The power supply converts AC current from the wall outlet to the appropriate DC voltages for the various components of the computer. A power supply outputs power rated in watts. A PC's demand for wattage depends upon the number of components it contains. Each of the drives, the motherboard, CPU and accessory cards draw power. A power supply that is too small for the power demands of the PC will result in performance problems or malfunction.
A supply rated at 150 watts is small and suitable only for a very basic PC. Supplies rated for 225 to 250 watts are ample for most PCs. However, if you are running multiple hard drives, a CD-ROM, a tape drive, a Zip drive and a DVD player or some other large group of devices, then a power supply of 300 watts or more is called for. The bottom line is that you can never have a power supply that is too big. So when in doubt, choose the larger power supply.
Power supplies are often integrated into and sold with the case. They are also available individually to replace or upgrade an existing unit. In some computers, especially "bargain" PCs, the power supply is permanently fastened to the case and cannot be replaced without replacing the entire case as well. Some high end cases are sold without a power supply because the user may prefer to choose the quality and capacity to suit their needs.
The power supply provides several connectors for powering specific devices. These connectors carry different voltages and are keyed to fit only the device for which they are intended. Some power supplies are designed only for certain motherboard form factors while others provide multiple connectors to choose from for use with any motherboard.
Note: Some difficult to diagnose PC problems such as lockups and other glitches could be the fault of a damaged or failing power supply.