Centrally define Watt (#21570)

* centralize Watt definition

* lint

homeassistant/components/enocean/sensor.py

@@ -4,7 +4,7 @@ import logging
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_NAME, CONF_ID)
+from homeassistant.const import (CONF_NAME, CONF_ID, POWER_WATT)
 from homeassistant.helpers.entity import Entity
 import homeassistant.helpers.config_validation as cv
 from homeassistant.components import enocean
@@ -59,4 +59,4 @@ class EnOceanSensor(enocean.EnOceanDevice, Entity):
     @property
     def unit_of_measurement(self):
         """Return the unit of measurement."""
-        return 'W'
+        return POWER_WATT