The USB specification defines how much power a device is allowed to consume. So back in the days, some USB-HDD came with 2 USB-A cable (500mA per port), to feed the hungry device on USB 'only'. With USB 3 this was increased to 900mA iirc - it is written in the internet or in one of TKs posts, ahh wait, he deleted everything here. So you must look it up in the net.