diff --git a/homeassistant/components/apcupsd/sensor.py b/homeassistant/components/apcupsd/sensor.py
index 4ebe0ac8aaf..09f9b324bdd 100644
--- a/homeassistant/components/apcupsd/sensor.py
+++ b/homeassistant/components/apcupsd/sensor.py
@@ -6,7 +6,7 @@ import voluptuous as vol
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 import homeassistant.helpers.config_validation as cv
 from homeassistant.components import apcupsd
-from homeassistant.const import (TEMP_CELSIUS, CONF_RESOURCES)
+from homeassistant.const import (TEMP_CELSIUS, CONF_RESOURCES, POWER_WATT)
 from homeassistant.helpers.entity import Entity
 
 _LOGGER = logging.getLogger(__name__)
@@ -57,7 +57,7 @@ SENSOR_TYPES = {
     'nombattv': ['Battery Nominal Voltage', 'V', 'mdi:flash'],
     'nominv': ['Nominal Input Voltage', 'V', 'mdi:flash'],
     'nomoutv': ['Nominal Output Voltage', 'V', 'mdi:flash'],
-    'nompower': ['Nominal Output Power', 'W', 'mdi:flash'],
+    'nompower': ['Nominal Output Power', POWER_WATT, 'mdi:flash'],
     'nomapnt': ['Nominal Apparent Power', 'VA', 'mdi:flash'],
     'numxfers': ['Transfer Count', '', 'mdi:counter'],
     'outcurnt': ['Output Current', 'A', 'mdi:flash'],
@@ -93,7 +93,7 @@ INFERRED_UNITS = {
     ' Volts': 'V',
     ' Ampere': 'A',
     ' Volt-Ampere': 'VA',
-    ' Watts': 'W',
+    ' Watts': POWER_WATT,
     ' Hz': 'Hz',
     ' C': TEMP_CELSIUS,
     ' Percent Load Capacity': '%',
diff --git a/homeassistant/components/edp_redy/sensor.py b/homeassistant/components/edp_redy/sensor.py
index 926a073832c..389ae77f35b 100644
--- a/homeassistant/components/edp_redy/sensor.py
+++ b/homeassistant/components/edp_redy/sensor.py
@@ -2,6 +2,7 @@
 import logging
 
 from homeassistant.helpers.entity import Entity
+from homeassistant.const import POWER_WATT
 
 from homeassistant.components.edp_redy import EdpRedyDevice, EDP_REDY
 
@@ -29,7 +30,7 @@ async def async_setup_platform(
 
     # Create a sensor for global active power
     devices.append(EdpRedySensor(session, ACTIVE_POWER_ID, "Power Home",
-                                 'mdi:flash', 'W'))
+                                 'mdi:flash', POWER_WATT))
 
     async_add_entities(devices, True)
 
@@ -89,7 +90,7 @@ class EdpRedyModuleSensor(EdpRedyDevice, Entity):
     @property
     def unit_of_measurement(self):
         """Return the unit of measurement of this sensor."""
-        return 'W'
+        return POWER_WATT
 
     async def async_update(self):
         """Parse the data for this sensor."""
diff --git a/homeassistant/components/enocean/sensor.py b/homeassistant/components/enocean/sensor.py
index d2e88ed3825..8d79de2c50d 100644
--- a/homeassistant/components/enocean/sensor.py
+++ b/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
diff --git a/homeassistant/components/homematic/sensor.py b/homeassistant/components/homematic/sensor.py
index c4d97dca3fe..1bdd2323108 100644
--- a/homeassistant/components/homematic/sensor.py
+++ b/homeassistant/components/homematic/sensor.py
@@ -2,7 +2,7 @@
 import logging
 
