I know this has been coming up for awhile now, and I'm actually taking the time to sit down and type this up. What's so important about a good power supply? A good power supply uses higher quality parts and manufacturing for starters. Secondly, they tend to be more conservative on their ratings, where as lesser quality units may rate their units based on an output that can only be attained at a temperature of 25*C. On average, a power supply will run between 35-45*C, and good units tend to be rated for their stated output at around 50*C. Third, bad power supplies have a nasty habit of taking other parts with it and/or putting on a show on the way out. This isn't to say a good brand will never do this, but for previously mentioned reasons, there's a much lower likelyhood. A power supply is afforded a tolerance of +/-5% for each of the rails. Some companies rate their tolerances for 10%, which gives them freedom for lower quality control. Why is this a big deal? Components require a certain voltage level to operate properly. If this is not met, then these parts may not do the proper calculations, cause errors, shut off, or even fail. In extreme cases, there's the risk of damage to that component. The main issue is simply stability, but that can have far reaching consequences if something like the CPU, RAM, or hard drive is getting shafted. Brands we recommend: -Enermax (except for the Liberty series---high failure rate) -Antec -Corsair -OCZ -PC Power & Cooling (for those who want the best money can buy) -Seasonic (very quiet too) -Silverstone -Sparkle (FSP Group brand) -Thermaltake Purepower (for Socket A/Northwood P4s) or Toughpower (A64, Prescott, Core2) -Ultra (except X-Connect) -XClio (rebadged Channelwell, like some Antec units) -Zippy/Emacs Of course, there's a few brands you want to avoid like hell, and here they are: -Apevia (formerly Aspire) -Coolmax -Tagan -Enermax Liberty series (only the Liberty series) -Fortron (FSP Group brand) -Hiper (easy to get ahold of in the UK) -Logisys -MGE -NZXT -Powmax -Q-Tec (also, Q-Technology) -Ultra X-Connect -Anything that comes bundled in a case Now, there are exceptions to the rule, but generally So far, good PSU reviews are pretty hard to come by. A good PSU review involves much more than measuring the unit with a multimeter and seeing the system not crash. Very few sites actually take the time to put the PSU under various loads to see if they pass the muster, so here they are: -PC Perspective -JonnyGuru -X-bit Labs -Hardware Secrets -[H]ard|OCP -Anandtech You may wonder why so few sites are listed. Hardware Secrets has an excellet article, located here, regarding this. As I find more sites with proper power supply reviews, I'll list them. Also, as a rule of thumb, if the power supply is bundled with your case, don't use it. You cannot expect a quality unit in a case costing $60 for the case/power supply combo, when you're looking at $80-100 in most cases on the inexpensive end of power supply costs. I realize that not everyone has $3k to spend on a new box, but make sure you're spending it on quality parts. If you're looking at an SLI rig, nVidia's list of SLI-Certified PSU's is over here. The power supply didn't use to matter much. If it powered the system on, that was good enough. As computers became more powerful, they eventually needed more juice. Within the past year or so, we've seen the Prescott Pentiums and the Athlon 64/FX/Opteron become the big dogs. No problem there, more power is good. While Core 2 is now out, the power consumption still is there, but more of it is coming from the video cards, so even if you get a low-power CPU, don't skimp---especially if you're purchasing a high-end video card or using a multi-GPU setup. So, what's the problem? The problem is that, due to the power requirements of our computers, it's becoming more and more obvious that the best PSU's are far and few between. If the system isn't getting enough power, you can have anything from stability issues and boot problems to damaging the system. The power supply is a component that's overlooked by many, but is extremely critical. Watts don't mean jack. A common misconception is that higher watts mean lots of power. Actually no. I have a Sparkle 350W that has the same amperage as some so-called 500-600W power supplies. The thing is that there's no temperatures in the wattage rating equation, so the companies can spin this any way they want. That generic 500W may only provide that 500W at 50*C, whereas a Sparkle or Enermax may be able to pump out 500W at 85*C. There's also the weight of a power supply. While it's not the only thing you should check out, its generally a good sign to have a heavy PSU. This means that there's likely more capacitors, larger heatsinks, and other power-regulating components inside the unit to provide better, cleaner power. What should I look for? For a Socket A or Socket 478 Pentium 4 (Northwood core), in many cases, the following should be fine. -18A on the 12V rail -30A (or so) on the 5V and 3.3V rails A 350-400W unit from the list of companies I gave earlier