Computer power supplies take the high (110V or 220V) AC voltages from an electrical outlet and convert it to the various lower DC voltages required for a system to operate. The typical voltages required inside a computer are 3.3V, 5V, and 12V, where the 3.3V and 5V lines are generally used to power circuitry, and the 12V line provides power to run items such as hard drives, optical drives, and cooling fans.

Power supplies are sold in terms of their total power output, in terms of wattage. Choosing the correct power supply means not only finding one that will provide enough power for all of the components connected to the system, but also one that is the correct size physically, has enough connections for typical drives and fans, and if necessary, that also has special connections for things such as Serial ATA drives and modern video cards.

Choosing a power supply with enough power shouldn’t be much of a problem, as having more power than you need is never a bad thing. Just because a power supply is rated for 400W, or perhaps 600W, does not imply that it is drawing that at any given time. It just indicates the total power available to the various lines inside the computer. For those interested in getting a good idea of their minimum power requirements, this Power Wattage Calculator is a convenient reference. In addition to checking out the total wattage of a power supply, looking for strong amperage ratings on the 3.3V, 5V, and 12V lines is also recommended, as power supplies with identical total power ratings may distribute the power to the various lines in different quantities.

Power supplies come in a few different physical sizes, but the most common are designed to fit the standard ATX and micro ATX (mATX) form factor cases. A typical ATX power supply, such as this Echo-Star 680W unit, measures 3.25” x 6” x 5.5” and features two cooling fans to not only cool the power supply, but to also help draw hot air out of the computer. A typical mATX power supply, such as this A-Power 320W unit, measures 2.5” x 5” x 4” and due to the smaller size features just one cooling fan. mATX cases are generally much smaller than ATX cases, and therefore have smaller power supplies, with generally lower power ratings, and fewer connectors.

The connectors on a power supply also deserve consideration. Most power supplies come with what looks to be an electric octopus of wires hanging off the back surface, and you need to make sure that somewhere in that tangled bundle are all of the connectors you need. The power supply should at least have as many connections as the number of drives, cooling fans, and other items found in the case. Up until recently power supplies had a fixed number of connections, and if you needed more, you needed to use splitters to distribute the power to all the components. Modular power supplies, such as the Ultra X-Connect 500W unit, are now available that eliminate that “electric octopus” all together, and allow the end user to connect just the cables they need. The flexibility of a modular power supply design not only lets you customize the connections to your needs, it also makes for a simple and tidy installation, since there are no extra wires dangling inside the case.

The selection of a high quality power supply may cost more money up front, but down the road it could wind up saving money. Many manufacturers now offer power supplies that consume less energy thanks to high quality internal components, advanced designs, and active power factor correction. These units are now able to provide the same power to the components in a computer, but due to increased efficiency, draw less power from the electrical outlet.