USB Interface
Many USB decorations do not use the communications features of the USB bus, or fully comply with USB standard power discipline. This can interfere with the purposes for which the USB bus was designed, especially if the device suddenly attempts to draw the full 500 mA of power nominally available. The default power consumption is only 100 mA, which should not impact the computer's PSU if a device is suddenly connected without identifying itself first. A newly-connected device is supposed to then use the USB protocol to communicate a request for more DC power, if needed.
Often, several decorations are connected to a single USB hub, which is limited to 400 mA if it has no external power connection and is drawing all its power from a host computer. With each device drawing 100 mA, it is therefore only possible to run four low-power decorations.
If instead a USB hub has an external power adapter, it may be able to support much higher power loads. In many cases, the powered hub may not even need to be connected to a computer host, but can serve as a stand-alone power source for USB decorations that need only DC power.
Read more about this topic: USB Decoration