What Is the Power Output for a USB Port?
The USB port is universal. All computers use it to access removable drives -- such as flash drives and external drives. While most devices are optimized to run and charge on a USB port, some devices require more power than a USB port provides.
Average Power Output
The average power of a USB port is about 5 volts. Your USB device will be able to draw out a maximum of 500 milliamperes (mA), but most default to 100 mA until prompted to provide more power by the device software. If your device requires more power than 500 mA, it may work slower or not at all while plugged into your computer.
What It Means
This means that by and large, most USB-connectible devices will power on and become accessible when plugged into the USB port of a computer as long as that computer is on.
There are a number of devices that use USB ports to power on and run, including flash drives and external hard drives; these devices are turned off when unplugged from the computer. Devices such as MP3 players and cameras use USB power for recharging.