 from homeassistant.components.homematic import ATTR_DISCOVER_DEVICES, HMDevice
-from homeassistant.const import STATE_UNKNOWN
+from homeassistant.const import STATE_UNKNOWN, POWER_WATT
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -24,7 +24,7 @@ HM_UNIT_HA_CAST = {
     'TEMPERATURE': '°C',
     'ACTUAL_TEMPERATURE': '°C',
     'BRIGHTNESS': '#',
-    'POWER': 'W',
+    'POWER': POWER_WATT,
     'CURRENT': 'mA',
     'VOLTAGE': 'V',
     'ENERGY_COUNTER': 'Wh',
diff --git a/homeassistant/components/homematicip_cloud/sensor.py b/homeassistant/components/homematicip_cloud/sensor.py
index d755735e0e0..f1a20e46922 100644
--- a/homeassistant/components/homematicip_cloud/sensor.py
+++ b/homeassistant/components/homematicip_cloud/sensor.py
@@ -5,7 +5,7 @@ from homeassistant.components.homematicip_cloud import (
     DOMAIN as HMIPC_DOMAIN, HMIPC_HAPID, HomematicipGenericDevice)
 from homeassistant.const import (
     DEVICE_CLASS_HUMIDITY, DEVICE_CLASS_ILLUMINANCE, DEVICE_CLASS_TEMPERATURE,
-    TEMP_CELSIUS)
+    TEMP_CELSIUS, POWER_WATT)
 
 _LOGGER = logging.getLogger(__name__)
 
@@ -223,4 +223,4 @@ class HomematicipPowerSensor(HomematicipGenericDevice):
     @property
     def unit_of_measurement(self):
         """Return the unit this state is expressed in."""
-        return 'W'
+        return POWER_WATT
diff --git a/homeassistant/components/isy994/sensor.py b/homeassistant/components/isy994/sensor.py
index 60212d081de..2115c19f496 100644
--- a/homeassistant/components/isy994/sensor.py
+++ b/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 (°)',
diff --git a/homeassistant/components/juicenet/sensor.py b/homeassistant/components/juicenet/sensor.py
index e3786627075..35cf5628d6f 100644
--- a/homeassistant/components/juicenet/sensor.py
+++ b/homeassistant/components/juicenet/sensor.py
@@ -1,7 +1,7 @@
 """Support for monitoring juicenet/juicepoint/juicebox based EVSE sensors."""
 import logging
 
-from homeassistant.const import TEMP_CELSIUS
+from homeassistant.const import TEMP_CELSIUS, POWER_WATT
 from homeassistant.helpers.entity import Entity
 from homeassistant.components.juicenet import JuicenetDevice, DOMAIN
 
@@ -14,7 +14,7 @@ SENSOR_TYPES = {
     'temperature': ['Temperature', TEMP_CELSIUS],
     'voltage': ['Voltage', 'V'],
     'amps': ['Amps', 'A'],
-    'watts': ['Watts', 'W'],
+    'watts': ['Watts', POWER_WATT],
     'charge_time': ['Charge time', 's'],
     'energy_added': ['Energy added', 'Wh']
 }
diff --git a/homeassistant/components/mysensors/sensor.py b/homeassistant/components/mysensors/sensor.py
index ce6d5da2b4c..7aa19e2157c 100644
--- a/homeassistant/components/mysensors/sensor.py
+++ b/homeassistant/components/mysensors/sensor.py
@@ -1,7 +1,7 @@
 """Support for MySensors sensors."""
 from homeassistant.components import mysensors
 from homeassistant.components.sensor import DOMAIN
-from homeassistant.const import TEMP_CELSIUS, TEMP_FAHRENHEIT
+from homeassistant.const import TEMP_CELSIUS, TEMP_FAHRENHEIT, POWER_WATT
 
 SENSORS = {
     'V_TEMP': [None, 'mdi:thermometer'],
@@ -12,7 +12,7 @@ SENSORS = {
     'V_WEIGHT': ['kg', 'mdi:weight-kilogram'],
     'V_DISTANCE': ['m', 'mdi:ruler'],
     'V_IMPEDANCE': ['ohm', None],
-    'V_WATT': ['W', None],
+    'V_WATT': [POWER_WATT, None],
     'V_KWH': ['kWh', None],
     'V_FLOW': ['m', None],
     'V_VOLUME': ['m³', None],
diff --git a/homeassistant/components/rfxtrx/__init__.py b/homeassistant/components/rfxtrx/__init__.py
index a7b703ef2ab..411f0538bde 100644
--- a/homeassistant/components/rfxtrx/__init__.py
+++ b/homeassistant/components/rfxtrx/__init__.py
@@ -6,7 +6,8 @@ import voluptuous as vol
 
