Power Supply

See examples at Newegg. 

The Power Supply takes the 110 volt alternating current that you get from the wall plug and turns it into 12 or 5 volt DC current that can be used by the CPU, mainboard and disks.

The Power Supply has a maximum number of watts that is a limit to the number of devices you can plug into it. The CPU, memory, and mainboard may use 100 watts at low activity and then jump up 20 to 40 watts higher during heavy CPU use. Disks use less than 8 watts each. So a 350 watt power supply will handle an ordinary computer.

However, a high end video card can use 250 watts of power by itself, and gaming enthusiasts put two video cards in a machine. So for unusual systems you can buy power supplies rated at 1000 watts or more.

The Power Supply is the component most likely to fail during the life of the computer. It must deal with surges if the power cuts off unexpectedly and is then restored. If not kept clean, it can overheat.

There are $30 no name power supplies built in China that have a pretty good chance of working for the life of the machine. If one of them fails, it can be replaced in 10 minutes. Or you can buy a quality brand name power supply for $100. The author has had some good luck with cheap units and some bad luck with expensive units. If you don't have some monster demand from the video cards, choose low noise (large fan) medium priced ($50-60), 350 to 480 watt power supplies from some reputable brand (Antec, Asus, SeaSonic, Silverstone) and always have a spare.

This is one of the simplest components to replace. Four screws attach it to the case. You have to disconnect all the cables, put in the new unit, and reconnect all the cables. This is fine if the thing dies outright. However, since every other part of the computer draws power, if the power supply doesn't fail outright but just drifts out of its standard operating range, the symptoms can incorrectly point to other components. Experts who deal with lots of computers tend to invest more on the power supply and get higher quality here than on other components. If you are unsure, read the comment section of Newegg to see how other people feel about a particular brand and model. If you have enough machines to stock spare parts, this is the first item where you buy extra and put one on the shelf.

If you deal with five or more computers, it becomes worthwhile to get a Power Supply Tester. They cost from $6 to $20 and typically have LEDs that indicate if each voltage generated by the power supply is within acceptable limits. If you are having a problem, you can test a power supply in seconds rather than spending minutes swapping it out for another unit only to find that is not the problem.