0.5 A is the max current in the USB-standard.
The USB connector provides a single nominally 5 volt wire from which connected USB devices may power themselves. In practice, delivered voltage can drop well below 5 V, to only slightly above 4 V. The compliance spec requires no more than 5.25 V anywhere and no less than 4.375 V at the worst case; a low-power function after a bus-powered hub. In typical situations the voltage is close to 5 V.
A given segment of the bus is specified to deliver up to 500 mA. This is often enough to power several devices, although this budget must be shared among all devices downstream of an unpowered hub. A bus-powered device may use as much of that power as allowed by the port it is plugged into.
Googled it: from Wikipedia ( http://en.wikipedia.org/wiki/USB#Power_supply
power = voltage x current
P=V x I
P max for USB = 5V x 0.5A = 2.5Watts)