 from homeassistant.const import (
     ATTR_ENTITY_ID, ATTR_NAME, ATTR_STATE, CONF_DEVICE, CONF_DEVICES,
-    EVENT_HOMEASSISTANT_START, EVENT_HOMEASSISTANT_STOP, TEMP_CELSIUS)
+    EVENT_HOMEASSISTANT_START, EVENT_HOMEASSISTANT_STOP, TEMP_CELSIUS,
+    POWER_WATT)
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import slugify
@@ -40,8 +41,8 @@ DATA_TYPES = OrderedDict([
     ('Barometer', ''),
     ('Wind direction', ''),
     ('Rain rate', ''),
-    ('Energy usage', 'W'),
-    ('Total usage', 'W'),
+    ('Energy usage', POWER_WATT),
+    ('Total usage', POWER_WATT),
     ('Sound', ''),
     ('Sensor Status', ''),
     ('Counter value', ''),
diff --git a/homeassistant/components/sense/sensor.py b/homeassistant/components/sense/sensor.py
index 2995b860e5b..8fefea03506 100644
--- a/homeassistant/components/sense/sensor.py
+++ b/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'
 
diff --git a/homeassistant/components/sensor/efergy.py b/homeassistant/components/sensor/efergy.py
index 54666c74f96..18a8fa2e02d 100644
--- a/homeassistant/components/sensor/efergy.py
+++ b/homeassistant/components/sensor/efergy.py
@@ -10,7 +10,7 @@ import requests
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import CONF_CURRENCY
+from homeassistant.const import CONF_CURRENCY, POWER_WATT
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.entity import Entity
 
@@ -34,11 +34,11 @@ DEFAULT_PERIOD = 'year'
 DEFAULT_UTC_OFFSET = '0'
 
 SENSOR_TYPES = {
-    CONF_INSTANT: ['Energy Usage', 'W'],
+    CONF_INSTANT: ['Energy Usage', POWER_WATT],
     CONF_AMOUNT: ['Energy Consumed', 'kWh'],
     CONF_BUDGET: ['Energy Budget', None],
     CONF_COST: ['Energy Cost', None],
-    CONF_CURRENT_VALUES: ['Per-Device Usage', 'W']
+    CONF_CURRENT_VALUES: ['Per-Device Usage', POWER_WATT]
 }
 
 TYPES_SCHEMA = vol.In(SENSOR_TYPES)
diff --git a/homeassistant/components/sensor/eliqonline.py b/homeassistant/components/sensor/eliqonline.py
index 8e750a8d5e1..b03164a30d4 100644
--- a/homeassistant/components/sensor/eliqonline.py
+++ b/homeassistant/components/sensor/eliqonline.py
@@ -10,7 +10,7 @@ import logging
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_ACCESS_TOKEN, CONF_NAME)
+from homeassistant.const import (CONF_ACCESS_TOKEN, CONF_NAME, POWER_WATT)
 from homeassistant.helpers.entity import Entity
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.aiohttp_client import async_get_clientsession
@@ -27,7 +27,7 @@ ICON = 'mdi:gauge'
 
 SCAN_INTERVAL = timedelta(seconds=60)
 
-UNIT_OF_MEASUREMENT = 'W'
+UNIT_OF_MEASUREMENT = POWER_WATT
 
 PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
     vol.Required(CONF_ACCESS_TOKEN): cv.string,
diff --git a/homeassistant/components/sensor/emoncms.py b/homeassistant/components/sensor/emoncms.py
index 7546224d4c5..5d619878d98 100644
--- a/homeassistant/components/sensor/emoncms.py
+++ b/homeassistant/components/sensor/emoncms.py
@@ -14,7 +14,7 @@ import homeassistant.helpers.config_validation as cv
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.const import (
     CONF_API_KEY, CONF_URL, CONF_VALUE_TEMPLATE, CONF_UNIT_OF_MEASUREMENT,
-    CONF_ID, CONF_SCAN_INTERVAL, STATE_UNKNOWN)
+    CONF_ID, CONF_SCAN_INTERVAL, STATE_UNKNOWN, POWER_WATT)
 from homeassistant.helpers.entity import Entity
 from homeassistant.helpers import template
 from homeassistant.util import Throttle
@@ -34,7 +34,7 @@ CONF_ONLY_INCLUDE_FEEDID = 'include_only_feed_id'
 CONF_SENSOR_NAMES = 'sensor_names'
 
