diff --git a/homeassistant/components/forecast_solar/sensor.py b/homeassistant/components/forecast_solar/sensor.py
index a6b1927926e..b32f1f341be 100644
--- a/homeassistant/components/forecast_solar/sensor.py
+++ b/homeassistant/components/forecast_solar/sensor.py
@@ -1,6 +1,8 @@
 """Support for the Forecast.Solar sensor service."""
 from __future__ import annotations
 
+from datetime import datetime
+
 from homeassistant.components.sensor import DOMAIN as SENSOR_DOMAIN, SensorEntity
 from homeassistant.config_entries import ConfigEntry
 from homeassistant.const import ATTR_IDENTIFIERS, ATTR_MANUFACTURER, ATTR_NAME
@@ -64,5 +66,7 @@ class ForecastSolarSensorEntity(CoordinatorEntity, SensorEntity):
     @property
     def state(self) -> StateType:
         """Return the state of the sensor."""
-        state: StateType = getattr(self.coordinator.data, self._sensor.key)
+        state: StateType | datetime = getattr(self.coordinator.data, self._sensor.key)
+        if isinstance(state, datetime):
+            return state.isoformat()
         return state
diff --git a/tests/components/forecast_solar/test_sensor.py b/tests/components/forecast_solar/test_sensor.py
index a3513b86a5d..31c367678c1 100644
--- a/tests/components/forecast_solar/test_sensor.py
+++ b/tests/components/forecast_solar/test_sensor.py
@@ -70,7 +70,7 @@ async def test_sensors(
     assert entry
     assert state
     assert entry.unique_id == f"{entry_id}_power_highest_peak_time_today"
-    assert state.state == "2021-06-27 13:00:00+00:00"
+    assert state.state == "2021-06-27T13:00:00+00:00"
     assert state.attributes.get(ATTR_FRIENDLY_NAME) == "Highest Power Peak Time - Today"
     assert state.attributes.get(ATTR_STATE_CLASS) is None
     assert state.attributes.get(ATTR_DEVICE_CLASS) == DEVICE_CLASS_TIMESTAMP
@@ -82,7 +82,7 @@ async def test_sensors(
     assert entry
     assert state
     assert entry.unique_id == f"{entry_id}_power_highest_peak_time_tomorrow"
-    assert state.state == "2021-06-27 14:00:00+00:00"
+    assert state.state == "2021-06-27T14:00:00+00:00"
     assert (
         state.attributes.get(ATTR_FRIENDLY_NAME) == "Highest Power Peak Time - Tomorrow"
     )