will be fine. For an Athlon 64 or LGA775/Prescott PSU, the above isn't going to cut it, so get the following, at bare minimum. You can try a little less, but you're likely to run into trouble if you do. -24-pin ATX power connector -25A on the 12V rail -30A-40A on the 5V and 3.3V rails -4- or 8-pin 12V auxillary power connector -24-pin ATX power connector A 480W or better unit from the listed companies that meet these qualifications should be fine. High-end SLI, Crossfire, or single DX10 card (GeForce 8) setups will want to go with a 600W or better unit featuring: -30A on the 12V or combination thereof -30-40A on the 5V and 3.3V rails -4- or 8-pin 12V auxillary power connector -24-pin ATX power connector Why multiple 12V rails? The general idea behind multiple 12V rails is to give different components their own line that they don't have to share, minimizing fluctuations caused by having additional components on that rail. For example, DFI's NF4 motherboards take one 12V rail and give it to the CPU exclusively, while the second one powers the video card(s) and other components that might need the 12V rail. Without going into electronic theory, there's the factor of a voltage drop between points, depending on the circuit, and if it drops too much, the parts that need a certain voltage just aren't going to work, or they're going to work incorrectly. This is a problem when you have screwy work on the math that the CPU does. The last piece is cost and heat. A single 30A 12V rail is harder to produce and is going to put out more heat as the load goes up. Having two 15A 12V rails in this case would be cheaper and easier to produce. The majority of the heat might only be coming from one while the other isn't putting off as much, reducing stress on the 12V rails, which tends to be good for the lifespan. Modular power supplies? Y/N? [strike]To be blunt, no, they're not recommended. The biggest reason is that the connectors are a point of resistance (to the flow of electricity). This makes the power supply work harder to get the power to the motherboard and other parts, and if there isn't enough power, then you start getting into trouble. Another point, which may not matter to some of you, is that they do cost a bit more than a non-modular power supply of equivalent specifications. If you're dead set on a modular unit, get one that has the main power connector permanently attached. There's no good reason for this one to be modular as it is always necessary. As of right now, the only brands that are recommended in the modular power supply area are Antec, Enermax (Galaxy only, the Liberty series appear to have major issues), OCZ and Hiper.[/strike] While it's been believe by many, including myself, that modular PSU's create a problem, this seems to be less of an actual problem, if at all, than originally believed. While there is the issue of power still needing to jump, it appears that it is not a cause for alarm or worry. So long as the internals are in good shape, standard or modular cabling is really up to you. I'm worried about my rails being low, so what's acceptable 5% above or below is within reason. Software is not the most reliable means of measuring. Use a Digital Multimeter to test this out. Gigabyte's Odin series does feature a software component, but that is measured directly from the unit itself, not via the motherboard. Unfortunately, the software seems to need a little work. This link at DevHardware has a visual guide to testing your PSU. Who makes what power supply? While it might come as a shock, many companies simply rebadge a power supply. They may tweak it a tad, but ultimately, there's very few companies that actually make the power supply. I'm going to sum up the data found at nVidia's Forums. Note that this is not definitive for every single PSU produced by every company. However, it is a pretty good guide for a general rule of thumb. #1a Topower (some TTGI - Super-Flower, OCZ, EPower/Tagan, RaidMax, Vantec, ACI) #1b FSP - Fortron Source Power (Fortron, Sparkle, Cooler Master*, Zalman, Aopen) #1c CWT - Channel Well Technology (Antec, Lead Power, Enermax, Xclio, Turbolink) #2a Sea Sonic (Seasonic) #2b Wintech (Ultra X-finity) (early models) #2c Acbel Polytech (Stateside, just the Cooler Master True Power) #3a Sirtec (Chieftec, Enlight, ThermalTake, High Power) #3b HEC - HeroIchi Electronic Co. (HEC, CompuCase) #3c AMS - American Media Systems (Mercury) #4a Youngyear (Aspire, Logisys, MGE, Ultra X-Connect [later versions], Rosewill) #4b ATNG (Coolink, CoolMax, Rosewill, StarTech) #5 CRAP! (L&C, Deer, Allied, Eagle, CodeGen/Foxconn, EverPower, Maxpower) Also, to the crap list, add Powmax and Q-tec/Q-tech/Q-technology. If you compare the prices, the ones at the top of the list are going to have more expensive units. That's for a good reason: they're built better, use higher quality parts, not to mention more of them. You're also likely to have them last longer as a side effects. JonnyGuru has a much more comprehensive list over here complete with links to the UL website for each model produced by said company.