fix(prediction): count each charging car's energy once in the house load

apps/predbat/prediction.py

@@ -889,8 +889,9 @@ def run_prediction(self, charge_limit, charge_window, export_window, export_limi
 
                         if self.car_energy_reported_load:
                             # Only add load if the car is reporting it as load, otherwise its outside the CT Clamp
+                            # Each car adds its own energy; car_amount_premium is the running total across cars
                             car_amount_premium += car_load_scale / self.car_charging_loss
-                            load_yesterday += car_amount_premium
+                            load_yesterday += car_load_scale / self.car_charging_loss
                         else:
                             car_load_energy_bypass += car_load_scale / self.car_charging_loss
 

apps/predbat/prediction_kernel.cpp

@@ -57,7 +57,9 @@
 // Parity 15: in PV10, an Intelligent dispatch minute assumed gone (io_flag, beyond 30 minutes) no longer
 // holds the battery for the car. The car still charges at the rate nominal pays, kept off the battery
 // by adding it to the grid balance after the battery has acted; the rest of the import pays rate_max.
-#define PK_PARITY_REVISION 15
+// Parity 16: each car charging in a step adds its own energy to the house load, not the running total
+// across cars, which counted the first car once per car (GH#5313).
+#define PK_PARITY_REVISION 16
 #define PK_MAX_CARS 8
 #define PK_RUN_EVERY 5 // const.py RUN_EVERY
 #define PK_EXPORT_MODE_TARGET 0 // const.py EXPORT_MODE_TARGET
@@ -899,9 +901,9 @@ static int32_t pk_run_one(const ContextStore *store, const PkScenario *s, PkResu
                     car_rate_premium = std::max(car_rate_premium, std::max(0.0, c->car_rate_flat[car_n * n_steps + k] - import_rate));
 
                     if (c->car_energy_reported_load) {
-                        // Note: mirrors the Python engine exactly - the cumulative premium amount is added per car
+                        // Each car adds its own energy; car_amount_premium is the running total across cars
                         car_amount_premium += car_load_scale / c->car_charging_loss;
-                        load_yesterday += car_amount_premium;
+                        load_yesterday += car_load_scale / c->car_charging_loss;
                     } else {
                         car_load_energy_bypass += car_load_scale / c->car_charging_loss;
                     }

apps/predbat/prediction_kernel.py

@@ -33,7 +33,7 @@
 
 # Expected ABI/parity revisions of the shared library (see prediction_kernel.cpp)
 KERNEL_ABI_VERSION = 7
-KERNEL_PARITY_REVISION = 15
+KERNEL_PARITY_REVISION = 16
 
 # Maximum number of cars supported by the kernel (PK_MAX_CARS in prediction_kernel.cpp)
 KERNEL_MAX_CARS = PREDBAT_MAX_CARS

apps/predbat/tests/test_multi_car_load.py

@@ -0,0 +1,83 @@
+# -----------------------------------------------------------------------------
+# Predbat Home Battery System
+# Copyright Trefor Southwell 2026 - All Rights Reserved
+# This application maybe used for personal use only and not for commercial use
+# -----------------------------------------------------------------------------
+# fmt off
+# pylint: disable=consider-using-f-string
+# pylint: disable=line-too-long
+# pylint: disable=attribute-defined-outside-init
+
+from const import PV_SCENARIO_NOMINAL
+from prediction import Prediction
+from tests.test_infra import reset_inverter, reset_rates
+from tests.test_kernel_parity import kernel_available
+
+# Each car charges this much over one hour, starting now
+CAR_KW = 7.0
+IMPORT_RATE = 10.0
+
+
+def _run_cars(my_predbat, cars, kernel, loss):
+    """Charge `cars` cars at CAR_KW for an hour with no PV, no house load and an empty battery; return (grid import kWh, cost, kernel used)."""
+    reset_inverter(my_predbat)
+    reset_rates(my_predbat, IMPORT_RATE, 5.0)
+    my_predbat.prediction_kernel_enable = kernel
+    my_predbat.soc_kw = 0.0
+    my_predbat.reserve = 0.0
+    now = my_predbat.minutes_now
+    my_predbat.num_cars = cars
+    my_predbat.car_charging_slots = [[{"start": now, "end": now + 60, "kwh": CAR_KW, "average": IMPORT_RATE, "octopus": False}] if car_n < cars else [] for car_n in range(4)]
+    my_predbat.car_charging_soc = [0.0] * 4
+    my_predbat.car_charging_limit = [100.0] * 4
+    my_predbat.car_charging_limit_model = None
+    my_predbat.car_charging_loss = loss
+    my_predbat.car_energy_reported_load = True
+    my_predbat.car_charging_from_battery = True
+    zero = {minute: 0.0 for minute in range(0, my_predbat.forecast_minutes, 5)}
+    my_predbat.prediction = Prediction(my_predbat, zero, zero, zero, zero)
+    result = my_predbat.run_prediction([], [], [], [], PV_SCENARIO_NOMINAL, end_record=my_predbat.forecast_minutes)
+    return result[1] + result[2], result[0], my_predbat.prediction.kernel_handle != 0
+
+
+def run_multi_car_load_tests(my_predbat):
+    """
+    Cars charging in the same step each add their own energy to the house load once (GH#5313).
+
+    The per-car loop used to add the running total of car energy to the load for every car, so the
+    first car was counted once per car: two 7 kWh cars modelled 21 kWh, three modelled 42 kWh.
+    Checked in both engines, and with a charging loss so it is the grid-side energy that is counted.
+    Only cars inside the CT clamp (car_energy_reported_load) are modelled as house load; a car outside
+    it goes through car_load_energy_bypass, which already accumulated each car once.
+    """
+    print("\n**** Multiple cars charging at once ****")
+    failed = 0
+    engines = [False]
+    available, required_failure = kernel_available()
+    if required_failure:
+        failed += 1
+    if available:
+        engines.append(True)
+
+    for kernel in engines:
+        engine = "kernel" if kernel else "python"
+        for loss in (1.0, 0.9):
+            for cars in (1, 2, 3):
+                expected = CAR_KW * cars
+                imported, cost, used_kernel = _run_cars(my_predbat, cars, kernel, loss)
+                label = "{} loss={} cars={}".format(engine, loss, cars)
+                if used_kernel != kernel:
+                    print("  ERROR: {}: expected the {} engine to run".format(label, engine))
+                    failed += 1
+                elif abs(imported - expected) > 0.01 or abs(cost - expected * IMPORT_RATE) > 0.1:
+                    print("  ERROR: {}: modelled {:.2f} kWh import costing {:.2f}p, expected {:.2f} kWh costing {:.2f}p".format(label, imported, cost, expected, expected * IMPORT_RATE))
+                    failed += 1
+                else:
+                    print("  OK: {}: {:.2f} kWh import".format(label, imported))
+
+    my_predbat.prediction_kernel_enable = False
+    if failed:
+        print("\n**** multi_car_load tests: FAILED ({} failures) ****".format(failed))
+    else:
+        print("\n**** multi_car_load tests: PASSED ****")
+    return failed

