You can't. Antec was one of the early players to destroy the rating system with their lower end power supplies dropping voltage around 70% of their rated load. Antec then came out with the "True Power" which actually put out what it was rated at, and now people claim Antec's "always been" a fine company.
Different companies have different duty cycles they rate their power supplies at. Fortron Source for example rates their supplies at 70% of the cuttoff load (the point when the power supply trips it's overload protection circuit). So they tend to put out far more than their rating. Antec's True Power put out a little more than their rating. Enermax puts out about the same as it's rated at. Enlight as I recall puts out just a hair less than it's rated at. And cheaper power supplies tend to be anyone's guess, but that guess had better be low.
Another thing about cheaper supplies is they don't filter the current as well, which can lead to stability problems and shortened component life. And they tend to use inferior capacitors, which means the power supply itself will likely fail within a short period of time (2 months to 2 years, rather than 5-15 years of the good ones).
There are other power supplies at or near the top, I'd read some reviews.
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