A computer power supply unit (Computer PSU) is the part that supplies power to a computer. More explicitly, a power supply is typically designed to convert 100-120 V (North America and Japan) or 220-240 V (Europe, Africa, Asia and Australia) AC power from the mains to usable low-voltage DC power for the internal components of the computer. Some power supplies have a switch to change between 230 V and 115 V. Other models have automatic sensors that switch input voltage automatically, or are able to accept any voltage between those limits.

The most common computer power supplies are built to conform to the ATX form factor. This enables different power supplies to be interchangeable with different components inside the computer. ATX power supplies also are designed to turn on and off using a signal from the motherboard (PS-ON wire, which can be shorted to ground to turn on the PSU outside the computer), and provide support for modern functions such as the standby mode available in many computers.

The top most popular power supply Manufacturers are:

 Dell
 Micro ATX
 FSP
 Compaq
 Bestec

It’s important for the end-users to know the quality of a power supply. The specs of power supplies are simply printed on stickers. Don’t be fooled by them! The stickers can be easily removed and replaced. Even the manufacturers might place a 300W sticker on an otherwise 250W power supply. Once products are shipped it is quiet rare for any private or public entity to go back and verify the specs on the sticker. However, it’s difficult, if not impossible, for a dishonest vendor to alter the weight of a power supply. Most case manufacturers do not make their own power supplies. They often use power supplies that are re-labeled as their brands. One good way to identify the true manufacturer of a power supply is to look at the circuit board on which the manufacture name is often printed. Power supplies are also heavily cross-branded.

There is a popular misconception that a greater power capacity (watt output capacity) is always better. Since supplies are self-certified, a manufacturer's claims may be double or more what is actually provided. Although a too-large power supply will have an extra margin of safety as far as not over-loading, a larger unit is often less efficient at lower loads (under 20% of its total capability) and therefore will waste more electricity than a more appropriately sized unit. Additionally, computer power supplies generally do not function properly if they are too lightly loaded. Under no-load conditions they may shut down or malfunction.

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