In a related article The PSU Factor, I said that it is inevitable a PSU purchase is looming on the horizon. The reason being, your current PSU is probably a few years old, it came with your computer system you bought online at a great price, and you have no idea what the specs are. Taking into consideration your computer manufacturer more than likely had a reliable power supply as being LAST on their list, or not on their list at all, pretty much guarantees something is about to happen shortly you won't like.

The PSU Explosion

Over the last few years, the number of manufacturers producing power supply units have increased almost exponentially. If the company makes computer fans or computer cases, chances are they also manufacture PSUs. This makes for a confusing market for the consumer because of the massive selection of power supplies and the varying differences in the quality of those power supplies.

Determine What You Need

Before you begin any type of shopping for a PSU, find the amount of power required which your computer system is currently using. If you plan on upgrading your computer system and will be using the power supply, then it is wise to forecast the wattage for your future computer system. One particular way of determining this, is using an online PSU wattage calculator. The one I reference, eXtreme PSU Calculator, is in my opinion, the best one found online so far, and takes into account many of the latest systems such as dual- and quad-processors, overclocking, SCSI, SATA, and much more. In addition to the eXtreme PSU Calculator, eXtreme Outer Vision's website is also a great place to go for other calculators relating to other aspects of your PC.

Once you have a general idea of how much wattage your computer system will need, the amperage and the number of rails provided need to be determined.

Rails, Wattage And Amperage

Knowing the total amount of watts your system requires is certainly a good thing, but what's even more important, is knowing how many amps the PSU will provide, and on what rails the amperage will be distributed. The most important rail is the 12V line. Today's motherboards, processors, graphics cards, hard drives, and other components, use the 12V line. The 5V line is not so important anymore and computers built several years ago use them. When you see a 500W PSU, and notice there are 50A on the 5V line, then this should tell you there will be problems. Why? Because that's 250W which is basically available for your entire computer system--but it cannot use it because the newer PC components require a 12V line. So, now you're talking about 250W remaining that is actually usable. Puts things into perspective, doesn't it. Your 500W "SLi Ready" or "High Performance" PSU is far less than what the outside of the box says it is.

Be sure your 12V rail has enough amps to support your system. How do you know what the proper amount of amps should be? Well, suppose you find out after entering in all your component information in a PSU calculator, you come up with 625W. Take 625 and divide this by 12, which will give you the amps. This gives you 58A which should be on the 12V rails exclusively. You might also find it interesting to take manufacturer's PSU wattage claims and then divide them by 12 and see how many amps are actually offered on the 12V rails versus how much you need.

The Rails

When discussing a PSU or reading an article/post about power supplies and especially figuring out which PSU to purchase, the term rails will more than likely come up. A rail is basically the voltage output of a PSU, and there can be several voltage outputs. Ideally, there should be separate rails for hard drives, graphics cards, and CD/DVD drives. Why? Well, because if you have only one "rail" delivering power to all of these components, the power will be split up between all of these components. This is not the preferred method of power current delivery because all of these components have different requirements. One faulty device could send off a chain reaction to other devices since the power is being distributed through one cable. So, finding a power supply with separate rails and their own separate power current for specific devices, is what to look for.

Temperature, Temperature, Temperature!

All this preparation and know-how is great, but unfortunately, we still have to be careful when looking at the specs of power supplies. Many manufacturers are now competing with legitimate ones by simply changing a very important factor of PSU specs: the temperature the PSU was tested at when the specs were taken. Currently, only one PSU manufacturer tests their PSUs at realistic temps, which are tested at 50