diff --git a/custom_components/sigen/calculated_sensor.py b/custom_components/sigen/calculated_sensor.py
index 4dd7588..6c29b3f 100644
--- a/custom_components/sigen/calculated_sensor.py
+++ b/custom_components/sigen/calculated_sensor.py
@@ -239,10 +239,17 @@ def calculate_pv_power(
             try:
                 voltage_dec = safe_decimal(pv_voltage)
                 current_dec = safe_decimal(pv_current)
-                if voltage_dec and current_dec:
-                    power = voltage_dec * current_dec  # Already in Watts
-                else:
+                if voltage_dec is None or current_dec is None:
+                    return None
+
+                # The signed Modbus inputs can contain small negative offsets
+                # under low-light conditions. A PV string is a production source,
+                # so these offsets must not become negative power and make its
+                # accumulated or daily energy decrease.
+                if voltage_dec <= Decimal("0") or current_dec <= Decimal("0"):
                     return 0.0
+
+                power = voltage_dec * current_dec  # Already in Watts
             except (ValueError, TypeError, InvalidOperation):
                 _LOGGER.warning(
                     "[CS][PV Power] Error converting values to Decimal: V=%s, I=%s",
diff --git a/custom_components/sigen/manifest.json b/custom_components/sigen/manifest.json
index f7d8049..b93c618 100644
--- a/custom_components/sigen/manifest.json
+++ b/custom_components/sigen/manifest.json
@@ -31,5 +31,5 @@
   "requirements": [
     "pymodbus>=3.8.3"
   ],
-  "version": "1.2.7.3"
+  "version": "1.2.7.4"
 }
diff --git a/tests/test_pv_power_floor.py b/tests/test_pv_power_floor.py
new file mode 100644
index 0000000..593c063
--- /dev/null
+++ b/tests/test_pv_power_floor.py
@@ -0,0 +1,101 @@
+"""Regression tests for PV string power measurement offsets."""
+
+from __future__ import annotations
+
+import ast
+import copy
+import unittest
+from decimal import Decimal, InvalidOperation
+from pathlib import Path
+from unittest.mock import Mock
+
+
+SOURCE_PATH = (
+    Path(__file__).resolve().parents[1]
+    / "custom_components"
+    / "sigen"
+    / "calculated_sensor.py"
+)
+
+
+def _calculate_pv_power_function():
+    """Load calculate_pv_power without requiring Home Assistant."""
+    tree = ast.parse(SOURCE_PATH.read_text(encoding="utf-8"))
+    calculations_class = next(
+        node
+        for node in tree.body
+        if isinstance(node, ast.ClassDef) and node.name == "SigenergyCalculations"
+    )
+    function = copy.deepcopy(
+        next(
+            node
+            for node in calculations_class.body
+            if isinstance(node, ast.FunctionDef)
+            and node.name == "calculate_pv_power"
+        )
+    )
+    function.decorator_list = []
+    module = ast.fix_missing_locations(
+        ast.Module(
+            body=[
+                ast.ImportFrom(
+                    module="__future__",
+                    names=[ast.alias(name="annotations")],
+                    level=0,
+                ),
+                function,
+            ],
+            type_ignores=[],
+        )
+    )
+    namespace = {
+        "Decimal": Decimal,
+        "InvalidOperation": InvalidOperation,
+        "_LOGGER": Mock(),
+        "safe_decimal": lambda value: Decimal(str(value)),
+        "safe_float": float,
+    }
+    exec(compile(module, SOURCE_PATH, "exec"), namespace)
+    return namespace["calculate_pv_power"]
+
+
+class TestPVPowerFloor(unittest.TestCase):
+    """Ensure measurement offsets cannot produce negative PV power."""
+
+    @classmethod
+    def setUpClass(cls) -> None:
+        cls.calculate_pv_power = staticmethod(_calculate_pv_power_function())
+
+    def _calculate(self, voltage: float, current: float) -> float | None:
+        return self.calculate_pv_power(
+            None,
+            {
+                "inverters": {
+                    "Sigen Inverter": {
+                        "inverter_pv3_voltage": voltage,
+                        "inverter_pv3_current": current,
+                    }
+                }
+            },
+            {"pv_idx": 3, "device_name": "Sigen Inverter"},
+        )
+
+    def test_positive_voltage_and_current_produce_power(self) -> None:
+        self.assertAlmostEqual(2.245789, self._calculate(336.7, 6.67))
+
+    def test_non_positive_current_produces_zero_power(self) -> None:
+        for current in (0.0, -0.01, -0.04):
+            with self.subTest(current=current):
+                self.assertEqual(0.0, self._calculate(168.6, current))
+
+    def test_non_positive_voltage_produces_zero_power(self) -> None:
+        for voltage in (0.0, -0.1):
+            with self.subTest(voltage=voltage):
+                self.assertEqual(0.0, self._calculate(voltage, 0.01))
+
+    def test_negative_voltage_and_current_do_not_create_phantom_power(self) -> None:
+        self.assertEqual(0.0, self._calculate(-0.1, -0.01))
+
+
+if __name__ == "__main__":
+    unittest.main()