You can buy the best components to be found on the Web, and they’ll be useless if you can’t actually turn your computer on once you’ve finished building it. A power supply unit (or PSU) may be the most unappreciated of components, but without it, nothing else will work, so don’t forget to give it the thought it deserves.
What to look for?
Maximum power. This is the highest amount of power the supply is capable of directing to your components. The less complicated or intense the build, the lower a number you can get away with—for most people, 500 to 750 watts will be fine. But if you’re using high-end parts, particularly energy-sucking video cards (or more than one), your power needs could increase to 1,000 watts or more. Checking your components’ power usage or thermal design power (TDP) is vital—get a PSU that’s too weak, and your computer may not even turn on.
Voltage rails. Simply put, voltage rails are like individual power circuits within your PSU, with each of the major varieties (+3.3V, +5V, and +12V) powering different kinds of components. In most cases, the most important one to pay attention to is the +12V rail, as that’s what will be driving your video cards; one of these capable of supplying 34 to 40 amps should be enough for the most power-hungry cards you can currently buy, and is likely to be more reliable than using multiple +12V rails for the same job.
Form factor. Like other components we’ve covered here, power supplies come in a variety of form factors that determine the kinds of hardware you can use with them, and under what circumstances. The most common for mainstream motherboards right now is the ATX12V, but you may also see others (such as EPS12V), and you may need to buy a smaller power supply if you’re building a system too miniature to fit a full-size ATX power supply, say.
Connectors. Power supplies come in two varieties: one in which all the cables are pre-attached, and another (called modular) that lets you hook up only the cables you need. In either case, your PSU still has to have the right connectors, whether six- and eight-pin PCIe for video cards, SATA for newer hard drives and SSDs, or Molex for older drives and other devices. The good news is that if you don’t have all the connectors you need, adapters aren’t too tough to find. Still, it’s easiest to verify that you have what you need ahead of time; the video card (or PCIe) connectors are most likely to trip you up, so find out what your card needs so your PSU can supply power in the proper way.