 DECIMALS = 2
-DEFAULT_UNIT = 'W'
+DEFAULT_UNIT = POWER_WATT
 
 MIN_TIME_BETWEEN_UPDATES = timedelta(seconds=5)
 
diff --git a/homeassistant/components/sensor/enphase_envoy.py b/homeassistant/components/sensor/enphase_envoy.py
index 4bbf7eec01b..1bfee88d41c 100644
--- a/homeassistant/components/sensor/enphase_envoy.py
+++ b/homeassistant/components/sensor/enphase_envoy.py
@@ -11,14 +11,15 @@ import voluptuous as vol
 from homeassistant.helpers.entity import Entity
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 import homeassistant.helpers.config_validation as cv
-from homeassistant.const import (CONF_IP_ADDRESS, CONF_MONITORED_CONDITIONS)
+from homeassistant.const import (
+    CONF_IP_ADDRESS, CONF_MONITORED_CONDITIONS, POWER_WATT)
 
 
 REQUIREMENTS = ['envoy_reader==0.3']
 _LOGGER = logging.getLogger(__name__)
 
 SENSORS = {
-    "production": ("Envoy Current Energy Production", 'W'),
+    "production": ("Envoy Current Energy Production", POWER_WATT),
     "daily_production": ("Envoy Today's Energy Production", "Wh"),
     "seven_days_production": ("Envoy Last Seven Days Energy Production", "Wh"),
     "lifetime_production": ("Envoy Lifetime Energy Production", "Wh"),
diff --git a/homeassistant/components/sensor/greeneye_monitor.py b/homeassistant/components/sensor/greeneye_monitor.py
index 3793ea7846c..4dee9d69b42 100644
--- a/homeassistant/components/sensor/greeneye_monitor.py
+++ b/homeassistant/components/sensor/greeneye_monitor.py
@@ -6,7 +6,7 @@ https://home-assistant.io/components/sensors.greeneye_monitor_temperature/
 """
 import logging
 
-from homeassistant.const import CONF_NAME, CONF_TEMPERATURE_UNIT
+from homeassistant.const import CONF_NAME, CONF_TEMPERATURE_UNIT, POWER_WATT
 from homeassistant.helpers.entity import Entity
 
 from ..greeneye_monitor import (
@@ -33,7 +33,7 @@ DEPENDENCIES = ['greeneye_monitor']
 DATA_PULSES = 'pulses'
 DATA_WATT_SECONDS = 'watt_seconds'
 
-UNIT_WATTS = 'W'
+UNIT_WATTS = POWER_WATT
 
 COUNTER_ICON = 'mdi:counter'
 CURRENT_SENSOR_ICON = 'mdi:flash'
diff --git a/homeassistant/components/sensor/neurio_energy.py b/homeassistant/components/sensor/neurio_energy.py
index addb7925bc2..a9fbc316751 100644
--- a/homeassistant/components/sensor/neurio_energy.py
+++ b/homeassistant/components/sensor/neurio_energy.py
@@ -11,7 +11,7 @@ import requests.exceptions
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_API_KEY)
+from homeassistant.const import (CONF_API_KEY, POWER_WATT)
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import Throttle
 import homeassistant.helpers.config_validation as cv
@@ -148,7 +148,7 @@ class NeurioEnergy(Entity):
         self._state = None
 
         if sensor_type == ACTIVE_TYPE:
-            self._unit_of_measurement = 'W'
+            self._unit_of_measurement = POWER_WATT
         elif sensor_type == DAILY_TYPE:
             self._unit_of_measurement = 'kWh'
 
