[simias]

Isn't that limited at 0.5 amperes @5V? I believe it's not enough for all devices (or to charge them quickly enough).

I believe there's a hack in the apple products that can be used to send more current through USB but it's not standard. I might be wrong though, it's been a while since I looked into that.

Also, it's the reason why certain devices (notably external hard drives alimented through USB) come with two USB connectors in a Y configuration to be able to leech enough current from the host. At least that's how I think it works.

[mbell]

As a baseline it's limited to 100mA @ ~5V (150mA for 3.0). Devices can request additional power from the host, 500mA max for 1.X/2.X or 900mA max for 3.0. The host isn't required to be able to supply this much current, it can say 'no'. There are also some quasi standards and I believe now a real standard now for 'USB charging' which allows higher current (and voltage I believe).