Centrally define Watt (#21570)

* centralize Watt definition

* lint

homeassistant/components/sense/sensor.py

@@ -3,6 +3,7 @@ from datetime import timedelta
 import logging
 
 from homeassistant.components.sense import SENSE_DATA
+from homeassistant.const import POWER_WATT
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import Throttle
 
@@ -90,7 +91,7 @@ class Sense(Entity):
         self._state = None
 
         if sensor_type == ACTIVE_TYPE:
-            self._unit_of_measurement = 'W'
+            self._unit_of_measurement = POWER_WATT
         else:
             self._unit_of_measurement = 'kWh'