Apple computers and displays with USB 1.1 or USB 2 ports provide:
Up to 500 mA (Milliamps) at 5 V (Volts) to most Apple USB peripherals and all USB peripherals not made by Apple in compliance with USB specifications.
Apple computers with USB 3 ports provide:
Up to 900 mA (milliamps) at 5 V (Volts) to most Apple USB peripherals and all USB peripherals not made by Apple in compliance with USB specifications.
Note: Some USB peripheral devices not manufactured by Apple may have unique power requirements. These may exceed the power allotment on an individual USB port. Those devices may require you connect them to multiple USB ports or may have external power sources that must be present for them to function.
Apple peripheral devices may request more than 500 mA (Milliamps) at 5 V (Volts) from a port to function or to allow for faster charging. Such Apple peripheral devices include:
Apple Computers and Displays: Powering peripherals through USB - Apple Support