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

@property
def unit_of_measurement(self):

    try:
        tag = self._port.tag
    except ValueError:
        return 'State'
    if (tag == 'temperature'):
        return TEMP_CELSIUS
    elif (tag == 'active_pwr'):
        return 'Watts'
    elif (self._port.model == 'Input    Digital'):
        return 'State'
    return tag
