Centrally define Watt (#21570)

* centralize Watt definition

* lint

homeassistant/components/isy994/sensor.py

@@ -5,7 +5,8 @@ from typing import Callable
 from homeassistant.components.isy994 import (
     ISY994_NODES, ISY994_WEATHER, ISYDevice)
 from homeassistant.components.sensor import DOMAIN
-from homeassistant.const import TEMP_CELSIUS, TEMP_FAHRENHEIT, UNIT_UV_INDEX
+from homeassistant.const import (
+    TEMP_CELSIUS, TEMP_FAHRENHEIT, UNIT_UV_INDEX, POWER_WATT)
 from homeassistant.helpers.typing import ConfigType
 
 _LOGGER = logging.getLogger(__name__)
@@ -75,7 +76,7 @@ UOM_FRIENDLY_NAME = {
     '69': 'gal',
     '71': UNIT_UV_INDEX,
     '72': 'V',
-    '73': 'W',
+    '73': POWER_WATT,
     '74': 'W/m²',
     '75': 'weekday',
     '76': 'Wind Direction (°)',