Retina macbook max USB power output
With the System Profiler you'll be able to know how much power does the USB delivers, and how much is required for each device connected to a USB. Under the Hardware tab, you'll find, listed, all the USB ports, and, within, all the devices connected to them. Just select one of those to get detailed information.

To open System Profiler just choose Apple Menu / About this Mac and clicking More Info, or go to Applications / Utilities.