diff --git a/homeassistant/components/sensor/nut.py b/homeassistant/components/sensor/nut.py
index 79ad176e42e..1464c0d91c1 100644
--- a/homeassistant/components/sensor/nut.py
+++ b/homeassistant/components/sensor/nut.py
@@ -13,7 +13,8 @@ from homeassistant.components.sensor import PLATFORM_SCHEMA
 import homeassistant.helpers.config_validation as cv
 from homeassistant.const import (
     CONF_HOST, CONF_PORT, CONF_NAME, CONF_USERNAME, CONF_PASSWORD,
-    TEMP_CELSIUS, CONF_RESOURCES, CONF_ALIAS, ATTR_STATE, STATE_UNKNOWN)
+    TEMP_CELSIUS, CONF_RESOURCES, CONF_ALIAS, ATTR_STATE, STATE_UNKNOWN,
+    POWER_WATT)
 from homeassistant.exceptions import PlatformNotReady
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import Throttle
@@ -63,8 +64,8 @@ SENSOR_TYPES = {
     'ups.efficiency': ['Efficiency', '%', 'mdi:gauge'],
     'ups.power': ['Current Apparent Power', 'VA', 'mdi:flash'],
     'ups.power.nominal': ['Nominal Power', 'VA', 'mdi:flash'],
-    'ups.realpower': ['Current Real Power', 'W', 'mdi:flash'],
-    'ups.realpower.nominal': ['Nominal Real Power', 'W', 'mdi:flash'],
+    'ups.realpower': ['Current Real Power', POWER_WATT, 'mdi:flash'],
+    'ups.realpower.nominal': ['Nominal Real Power', POWER_WATT, 'mdi:flash'],
     'ups.beeper.status': ['Beeper Status', '', 'mdi:information-outline'],
     'ups.type': ['UPS Type', '', 'mdi:information-outline'],
     'ups.watchdog.status': ['Watchdog Status', '', 'mdi:information-outline'],
diff --git a/homeassistant/components/sensor/solaredge.py b/homeassistant/components/sensor/solaredge.py
index fa49cdb3bfe..be21316948a 100644
--- a/homeassistant/components/sensor/solaredge.py
+++ b/homeassistant/components/sensor/solaredge.py
@@ -12,7 +12,7 @@ import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.const import (
-    CONF_API_KEY, CONF_MONITORED_CONDITIONS, CONF_NAME)
+    CONF_API_KEY, CONF_MONITORED_CONDITIONS, CONF_NAME, POWER_WATT)
 import homeassistant.helpers.config_validation as cv
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import Throttle
@@ -36,7 +36,7 @@ SENSOR_TYPES = {
                           'mdi:solar-power'],
     'energy_today': ['lastDayData', "Energy today", 'Wh',
                      'mdi:solar-power'],
-    'current_power': ['currentPower', "Current Power", 'W',
+    'current_power': ['currentPower', "Current Power", POWER_WATT,
                       'mdi:solar-power']
 }
 
diff --git a/homeassistant/components/sensor/ted5000.py b/homeassistant/components/sensor/ted5000.py
index 82a1ec8bb68..23a20b3e830 100644
--- a/homeassistant/components/sensor/ted5000.py
+++ b/homeassistant/components/sensor/ted5000.py
@@ -11,7 +11,8 @@ import requests
 import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
-from homeassistant.const import (CONF_HOST, CONF_NAME, CONF_PORT)
+from homeassistant.const import (
+    CONF_HOST, CONF_NAME, CONF_PORT, POWER_WATT)
 from homeassistant.helpers import config_validation as cv
 from homeassistant.helpers.entity import Entity
 from homeassistant.util import Throttle
@@ -46,7 +47,7 @@ def setup_platform(hass, config, add_entities, discovery_info=None):
 
     dev = []
     for mtu in gateway.data:
-        dev.append(Ted5000Sensor(gateway, name, mtu, 'W'))
+        dev.append(Ted5000Sensor(gateway, name, mtu, POWER_WATT))
         dev.append(Ted5000Sensor(gateway, name, mtu, 'V'))
 
     add_entities(dev)
@@ -58,7 +59,7 @@ class Ted5000Sensor(Entity):
 
     def __init__(self, gateway, name, mtu, unit):
         """Initialize the sensor."""
-        units = {'W': 'power', 'V': 'voltage'}
+        units = {POWER_WATT: 'power', 'V': 'voltage'}
         self._gateway = gateway
         self._name = '{} mtu{} {}'.format(name, mtu, units[unit])
         self._mtu = mtu
@@ -114,4 +115,4 @@ class Ted5000Gateway:
                 voltage = int(doc["LiveData"]["Voltage"]["MTU%d" % mtu]
                               ["VoltageNow"])
 
-                self.data[mtu] = {'W': power, 'V': voltage / 10}
+                self.data[mtu] = {POWER_WATT: power, 'V': voltage / 10}
diff --git a/homeassistant/components/sensor/volkszaehler.py b/homeassistant/components/sensor/volkszaehler.py
index 47aa580e3d4..d81400d300e 100644
--- a/homeassistant/components/sensor/volkszaehler.py
+++ b/homeassistant/components/sensor/volkszaehler.py
@@ -11,7 +11,7 @@ import voluptuous as vol
 
 from homeassistant.components.sensor import PLATFORM_SCHEMA
 from homeassistant.const import (
-    CONF_HOST, CONF_NAME, CONF_PORT, CONF_MONITORED_CONDITIONS)
+    CONF_HOST, CONF_NAME, CONF_PORT, CONF_MONITORED_CONDITIONS, POWER_WATT)
 from homeassistant.exceptions import PlatformNotReady
 from homeassistant.helpers.aiohttp_client import async_get_clientsession
 import homeassistant.helpers.config_validation as cv
@@ -31,10 +31,10 @@ DEFAULT_PORT = 80
 MIN_TIME_BETWEEN_UPDATES = timedelta(minutes=1)
 
