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July 14, 2007

How to Buy a Power Supply

The power supply unit (PSU) is one of the few items in an electrical device that will affect the performance and reliability of your entire system. It is often the most undervalued, under-appreciated component within any device, yet one of the first components to cause failure.

This article focuses on the factors to look for when purchasing a PSU for a personal computer, but it may be applied to any application needing a regulated power supply. When following this guide, take into account your own application, and weigh each factor appropriately.

Steps Determine the wattage you need. Use a power supply calculator web page or software to help determine your requirements. Do not purchase a PSU just above your requirements unless you plan not to upgrade the system for two years or more. Also, PSUs age, losing power over time. Purchase a PSU that will take you through your next one or two upgrades, over a multiple-year period.

Research which connectors you need. Newer PSUs will often provide both a 24-pin ATX connector that doubles as a 20-pin connector. Higher-end models may only provide a 24-pin connector, and lower-end models may provide just a 20-pin connector. Typically, most Pentium 4 and Athlon 64 CPU-based motherboards (and earlier) will require a 20-pin ATX connector, while newer motherboards require a 24-pin ATX connector. Also, most PSUs will have both a 4-pin and 8-pin auxiliary 12V connector for motherboards, and only high-end PSUs will have one or more 6-pin PCI-E connectors for video cards.

Look for PSUs with high efficiency ratings, and ones rated under load load temperatures (not room temperature). Anything 80% and above is good. At 83%, approximately 17% of the wattage is lost as heat. Therefore, a PSU that may be advertised as a 500W PSU, may only supply 415W. Efficiency drops over time, and during the life of the power supply. A year-old PSU is most likely not capable of producing the same amount of energy when it was new.
Determine the robustness of the PSU. How well does the PSU handle changes in current? Bigger components (ie: capacitors) equates to a more tolerant and reliable PSU. PSUs with a 120mm fan built into the top (or bottom) of a PSU enclosure will not have room for the larger capacitors found in some units with 80mm fan(s) in the back and/or front. This isn't necessarily a measure of power, just a rough, visual estimate of a PSU's reliability and lifespan. Even though there are plenty of good PSUs with 120mm fans, never run these types on a simple surge protector; put them on a UPS only.

Check the number of rails. Multiple rails mean wasted power. There is nothing wrong with a single rail design if the component quality is good (ie: brand name manufacturers who dump money into design). So one power supply has dual 12V rails, and another has quad 12V rails. The quad rail model is worse off compared to others because a rail is isolated. It can't provide any more amps than it can generate unless it starts to dip the voltage supplied. To make matters worse with a multi-rail unit, you, as the the builder, have to decide what goes on which rail. Some higher end PSUs can switch on the fly, but most can't. So you have your video card(s) on one rail, and other things on another rail, etc. What happens when your video cards need more than one rail can provide? They may not get the power they need, and voltage drops to provide additional current, affecting stability and causing artifacts to appear in your game or DirectX/OpenGL application.

Get a modular PSU if possible, as it will help eliminate extra wires to get in the way of cooling. Ignore the claims by PC Power & Cooling that modular cables create more resistance due to corrosion of contacts. The additional resistance is negligible.
Compare the amperage of each voltage. A PSU's wattage rating isn't conducive to determining amperage at any specific voltage. All PSUs will have a sticker with its rated amperage at each voltage level. This information should be provided when purchasing a PSU from an online vendor, and visible on the unit's retail box. A 500W PSU may sound adequate, but if its 12V amperage is in the low 20s or less, it may not be properly balanced to supply adequate 12V amperage.

Tips Major labels such as Rosewill (a Newegg.com brand), offer value, but at the cost of performance and reliability. Labels like this are a good choice for low-end builds with minimal power requirements, but they are often over-rated for their wattage output, and under-protected for ensured reliability.
Look past bells and whistles, such as lighted fans, adjustable fan speeds, sleeved cables, and painted or polished enclosures. Although these are certainly desirable features, they do not make up for performance or reliability shortcomings.
Purchase a PSU tester (typically between $10 and $20 US) to confirm all of the voltage rails are working on your new PSU. Note that PSUs with 24-pin connectors no longer supply -5V.
Some brands release top quality models, some just release value line models, and others shouldn't be releasing models at all! Here's a categorized list of brands by quality grouping according to performance and reliability based on reputable, cross-brand reviews listed in the External Links. Note that not all models within a single brand label are of the same quality, and these are averages for the entire model range of brands, not just specific models.

Top Quality (based upon electrical capabilities): PC Power & Cooling, Seasonic, Zippy, Silverstone, Enermax, Antec, Acbel, Akasa, AMS, Channel, Corsair, Etasis, Hiper, Mushkin, OCZ, NZXT, Scythe, Tagan, Thermaltake, Ultra, Zalman.
Low Quality (based upon RMA rate and intended application of value-line computers): A-TOP, Aerocool, APEX, Aspire, Asus, ATADC, Athena Power, ATRIX, BFG, Broadway, CoolerMaster, Coolmax, Deer, Delta, Diablotek, Dynapower, EagleTech, Enhance, Enlight, E-Power, FOXCONN, Futurepower, Fortron, HIPRO, I-Star, InWin, JPAC, Just PC, Kingwin, Linkworld, Lite-On, Logisys, Masscool, MGE, MSI, NMEDIAPC, Norwood Micro, NorthQ, Powmax, Q-Tec, Raidmax, Rosewill, SFC, Sintek, Shuttle, Skyhawk, Sparkle, Spire, Star Micro, STARTECH, Sunbeam, TOPOWER, TTGI, Wintech, XClio, XION, YoungYear, Zebronics.


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