apps/predbat/unit_test.py

@@ -237,6 +237,7 @@
 from tests.test_rate_add_io_slots import run_rate_add_io_slots_tests
 from tests.test_iog_charge_skew import run_iog_charge_skew_tests
 from tests.test_iog_early_slot_flat_battery import run_iog_early_slot_flat_battery_tests
+from tests.test_multi_car_load import run_multi_car_load_tests
 from tests.test_dispatch_timeline import run_dispatch_timeline_tests
 from tests.test_log_rotation import run_log_rotation_tests
 from tests.test_battery_curve_keys import run_battery_curve_keys_tests
@@ -571,6 +572,7 @@ def main():
         ("rate_add_io_slots", run_rate_add_io_slots_tests, "Rate add IO slots tests", False),
         ("iog_charge_skew", run_iog_charge_skew_tests, "IOG earlier-charge skew characterisation tests", False),
         ("iog_early_slot_flat_battery", run_iog_early_slot_flat_battery_tests, "IOG bonus dispatch before the night window with a flat battery", False),
+        ("multi_car_load", run_multi_car_load_tests, "Cars charging at once each add their own energy to the load", False),
         ("dispatch_timeline", run_dispatch_timeline_tests, "Dispatch timeline diagnostic tests (#4516 Stage 1)", False),
         ("log_rotation", run_log_rotation_tests, "Configurable log rotation and two-digit naming (#5076)", False),
         ("rate_replicate", test_rate_replicate, "Rate replicate comprehensive tests (missing slots, IO, offsets, gas)", False),

