Add BackupMode to Fronius integration.

This binary sensor shows if the Fronius Gen24 inverter is running
in "Backup Mode", which means the grid power is not available, and
the device is running from battery (or using the PVPoint).

homeassistant/components/fronius/sensor.py

@@ -641,6 +641,10 @@ class FroniusSensorEntityDescription(SensorEntityDescription):
         native_unit_of_measurement=PERCENTAGE,
         state_class=SensorStateClass.MEASUREMENT,
     ),
+    FroniusSensorEntityDescription(
+        key='backup_mode',
+        entity_category=EntityCategory.DIAGNOSTIC,
+    ),
 ]
 
 STORAGE_ENTITY_DESCRIPTIONS: list[FroniusSensorEntityDescription] = [

homeassistant/components/fronius/strings.json

@@ -291,6 +291,9 @@
       },
       "temperature_cell": {
         "name": "[%key:component::sensor::entity_component::temperature::name%]"
+      },
+      "backup_mode": {
+        "name": "Backup Mode"
       }
     }
   }