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What does a power supply actually do?
The job of a power supply unit (PSU) is fairly simple: Take the 110 volt AC current from the wall and turn it into integrated circuit friendly DC, and at a fraction of the original voltage. A power supply, however, is much more than just a single transformer. Modern PSUs have at least five separate voltage outputs, and usually more. Each different power output is called a “rail”: the -12 volt rail, the -5 volt rail, the 3.3 volt rail, the 5 volt rail, and one or more 12 volt rails. Each rail is an independent power source in its own right, monitoring and adjusting the voltage on its outputs.
The -12 and -5 volt rails are small, really only used by the motherboard in a few applications. The 3.3 volt rail is also used by the motherboard, along with SATA hard drives. The 5-volt rail is used by most of the drives, along with the motherboard and other accessories.
The most important rail is, by far, the 12 volt rail. It has the lion’s share of the wattage, usually accounting for 75% or more of the power output of the PSU. It’s used by everything, from the processor (which has a dedicated 12 volt input on the motherboard) to video cards (many of which also have dedicated 12 volt inputs) to the case fans.
Most PSUs have multiple 12 volt rails, to share the work load that would otherwise have to be handled by a single rail. The entry level Seasonic 380 watt has two, while on the other end of the spectrum is the giant Enermax Galaxy 1000 watt, which has no less than four.
Picking the right PSU
While it is entirely possible to go through a system part-by-part and calculate the exact expected power usage, that process is long and boring. Instead, here are the most powerful systems I would trust with each PSU:
Some other considerations
The next consideration is far more noticable in day-to-day operation. Noise. Some power supplies are designed to be dead silent, some are rather loud and others sound more like little jet engines. While we have tried to keep our options as quiet as possible, some of our offerings, such as the Enermax Galaxy, are not very quiet. It’s not loud, per se, but it makes a noticable amount of noise compared to the very quiet Seasonics.
The last consideration is purely cosmetic. Some PSUs have modular cables: If you don’t need four cables with SATA power plugs, they can be unplugged and left out of the computer. This makes my job of hiding all the extra cables during a build quite easy (we still send the cables to the customer just in case they want to have future upgrades). Especially in systems with windows and lights, a modular PSU can make the interior of a PC look much cleaner. However, there are who insist that modular PSUs are a bad idea, because the connectors offer slightly more resistance, and provide another point of failure.