diff --git a/kasa/smart/smartdevice.py b/kasa/smart/smartdevice.py
@@ -66,6 +66,7 @@ def __init__(
         self._children: Mapping[str, SmartDevice] = {}
         self._last_update = {}
         self._last_update_time: float | None = None
+        self._on_since: datetime | None = None
 
     async def _initialize_children(self):
         """Initialize children for power strips."""
@@ -502,7 +503,15 @@ def on_since(self) -> datetime | None:
             return None
 
         on_time = cast(float, on_time)
-        return self.time - timedelta(seconds=on_time)
+        on_since = self.time - timedelta(seconds=on_time)
+        # Ensure slight variations between time module and on_time do not cause
+        # the on_since time to change. More likely to happen with child devices
+        # or P100s that do not do multi-queries.
+        if not self._on_since or timedelta(
+            seconds=0
+        ) < on_since - self._on_since > timedelta(seconds=5):
+            self._on_since = on_since
+        return self._on_since
 
     @property
     def timezone(self) -> dict: