The Big Power Supply Guide - Features
Power supply manufacturers, like many companies, go to great lengths to try to differentiate their products from others. This is especially difficult in an industry where a small number of factories produce all the products on the market. So here now is a breakdown of the features that matter, the features that don't, and what they all mean.
Many power supplies now provide more than one 12 V "rail". That is, there are two or more mostly-separate circuits to provide 12 V power. This is a safety feature that is marketed as a performance feature. Standards recommend that any single "hot" wire on a device not be able to deliver more than 240 W. The ATX spec incorporated this idea by indicating that supplies able to deliver large 12 V currents (above 20 A, which at 12 V would be 240 W) should have multiple "rails" that are separately current-limited.
The exact way this is implemented varies dramatically; ideally separate regulators would be used, providing true isolation. In practice, the "rails" usually just have separate overcurrent limiters, if that. Increasingly, supplies are being built with "quasi-rails" (sometimes marketed as "rail fusion", usually not mentioned at all) in which the manufacturer supplies ratings for multiple rails but the actual device is built with a single rail. Antec's Trio series (which purports to have three 12 V rails) is an example.
The safety value of this type of limiting was suspect from the beginning, but the lax manner that manufacturers have implemented the limiting has done more damage. For various complicated reasons, it has turned into a mild performance negative. Many manufacturers are migrating back to single-rail supplies that are marketed as single-rail supplies.
Hold-up time is a measure of how long a supply can keep providing power at specification if its input power is suddenly cut off. If the building's power briefly shuts off and comes back, or falls very low and comes back, a supply with longer hold-up time is more likely to continue operating normally. Longer hold-up times can be a general indication of higher-quality supplies, built with better materials.
Power supplies convert one voltage to another (several others); in the process, they also waste some energy as heat. Efficiency describes just how much energy is lost as heat. Higher efficiency means less waste. For some notes on the intricacies of efficiency, please see here.
Video cards these days use a lot of power. nVidia's SLI technology allows you to use two cards together in one system- and that uses twice as much power. To ensure things Just Work, nVidia has developed a program to test power supplies and make certain they can handle the load. That's not to say other supplies can't, but nVidia won't guarantee it.
The load on a power supply is not consistent. As it changes, the output voltage will tend to vary as well. Regulation is a measure of how much that output voltage will change. The current revision of the ATX specification requires at least 5% regulation in continuous operation. Some manufacturers (notably PC Power & Cooling) go further and ensure 1% regulation.
This is just what it sounds like. In the event of some malfunction that causes the output voltage to rise unusually high, the power supply can shut down rather than endanger the hardware it is connected to. Lower ratings indicate a supply that will shut down sooner.
Power supplies are a very standardized business; but there happen to be a number of standards. Virtually everything on the market now complies with some iteration of the ATX specification. There's a run-down of the different power supply standards on Wikipedia. If you're buying a power supply for a typical consumer system, you want an ATX12V 2.0 (often written ATX 2.0) supply with a "20+4" pin "ATX" connector. That is, the main connector will have 20 pins and a square connector with 4 pins that can latch onto its side. This ensures compatibility both with older, 20-pin motherboards and with newer, 24-pin motherboards.
Computers are not toasters. Computers do interesting things to the AC voltage waveform; toasters do not. This is inconvenient for power companies. Power factor correction makes your computer behave more like a toaster, as far as the power company is concerned. Simple passive PFC is a cheap enough way to do this, and is required in many countries; it's therefore included on most computer power supplies. Active PFC is more expensive and much more effective, and usually included only in higher-end supplies. But most likely, it doesn't matter to you. Some businesses are charged for having a non-unity power factor, and they may invest in active PFC to avoid that charge. But even still, it takes a lot of computers for it to really matter. To the average home user, the difference is very close to nothing. But hey, it doesn't hurt. For more see here.
A small set of people are very concerned about getting the output voltages they want. And they're control freaks. If this describes you, adjustable voltages are for you. For everyone else, they are a gimmick. But like active PFC, having 3 knobs to adjust voltages is not likely to do you much harm, other than possibly looking a bit odd.
Another one most people will file under "gimmick". Here's the idea. Different people put different devices in their computers. So they need different connectors and cables. But power supply manufacturers have to cater to everyone, so you end up with cables you don't need that add clutter. So now, there are power supplies with cables that can disconnect from the power supply itself and be stored elsewhere. Sounds great, right? The thing is, having one more point of connection means having one more point of failure. One more place where the cable can come detached. One more source of resistance that wastes power. If you really want them, then fine; but be warned.