coverage/cases/random_results.json

@@ -2,7 +2,7 @@
   "run_info": {
     "template_yaml": "cases/predbat_debug_agile1.yaml",
     "scenarios_file": "cases/random_scenarios.yaml",
-    "timestamp": "2026-09-29T08:01:31.271058+00:00"
+    "timestamp": "2026-09-30T07:25:57.325333+00:00"
   },
   "results": [
     {
@@ -19,7 +19,7 @@
       "soc_final": 5.9262,
       "battery_cycles": 22.5372,
       "carbon_g": 20398.29,
-      "runtime_s": 1.981,
+      "runtime_s": 2.008,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -38,7 +38,7 @@
       "soc_final": 4.762,
       "battery_cycles": 16.5804,
       "carbon_g": 20357.15,
-      "runtime_s": 1.159,
+      "runtime_s": 1.161,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -57,7 +57,7 @@
       "soc_final": 2.1289,
       "battery_cycles": 6.2672,
       "carbon_g": 10983.08,
-      "runtime_s": 1.03,
+      "runtime_s": 1.058,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -76,7 +76,7 @@
       "soc_final": 1.2819,
       "battery_cycles": 11.1137,
       "carbon_g": 9719.92,
-      "runtime_s": 0.374,
+      "runtime_s": 0.343,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -95,26 +95,26 @@
       "soc_final": 4.6176,
       "battery_cycles": 11.471,
       "carbon_g": 18921.81,
-      "runtime_s": 0.314,
+      "runtime_s": 0.322,
       "failed": false,
       "error": null,
       "end_record": 1440
     },
     {
       "id": 5,
       "seed": 5,
-      "metric": 1630.7577,
-      "cost": 1700.8368,
-      "cost_pv10": 1997.116,
-      "cost_pv90": 1327.0428,
+      "metric": 1285.3384,
+      "cost": 1356.2346,
+      "cost_pv10": 1647.9744,
+      "cost_pv90": 993.0577,
       "import_kwh_battery": 10.1977,
-      "import_kwh_house": 48.028,
-      "export_kwh": 9.6928,
+      "import_kwh_house": 33.688,
+      "export_kwh": 10.113,
       "soc_min": 0.38,
       "soc_final": 2.2769,
-      "battery_cycles": 21.6064,
-      "carbon_g": 30670.27,
-      "runtime_s": 0.209,
+      "battery_cycles": 22.1943,
+      "carbon_g": 24896.47,
+      "runtime_s": 0.24,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -133,7 +133,7 @@
       "soc_final": 0.38,
       "battery_cycles": 35.4755,
       "carbon_g": 29386.81,
-      "runtime_s": 3.312,
+      "runtime_s": 3.335,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -152,7 +152,7 @@
       "soc_final": 0.3949,
       "battery_cycles": 14.1532,
       "carbon_g": 22415.34,
-      "runtime_s": 1.899,
+      "runtime_s": 1.909,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -171,7 +171,7 @@
       "soc_final": 0.38,
       "battery_cycles": 8.1686,
       "carbon_g": 28892.33,
-      "runtime_s": 0.268,
+      "runtime_s": 0.258,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -190,7 +190,7 @@
       "soc_final": 7.7805,
       "battery_cycles": 29.4133,
       "carbon_g": 11557.84,
-      "runtime_s": 0.292,
+      "runtime_s": 0.295,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -209,7 +209,7 @@
       "soc_final": 1.0081,
       "battery_cycles": 26.4498,
       "carbon_g": 20579.27,
-      "runtime_s": 1.077,
+      "runtime_s": 1.082,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -228,7 +228,7 @@
       "soc_final": 1.5231,
       "battery_cycles": 5.0652,
       "carbon_g": 18871.77,
-      "runtime_s": 0.404,
+      "runtime_s": 0.414,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -247,7 +247,7 @@
       "soc_final": 0.38,
       "battery_cycles": 20.7648,
       "carbon_g": 36713.81,
-      "runtime_s": 0.796,
+      "runtime_s": 0.816,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -266,26 +266,26 @@
       "soc_final": 0.38,
       "battery_cycles": 12.0194,
       "carbon_g": 37955.25,
-      "runtime_s": 0.31,
+      "runtime_s": 0.359,
       "failed": false,
       "error": null,
       "end_record": 1440
     },
     {
       "id": 14,
       "seed": 14,
-      "metric": 2887.1673,
-      "cost": 2873.2563,
-      "cost_pv10": 3003.1288,
-      "cost_pv90": 2251.1648,
-      "import_kwh_battery": 31.8142,
-      "import_kwh_house": 66.4699,
+      "metric": 2376.0241,
+      "cost": 2362.1131,
+      "cost_pv10": 2491.9856,
+      "cost_pv90": 1740.0216,
+      "import_kwh_battery": 24.6779,
+      "import_kwh_house": 53.6319,
       "export_kwh": 0.0009,
       "soc_min": 0.38,
       "soc_final": 0.4023,
       "battery_cycles": 6.7459,
-      "carbon_g": 48852.41,
-      "runtime_s": 0.325,
+      "carbon_g": 41033.45,
+      "runtime_s": 0.332,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -304,7 +304,7 @@
       "soc_final": 3.7345,
       "battery_cycles": 10.5722,
       "carbon_g": 31438.76,
-      "runtime_s": 0.791,
+      "runtime_s": 0.796,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -323,7 +323,7 @@
       "soc_final": 0.38,
       "battery_cycles": 3.7054,
       "carbon_g": 23055.97,
-      "runtime_s": 0.173,
+      "runtime_s": 0.176,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -342,7 +342,7 @@
       "soc_final": 0.38,
       "battery_cycles": 23.0298,
       "carbon_g": 22132.39,
-      "runtime_s": 0.867,
+      "runtime_s": 0.885,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -361,7 +361,7 @@
       "soc_final": 0.5188,
       "battery_cycles": 10.6203,
       "carbon_g": 15987.78,
-      "runtime_s": 0.269,
+      "runtime_s": 0.259,
       "failed": false,
       "error": null,
       "end_record": 1440
@@ -380,7 +380,7 @@
       "soc_final": 19.9987,
       "battery_cycles": 66.0439,
       "carbon_g": 27244.36,
-      "runtime_s": 3.408,
+      "runtime_s": 3.402,
       "failed": false,
       "error": null,
       "end_record": 1440