When shopping for PS's, look at the specification sheet that came with it, is printed on the side, or is at the manufacturers Web Site.

The specification that tells the most about your PSU is the Combined +3.3V & +5V maximum output power. First off, if your unit does not have that specification, it is instantly suspect. This is the equivalent to buying a car that does not have a specification of the Brake Horsepower for the engine.

So here's the magic numbers to compare to.

If your unit claims to be:

300W, the Combined 3.3 & 5V output should be 150-160W
350W, the Combined 3.3 & 5V output should be 180-185W
400W, the Combined 3.3 & 5V output should be 220-245W
500W, the Combined 3.3 & 5V output should be 270W

If your PSU falls short on this specification, you need to question what other specifications does it falls short on.

-Hoot

here is a general guide to PS's:
http://pages.sbcglobal.net/adunlap/psudatasheet.html

 

i also saw one ad saying how much power output is also directly related to temperature...