An electronic apparatus capable of effectively using power of an AC/DC adaptor including a host and an AC/DC adaptor is provided. The AC/DC adaptor is used for receiving an AC power and converting it into a DC power for the host. At the same time, the AC/DC adaptor outputs a controlling signal to control the power consumption of the host according to the power output at that time. The AC/DC adaptor includes a switching power converter for converting the AC power into the DC power, and a power supply controller for outputting a controlling signal according to the power output by the switching power converter.