applies
+    # "for tariffs providing at least 10 hours ... at the low rate", so an
+    # 18-hour tariff resolves to the 10-hour equations, not a separate
+    # column.
+
+    # Act
+    eighteen = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=False, tariff=Tariff.EIGHTEEN_HOUR,
+    )
+    ten = electric_dhw_high_rate_fraction(
+        cylinder_volume_l=110.0, occupancy_n=_N_AT_TFA_100,
+        single_immersion=False, tariff=Tariff.TEN_HOUR,
+    )
+
+    # Assert
+    assert abs(eighteen - ten) <= 1e-9