A USB cable provides a data transfer connection and provides power to the device.
USB devices are either powered via the USB cable that connects them directly to the PC or laptop, such as wired or wireless mice and keyboards, flash drives, cameras, mobile phones, etc., or by an independent power supply supplied with the device that connects them to the mains electricity supply, such as external hard disk, external CD/DVD drives, printers, scanners, MFPs (multi-function printer, scanner, copier), monitors, etc.
Each USB port provided by a computer can connect a single device or a non-powered or a self-powered hub to which several USB devices can be connected. A non-powered hub just splits access to the port to as many USB ports as it provides. If the hub connected to a single USB port has four ports, four USB devices can be connected via it. A self-powered hub is just like a self-powered USB device, such as a printer, with its own power supply. It adds additional USB ports but also provides their power.
Self-powered hubs usually provide up to 500 mA (milliamps) of power to each of its ports, while non-powered hubs split the total 500 mA provided via the USB cable attached to the port among all of ports and the hub itself, which can draw up to 100 mA.
If too many non-self-powered, power-hungry devices (not including self-powered printers, external drives, etc.) are connected to a non-powered hub, collectively they can exceed the maximum of 500 mA made available from the computer. Therefore, if you have problems with many devices connected to a non-powered hub, try using a self-powered hub.
Windows 7 provides a way to find out how much power is available per USB port. To access that information, open the Device Manager by typing device in the Start => Search.. box and click the link called Device Manager. Scroll down to the last of the devices and open Universal Serial Bus controllers. There should be several items called USB Root Hub. Right click on any of them, click Properties in the menu that is presented and open its Power tab. You should see Total power available: 500 mA per port. That information is also provided by Windows 8.1 from its Device Manager.
So, if you had everything working from, say, a non-powered hub and then when you add another device and things start to go wrong, check that the maximum power limit for the port has not been exceeded by the addition.
My self-powered HP PSC 1410 printer-scanner-copier registers as using only 100 mA.
View USB Power and Bandwidth Allocations for a Device [Windows 7] –