Apply unit_of_measurement to energy combined power sensor (#170404)

homeassistant/components/energy/sensor.py

@@ -664,6 +664,12 @@ class EnergyPowerSensor(SensorEntity):
         self._is_inverted = "stat_rate_inverted" in config
         self._is_combined = "stat_rate_from" in config and "stat_rate_to" in config
 
+        # Combined mode always emits Watts because _update_state converts
+        # heterogeneous source units to W internally. Inverted mode copies
+        # the source unit in _update_state to track source changes.
+        if self._is_combined:
+            self._attr_native_unit_of_measurement = UnitOfPower.WATT
+
         # Determine source sensors
         if self._is_inverted:
             self._source_sensors = [config["stat_rate_inverted"]]
@@ -764,11 +770,6 @@ class EnergyPowerSensor(SensorEntity):
             # Check first sensor
             if source_entry := entity_reg.async_get(self._source_sensors[0]):
                 device_id = source_entry.device_id
-                # Combined mode always emits Watts because we convert
-                # heterogeneous source units internally. For inverted mode the
-                # unit is copied from the source state in _update_state.
-                if self._is_combined:
-                    self._attr_native_unit_of_measurement = UnitOfPower.WATT
                 # Get source name from registry
                 source_name = source_entry.name or source_entry.original_name
             # Assign power sensor to same device as source sensor(s)

tests/components/energy/test_sensor.py

@@ -1615,6 +1615,7 @@ async def test_power_sensor_grid_combined(
     assert state is not None
     # 500 - 200 = 300 (net import)
     assert float(state.state) == 300.0
+    assert state.attributes[ATTR_UNIT_OF_MEASUREMENT] == UnitOfPower.WATT
 
 
 async def test_power_sensor_device_assignment(
@@ -1988,6 +1989,7 @@ async def test_power_sensor_combined_unit_conversion(
     assert state is not None
     # 1500 W - 500 W = 1000 W (units are converted to W internally)
     assert float(state.state) == 1000.0
+    assert state.attributes[ATTR_UNIT_OF_MEASUREMENT] == UnitOfPower.WATT
 
 
 async def test_power_sensor_inverted_negative_values(