Wow, this is a mega-uber guide to using USB as a power source, handy!
The Universal Serial Bus (USB) has become a popular method of connecting computer peripherals. The original USB 1.1 specification limited data rates to about 12 Mbits/sec. The updated USB 2.0 spec increases those rates to up to 480 Mbit/sec. Both versions of USB can provide DC power to peripheral devices. This power form is the subject of this page.
USB is less expensive than FireWire because a dedicated FireWire controller chip set isn’t needed. The computer’s CPU does the USB controller function in software. Under good conditions, USB can work nearly as well as FireWire, but at sustained high data rates, USB will sag under the strain. FireWire tends to work better but cost more therefore USB 2.0 is more common, especially in lower end hardware.
USB As A Power Source – Link.