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Power to the PC: How to Select a Computer Power Supply
Rails, Modular, Certified, Watts, Volts
September 16, 2010
Power supplies are one of those things you don't think about until yours dies. If you've been using personal computers for a while, there's a pretty good chance you've had a power supply fail on you. It's probably not as common as it once was, but that crisis does still happen. The other issue for some is what to do if you want to upgrade your graphics capabilities but don't have a sufficiently beefy power supply?
We'll try to answer questions like how does one choose a power supply; how do I size one; what are rails; what is "modular"; what does "green" mean; and what is "certified" and does it matter. We'll do that by looking at a number of vendors and how they help you choose one of their products.
The first thing we need to do is define a few terms. Every power supply unit (PSU) converts the 120 Volts coming out of a wall socket (at least in the US) into three voltage levels. Each of these voltages should be at one of three levels and are commonly referred to as the "3.3 volt rail", the "5 volt rail" and the "12 volt rail". Each rail, or voltage level, will have a maximum current rating. To determine the total wattage of a power supply, multiply the voltage level by the current rating to get maximum watts per rail. Then add up the watts to get the total maximum wattage of the PSU.
Every PSU you'll find for sale will have a rating in watts. This really doesn't tell the whole story as no two power supplies will have the same efficiency. In the process of converting from one voltage level (120 V AC) to another there is some loss typically in the form of heat. A really cheap power supply will most likely be equally inefficient, meaning it will generate more heat and consume more power. A quick Google search for "power supply efficiency" will produce links to multiple articles on the subject.
Most of the power in a modern PC is consumed by the hard drives, video card(s) and the motherboard. If you plan on installing multiple hard drives, you'll want to make sure your PSU can handle the additional load. A typical SATA hard drive requires all three voltages at gradually higher current levels. You'll find specifications for various hard drives on both the Seagate and Western Digital web sites. The Serial ATA International Organization website has everything you ever wanted to know about the SATA specification on their web site.
Corsair is probably better known for their memory products, but they also sell power supplies. Their product line is broken down by modular or non-modular meaning the cables are either removable or not. You'll typically pay more for a modular power supply, but it does give you added flexibility if you have any plans for upgrading or expanding your system at a later date.
They have a PSU finder tool to help you find the right product to meet your needs. Their AX1200 boasts a hefty 1200 watts of available power and runs at least 90% efficient under a 50% load. It also comes with a 7-year warranty and utilizes a super-quiet fan to reduce noise. They have a white paper on their web site with a full technical description of the product. Another Application Note available on their web site discusses why a high-quality PSU should be on the top of your list for system upgrades.