diff --git a/homeassistant/components/sofar/sensor.py b/homeassistant/components/sofar/sensor.py
index f446a9af5e4d..01d78cedfb9a 100644
--- a/homeassistant/components/sofar/sensor.py
+++ b/homeassistant/components/sofar/sensor.py
@@ -7,6 +7,7 @@ from enum import IntEnum
 from typing import cast, override
 
 from sofar_modbus.modern.device import SofarInverter
+from sofar_modbus.modern.enums import SystemState
 
 from homeassistant.components.sensor import (
     RestoreSensor,
@@ -142,6 +143,9 @@ class SofarSensor(SofarEntity, SensorEntity):
 class SofarCountdownSensor(SofarSensor):
     """Defines a Sofar countdown, published as the moment it runs out."""
 
+    # A positive register alone doesn't mean the countdown is active.
+    _ACTIVE_STATES = (SystemState.WAITING, SystemState.CHECKING)
+
     def __init__(
         self,
         runtime_data: SofarRuntimeData,
@@ -156,7 +160,11 @@ class SofarCountdownSensor(SofarSensor):
     def native_value(self) -> datetime | None:
         component = getattr(self.coordinator.device, self.entity_description.component)
         seconds = getattr(component, self.entity_description.key)
-        if not isinstance(seconds, int) or seconds <= 0:
+        if (
+            not isinstance(seconds, int)
+            or seconds <= 0
+            or component.system_state not in self._ACTIVE_STATES
+        ):
             # A restart must not land inside the finished countdown's slack.
             self._deadline = _deadline_filter()
             return None
diff --git a/tests/components/sofar/test_sensor.py b/tests/components/sofar/test_sensor.py
index 86998a9b50ac..ab325c2584c5 100644
--- a/tests/components/sofar/test_sensor.py
+++ b/tests/components/sofar/test_sensor.py
@@ -426,6 +426,7 @@ async def test_countdown_holds_its_deadline_until_it_restarts(
     """Test a countdown ticking with the clock keeps one deadline."""
     mock_config_entry.add_to_hass(hass)
     unit = mock_connection.for_unit(1)
+    unit.holding[0x0404] = 0  # Waiting
     unit.holding[0x0417] = 300
 
     with patch(
@@ -460,6 +461,34 @@ async def test_countdown_holds_its_deadline_until_it_restarts(
     assert hass.states.get(entity_id).state != deadline
 
 
+async def test_countdown_ignores_a_stale_register_while_grid_connected(
+    hass: HomeAssistant,
+    entity_registry: er.EntityRegistry,
+    mock_connection: MockModbusConnection,
+    mock_config_entry: MockConfigEntry,
+) -> None:
+    """Test a positive register is ignored once the inverter is connected."""
+    mock_config_entry.add_to_hass(hass)
+    unit = mock_connection.for_unit(1)
+    unit.holding[0x0404] = 2  # Grid connected
+    unit.holding[0x0417] = 60  # Left over from the last startup wait
+
+    with patch(
+        "homeassistant.components.sofar.async_get_unit",
+        side_effect=lambda hass, entry, params, unit_id: mock_connection.for_unit(
+            unit_id
+        ),
+    ):
+        await hass.config_entries.async_setup(mock_config_entry.entry_id)
+        await hass.async_block_till_done(wait_background_tasks=True)
+
+    entity_id = entity_registry.async_get_entity_id(
+        SENSOR_DOMAIN, DOMAIN, f"{MOCK_SERIAL}_waiting_time"
+    )
+    assert entity_id is not None
+    assert hass.states.get(entity_id).state == STATE_UNKNOWN
+
+
 async def test_countdown_restarting_after_idle_gets_a_new_deadline(
     hass: HomeAssistant,
     freezer: FrozenDateTimeFactory,
@@ -470,6 +499,7 @@ async def test_countdown_restarting_after_idle_gets_a_new_deadline(
     """Test a finished countdown's deadline is not reused by the next one."""
     mock_config_entry.add_to_hass(hass)
     unit = mock_connection.for_unit(1)
+    unit.holding[0x0404] = 0  # Waiting
     unit.holding[0x0417] = 10
 
     with patch(