Power supplies are computer components that provide electricity to the system by converting AC (alternating current) from a wall outlet to DC (direct current) for the computer. Power supplies are located at the rear of the computer case and usually contain one or more cooling fans. The back plate features a power cord receptacle and off/on switch. Most power supplies also have a rear voltage switch that can be changed for operating in different countries. Some power supplies come with LED lights, popular with modders.

Various components in the computer have different voltage requirements. Power supplies typically provide 3.3v and 5v rails for digital circuitry, and a 12v rail for running drives and fans. Since more components today, including processors, are feeding off the 12v rail, many power supplies now provide multiple 12v rails.

Power supplies are rated for power by wattage. As computers become laden with more components, such as RAID (Redundant Array of Independent Disks), the need for larger power supplies grows. A typical power supply has gone from 150 watts in the early nineties to 450 watts and greater. However, the public might be over-buying, according to some experts.

Power supplies draw only the current they require at any given time. A 450w model won't draw more than 200 watts if that is all the system is demanding. All system components are not in use simultaneously, so the amount of power required varies. This has led many to believe an advantage exists for getting the largest power supply possible, as it safeguards against under-powering the system, while only drawing the electricity required. But some contend this notion has gone too far, as larger power supplies generate more heat. The situation becomes one of diminishing returns, considering more wattage costs more money. In many cases, this extra money may be wasted when a smaller power supply would do the same job while generating less heat.

In the past, power supply manufacturers followed a number of different standards. Today, the predominant standard is the ATX-form factor, made for standard ATX cases and motherboards. This makes it very easy to swap out a power supply when needed. There are also less common standards, like TFX (Thin Form factor) for slim workstations or small PCs, and BTX (Balance Technology Extended). This latter standard, new as of mid-2005, will work in upcoming BTX cases. These newly designed cases incorporate computer components on a 3-D plane, rather than the 2-D arrangement of ATX cases. The design purportedly increases airflow.

Power supplies feature Molex connectors for the motherboard, drives, fans and other components. The connectors are color-coded and only fit one way to make installation easy. If your motherboard has a 24-pin power connector, be sure to get a power supply with a 24-pin connector. The old standard was 20-pin, while 24-pin was usually reserved for motherboards used in servers. However, high-end motherboards now come with 24-pin interfaces. Check for SATA (Serial ATA) connectors as well, even if you do not currently own SATA drives. This will help future-proof your investment. Note that inexpensive power supplies can be noisier than more expensive models.

In some cases, AMD (Advanced Micro Devices) and Intel will approve specific models of power supplies that can be safely used with their high-end processors. These models are not required, just recommended, insofar as they have been officially tested with the CPU and received a stamp of approval. This information can be found on their respective websites.