A wireless Internet of things (IoT) tag and a method for measuring energy-charging rate of the energy harvester is provided. The wireless IoT includes at least one antenna configured to harvest an ambient energy; an energy harvester coupled to the at least one antenna; an energy storage coupled to the energy harvester and configured to store harvested energy; and an energy detector configured to the energy storage, wherein the energy detector is configured to measure an energy-charging rate of the energy harvester.