 SENSOR_TYPES = {
-    'average': ['Average', 'W', 'mdi:power-off'],
+    'average': ['Average', POWER_WATT, 'mdi:power-off'],
     'consumption': ['Consumption', 'Wh', 'mdi:power-plug'],
-    'max': ['Max', 'W', 'mdi:arrow-up'],
-    'min': ['Min', 'W', 'mdi:arrow-down'],
+    'max': ['Max', POWER_WATT, 'mdi:arrow-up'],
+    'min': ['Min', POWER_WATT, 'mdi:arrow-down'],
 }
 
 PLATFORM_SCHEMA = PLATFORM_SCHEMA.extend({
diff --git a/homeassistant/components/smappee/sensor.py b/homeassistant/components/smappee/sensor.py
index 67213ab15bf..afe06d83f47 100644
--- a/homeassistant/components/smappee/sensor.py
+++ b/homeassistant/components/smappee/sensor.py
@@ -4,6 +4,7 @@ from datetime import timedelta
 
 from homeassistant.components.smappee import DATA_SMAPPEE
 from homeassistant.helpers.entity import Entity
+from homeassistant.const import POWER_WATT
 
 DEPENDENCIES = ['smappee']
 
@@ -12,9 +13,10 @@ _LOGGER = logging.getLogger(__name__)
 SENSOR_PREFIX = 'Smappee'
 SENSOR_TYPES = {
     'solar':
-        ['Solar', 'mdi:white-balance-sunny', 'local', 'W', 'solar'],
+        ['Solar', 'mdi:white-balance-sunny', 'local', POWER_WATT, 'solar'],
     'active_power':
-        ['Active Power', 'mdi:power-plug', 'local', 'W', 'active_power'],
+        ['Active Power', 'mdi:power-plug', 'local', POWER_WATT,
+         'active_power'],
     'current':
         ['Current', 'mdi:gauge', 'local', 'A', 'current'],
     'voltage':
diff --git a/homeassistant/components/smartthings/sensor.py b/homeassistant/components/smartthings/sensor.py
index 50beefdb5b2..680098adc7e 100644
--- a/homeassistant/components/smartthings/sensor.py
+++ b/homeassistant/components/smartthings/sensor.py
@@ -5,7 +5,7 @@ from typing import Optional, Sequence
 from homeassistant.const import (
     DEVICE_CLASS_BATTERY, DEVICE_CLASS_HUMIDITY, DEVICE_CLASS_ILLUMINANCE,
     DEVICE_CLASS_TEMPERATURE, DEVICE_CLASS_TIMESTAMP, MASS_KILOGRAMS,
-    TEMP_CELSIUS, TEMP_FAHRENHEIT)
+    TEMP_CELSIUS, TEMP_FAHRENHEIT, POWER_WATT)
 
 from . import SmartThingsEntity
 from .const import DATA_BROKERS, DOMAIN
@@ -85,7 +85,7 @@ CAPABILITY_TO_SENSORS = {
     'ovenSetpoint': [
         Map('ovenSetpoint', "Oven Set Point", None, None)],
     'powerMeter': [
-        Map('power', "Power Meter", 'W', None)],
+        Map('power', "Power Meter", POWER_WATT, None)],
     'powerSource': [
         Map('powerSource', "Power Source", None, None)],
     'refrigerationSetpoint': [
diff --git a/homeassistant/components/switch/fritzdect.py b/homeassistant/components/switch/fritzdect.py
index a04de7618af..2aefcec9670 100644
--- a/homeassistant/components/switch/fritzdect.py
+++ b/homeassistant/components/switch/fritzdect.py
@@ -12,7 +12,7 @@ import voluptuous as vol
 
 from homeassistant.components.switch import (SwitchDevice, PLATFORM_SCHEMA)
 from homeassistant.const import (
-    CONF_HOST, CONF_PASSWORD, CONF_USERNAME)
+    CONF_HOST, CONF_PASSWORD, CONF_USERNAME, POWER_WATT)
 import homeassistant.helpers.config_validation as cv
 from homeassistant.const import TEMP_CELSIUS, ATTR_TEMPERATURE
 
