diff --git a/docs/sap-spec/HANDOVER_NEXT.md b/docs/sap-spec/HANDOVER_NEXT.md
index ba6eb72c..1a10dd3a 100644
--- a/docs/sap-spec/HANDOVER_NEXT.md
+++ b/docs/sap-spec/HANDOVER_NEXT.md
@@ -133,12 +133,12 @@ Two test files contain the strict pins:
 Total: **169 PASS / 83 FAIL** across the strict pins. 4 of 6 fixtures fully
 close §1+§2+§4. 000487 is the worst (RR fixture defect propagates everywhere).
 
-(Post-slice-25c: section_cascade_pins 293 PASS / 19 FAIL, e2e SapResult
-32 PASS / 40 FAIL. §3 closes 24/24. §4 closes 53/54 (only 000487
-LINE_65 remains — Appendix J step 8 electric-shower kWh derivation,
-slice 25d). §5 52/54, §6 11/12 — only 000487 cascade. 000477's §4
-combi-loss cluster was closed by fixing the SAP10.2 Table 3c (p.162)
-M+L lower bound: my DVF used `V < 100.0` where spec says `V < 100.2`.)
+(Post-slice-25d: section_cascade_pins 304 PASS / 8 FAIL, e2e SapResult
+33 PASS / 39 FAIL. §3 + §4 + §5 + §6 ALL fully close for all 6
+fixtures. §7 closes 52/60 — only LINE_92/93 marginal ~0.0001 K
+residual on 000474/477/480/490 remains (the precision artefact
+discovered in slice 26c, no spec-grounded fix identified). Cascade
+total closure: 312 tests, 304 PASS = 97.4%.)
 
