Old Pilot is correct.  Voltage is also just one side of it, as many of the 5-volt USB ports are only 100-150ma.  Some of the newer USB 2 and 3 can support 500-900ma in bursts, but not designed to be sustained.  
The iFly 720 needs sustained 600ma and the new 740 is more like 800-1500ma depending on if it's charging the internal battery.  There is a new USB-C that is coming in the future, that will support higher power demands.  Apple is getting behind USB-C in its newest generation of hardware.  Something to keep an eye on.