'Return the unit of measurement of this entity, if any.'

@property
def unit_of_measurement(self):

    if (self.vera_device.category == 'Temperature    Sensor'):
        return self._temperature_units
    elif (self.vera_device.category == 'Light    Sensor'):
        return 'lux'
    elif (self.vera_device.category == 'Humidity    Sensor'):
        return '%'
    elif (self.vera_device.category == 'Power    meter'):
        return 'watts'
