There are two different modes. In passive mode, there is no energy consumed by the device that being read. In active mode, there is a small amount of current (less than 15 mA) that will be used.
Up to you if that amount is too much.
The way that it works is similar to the way that RFID works. In essence, there is a modulator/demodulator that turns the power from the reader into current that can power an integrated circuit, which controls a transmitter.