@@ -25,7 +25,7 @@ DEFAULT_HOST = 'fritz.box'
 
 ATTR_CURRENT_CONSUMPTION = 'current_consumption'
 ATTR_CURRENT_CONSUMPTION_UNIT = 'current_consumption_unit'
-ATTR_CURRENT_CONSUMPTION_UNIT_VALUE = 'W'
+ATTR_CURRENT_CONSUMPTION_UNIT_VALUE = POWER_WATT
 
 ATTR_TOTAL_CONSUMPTION = 'total_consumption'
 ATTR_TOTAL_CONSUMPTION_UNIT = 'total_consumption_unit'
diff --git a/homeassistant/components/tellduslive/sensor.py b/homeassistant/components/tellduslive/sensor.py
index 48133fd69e6..42c93aa52f9 100644
--- a/homeassistant/components/tellduslive/sensor.py
+++ b/homeassistant/components/tellduslive/sensor.py
@@ -5,7 +5,7 @@ from homeassistant.components import sensor, tellduslive
 from homeassistant.components.tellduslive.entry import TelldusLiveEntity
 from homeassistant.const import (
     DEVICE_CLASS_HUMIDITY, DEVICE_CLASS_ILLUMINANCE, DEVICE_CLASS_TEMPERATURE,
-    TEMP_CELSIUS)
+    TEMP_CELSIUS, POWER_WATT)
 from homeassistant.helpers.dispatcher import async_dispatcher_connect
 
 _LOGGER = logging.getLogger(__name__)
@@ -33,7 +33,7 @@ SENSOR_TYPES = {
     SENSOR_TYPE_WINDAVERAGE: ['Wind average', 'm/s', '', None],
     SENSOR_TYPE_WINDGUST: ['Wind gust', 'm/s', '', None],
     SENSOR_TYPE_UV: ['UV', 'UV', '', None],
-    SENSOR_TYPE_WATT: ['Power', 'W', '', None],
+    SENSOR_TYPE_WATT: ['Power', POWER_WATT, '', None],
     SENSOR_TYPE_LUMINANCE: ['Luminance', 'lx', None, DEVICE_CLASS_ILLUMINANCE],
     SENSOR_TYPE_DEW_POINT:
     ['Dew Point', TEMP_CELSIUS, None, DEVICE_CLASS_TEMPERATURE],
diff --git a/homeassistant/components/zha/sensor.py b/homeassistant/components/zha/sensor.py
index 6dcdbb845dc..16ca8ec3d71 100644
--- a/homeassistant/components/zha/sensor.py
+++ b/homeassistant/components/zha/sensor.py
@@ -7,7 +7,7 @@ at https://home-assistant.io/components/sensor.zha/
 import logging
 
 from homeassistant.components.sensor import DOMAIN
-from homeassistant.const import TEMP_CELSIUS
+from homeassistant.const import TEMP_CELSIUS, POWER_WATT
 from homeassistant.helpers.dispatcher import async_dispatcher_connect
 from .core.const import (
     DATA_ZHA, DATA_ZHA_DISPATCHERS, ZHA_DISCOVERY_NEW, HUMIDITY, TEMPERATURE,
@@ -69,8 +69,8 @@ UNIT_REGISTRY = {
     TEMPERATURE: TEMP_CELSIUS,
     PRESSURE: 'hPa',
     ILLUMINANCE: 'lx',
-    METERING: 'W',
-    ELECTRICAL_MEASUREMENT: 'W',
+    METERING: POWER_WATT,
+    ELECTRICAL_MEASUREMENT: POWER_WATT,
     GENERIC: None
 }
 
diff --git a/homeassistant/const.py b/homeassistant/const.py
index 54901feb73b..49194c06c17 100644
--- a/homeassistant/const.py
+++ b/homeassistant/const.py
@@ -319,6 +319,9 @@ ATTR_DEVICE_CLASS = 'device_class'
 ATTR_TEMPERATURE = 'temperature'
 
 # #### UNITS OF MEASUREMENT ####
+# Power units
+POWER_WATT = 'W'
+
 # Temperature units
 TEMP_CELSIUS = '°C'
 TEMP_FAHRENHEIT = '°F'