 ### B.2 SapResult pin matrix (post-slice-22/23)
 
@@ -199,6 +199,7 @@ fixture | section §4 pin status
 ### B.5 Recent slices (in reverse order — newest first)
 
 ```
+Slice 25d: 000487 §4 LINE_65 closure — derive LINE_64A electric-shower kWh from cert (Appendix J step 8, p.82)
 Slice 25c: 000477 §4/§5/§6 closure — SAP10.2 Table 3c (p.162) M+L lower bound 100.0 → 100.2
 Slice 25b: 000487 §4 closure (7/8) — has_electric_shower + mixer/electric counts on SapHeating, Appendix J step 2a fix
 Slice 25a: 000487 §3 closure — detailed RR + gable_wall_external + Ext1 alt U=1.9 + §3.8 max-floor roof + half-up rounding
diff --git a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
index 2545c976..678d6b8c 100644
--- a/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
+++ b/packages/domain/src/domain/sap/rdsap/cert_to_inputs.py
@@ -719,6 +719,7 @@ def water_heating_section_from_cert(
         else None
     )
     has_electric_shower = _has_electric_shower_from_cert(epc)
+    electric_shower_count = _electric_shower_count_from_cert(epc)
     bootstrap = water_heating_from_cert(
         epc=epc,
         mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
@@ -726,6 +727,7 @@ def water_heating_section_from_cert(
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
         has_electric_shower=has_electric_shower,
+        electric_shower_count=electric_shower_count,
     )
     combi_loss_override = pcdb_combi_loss_override(
         pcdb_main,
@@ -740,6 +742,7 @@ def water_heating_section_from_cert(
         low_water_use=False,
         combi_loss_monthly_kwh_override=combi_loss_override,
         has_electric_shower=has_electric_shower,
+        electric_shower_count=electric_shower_count,
     )
 
 
@@ -979,6 +982,14 @@ def _has_electric_shower_from_cert(epc: EpcPropertyData) -> bool:
     return (n or 0) >= 1
 
 
+def _electric_shower_count_from_cert(epc: EpcPropertyData) -> int:
+    """Cert-lodged count of instantaneous electric showers. Drives the
+    LINE_64A energy derivation in `water_heating_from_cert` per SAP10.2
+    Appendix J (p.82) step 8."""
+    n = epc.sap_heating.electric_shower_count if epc.sap_heating is not None else None
+    return max(0, n or 0)
+
+
 def _has_bath_from_cert(epc: EpcPropertyData) -> bool:
     """True iff cert lodges ≥ 1 bath. `number_baths is None` is treated as
     bath present (modal UK lodging — bathless dwellings are rare and
@@ -1070,6 +1081,7 @@ def _water_heating_worksheet_and_gains(
     if epc.total_floor_area_m2 is None:
         return None, zero_monthly
     has_electric_shower = _has_electric_shower_from_cert(epc)
+    electric_shower_count = _electric_shower_count_from_cert(epc)
     bootstrap = water_heating_from_cert(
         epc=epc,
         mixer_shower_flow_rates_l_per_min=_mixer_shower_flow_rates_from_cert(epc),
@@ -1077,6 +1089,7 @@ def _water_heating_worksheet_and_gains(
         cold_water_temps_c=TABLE_J1_TCOLD_FROM_MAINS_C,
         low_water_use=False,
         has_electric_shower=has_electric_shower,
+        electric_shower_count=electric_shower_count,
     )
     combi_loss_override = pcdb_combi_loss_override(
         pcdb_record,
@@ -1091,6 +1104,7 @@ def _water_heating_worksheet_and_gains(
         low_water_use=False,
         combi_loss_monthly_kwh_override=combi_loss_override,
         has_electric_shower=has_electric_shower,
+        electric_shower_count=electric_shower_count,
     )
     return wh_result, wh_result.heat_gains_monthly_kwh
 
diff --git a/packages/domain/src/domain/sap/worksheet/water_heating.py b/packages/domain/src/domain/sap/worksheet/water_heating.py
index dac1b18f..16cc6748 100644
--- a/packages/domain/src/domain/sap/worksheet/water_heating.py
+++ b/packages/domain/src/domain/sap/worksheet/water_heating.py
@@ -586,6 +586,49 @@ def hot_water_other_uses_monthly_l_per_day(
     return tuple(annual_average * f for f in _TABLE_J2_MONTHLY_FACTORS)
 
 
+_ELECTRIC_SHOWER_DEFAULT_KW: Final[float] = 9.3
+_ELECTRIC_SHOWER_DURATION_HOURS: Final[float] = 0.1
+
+
+def electric_shower_monthly_kwh(
+    *,
+    n_occupants: float,
+    has_bath: bool,
+    n_outlets: int,
+    n_electric_showers: int,
+    rated_power_kw: float = _ELECTRIC_SHOWER_DEFAULT_KW,
+) -> tuple[float, ...]:
+    """SAP 10.2 §4 line (64a)m via Appendix J (p.82) step 8.
+
+    Per outlet for each month:
+        N_ES = N_shower / N_outlets    (eq J16)
+        E_ES,j,m = N_ES × f_beh × P_ES,j × 0.1 × n_m  (eq J17)
+    Summed across electric-shower outlets j (eq J18).
+
+    N_shower from step 1c (same branch as `hot_water_mixer_showers_monthly_
+    l_per_day`); N_outlets is the cert-lodged total of mixer + electric
+    outlets. P_ES,j defaults to Table J4 row "Instantaneous electric
+    shower" = 9.3 kW for assessments of existing dwellings.
+
+    Returns 12-tuple of zeros when there are no electric showers."""
+    if n_electric_showers <= 0 or n_outlets <= 0:
+        return tuple(0.0 for _ in range(12))
+    if has_bath:
+        n_shower = 0.45 * n_occupants + 0.65
+    else:
+        n_shower = 0.58 * n_occupants + 0.83
+    n_es_per_outlet = n_shower / n_outlets
+    return tuple(
+        n_electric_showers
+        * n_es_per_outlet
+        * fbeh
+        * rated_power_kw
+        * _ELECTRIC_SHOWER_DURATION_HOURS
+        * n_m
+        for fbeh, n_m in zip(TABLE_J5_BEHAVIOURAL_FACTOR, _DAYS_IN_MONTH)
+    )
+
+
 def water_heating_from_cert(
     *,
     epc: EpcPropertyData,
@@ -596,6 +639,7 @@ def water_heating_from_cert(
     combi_loss_monthly_kwh_override: Optional[tuple[float, ...]] = None,
     electric_shower_monthly_kwh_override: Optional[tuple[float, ...]] = None,
     has_electric_shower: bool = False,
+    electric_shower_count: int = 0,
 ) -> WaterHeatingResult:
     """SAP 10.2 §4 orchestrator — chain every line ref from (42) through
     (65) for a combi-gas dwelling with optional PCDB-backed combi loss.
@@ -687,11 +731,20 @@ def water_heating_from_cert(
         solar_monthly_kwh=zero12,
         fghrs_monthly_kwh=zero12,
     )
-    electric_shower = (
-        electric_shower_monthly_kwh_override
-        if electric_shower_monthly_kwh_override is not None
-        else zero12
-    )
+    if electric_shower_monthly_kwh_override is not None:
+        electric_shower = electric_shower_monthly_kwh_override
+    elif electric_shower_count > 0:
+        # Appendix J step 8 — N_outlets counts mixer + electric outlets
+        # together (eq J16).
+        n_outlets_total = len(mixer_shower_flow_rates_l_per_min) + electric_shower_count
+        electric_shower = electric_shower_monthly_kwh(
+            n_occupants=n,
+            has_bath=has_bath,
+            n_outlets=n_outlets_total,
+            n_electric_showers=electric_shower_count,
+        )
+    else:
+        electric_shower = zero12
     gains = heat_gains_from_water_heating_monthly_kwh(
         energy_content_monthly_kwh=energy_content,
         distribution_loss_monthly_kwh=distribution,