The first thing to do is go to the documentation for your motherboard and see what standards your power supply must meet to be compatible. Usually this is only a problem if you’re trying to re-use an older power supply.

Next, you need to figure out how much power you really need. The best way to do this is use a power supply calculator. If you google for one, you’ll get a lot of links to the eXtreme PSU calculator, http://extreme.outervision.com/psucalculator.jsp , which charges for advanced features. The advanced features include amperage requirements for each voltage, so it’s probably worth the $1.95 they ask for.

Antec has a free PSU calculator at http://www.antec.outervision.com/ which will let you list your CPU, video cards, etc.  Unfortunately, it calculates a very low value for my configuration, only 509 watts. It actually takes an 800W PSU for my system, and even that is somewhat borderline. The biggest problem is that it doesn’t tell you what amperage you need on the various voltages.

The key value, lately, is the 12V requirement. High end video cards have enormous appetites for power and most of it comes from the 12V line. Often, a single graphics card will demand more than the common 20A limit on most 12V rails. You can usually find this number listed in the detailed specifications for your video card. For instance, the latest nVidia video card, the 295, nVidia recommends a 590W PSU.

The nVidia GeForce 295 demands a whopping 289W all by itself. That means at least 24 A on the 12V line, and since you can only be sure of 80% efficiency, you want at least 30 A. (I got this number by taking the wattage requirement and dividing by 12 volts to get amps.) If you were to select a PSU with multiple rails, you’d have to make sure that at least one of the rails could carry 30 A.

A single rail design shares the power among all the users of the 12V line,

TERMINOLOGY: Many PSU vendors divide up the 12V line into multiple “rails,” often four or more.  Each rail has it’s own individual capacity and is isolated from the rest of the system. That means that if you have one device that needs more power than it’s rail is rated for, it’s out of luck, even if there are several unused rails. For this reason, it’s generally best to go with a single rail system. (Single rail does not mean there is only a single cable, just that all the cables draw from the same 12V source.)

PC Power & Cooling has a great page on terminology: http://www.pcpower.com/technology/terminology/

Modular cables are also attractive but can introduce noise and power fluctuations. I haven’t tried one yet, but probably will, despite the dire warnings, because of how difficult it can be to work with the huge mass of cables that come out of most PSUs.

Also, make sure that the wattage being reported is the power delivered for continuous, full-load operation. Often this isn’t reported, or the testing is done at artificially low temperatures, like 80 F when a more realistic temperature is 104 F .

Another consideration is the efficiency of your new PSU.  These days,the better PSU are certified as being 80% efficient or better. Typical PSUs of the past often didn’t get into the high seventies. That can make a big difference.  A 750@ PSU at 83% efficiency really gives you 662 Watts. At 70% you get only 525 W.

Enthusiast System Architecture

nVidia has come out with something called the Enthusiast System Architecture, or ESA, which is meant to enable PC components like the chassis, PSU, and water cooling systems, to communicate their state in real time, making it possible to react to changes rapidly. This is a real boon for people who want to try overclocking, or just those who want to build a super reliable system. But it has to be supported by your motherboard, too.

 Recommendations

In the future, I’ll be buying PC Power and Cooling power supplies because they have an excellent reputation and everyone I talk to who has had one has sworn they’ll never go with anything else.

I hope this rambling helps someone. I’ll try to find some more good related links and post them.

LINKS:

I found a great PSU calculator that also figures what you need for your backup UPS:

http://web.aanet.com.au/SnooP/psucalc.php