If the PSU weighs less than 3 pounds it's probably trash (for mining at least). Compare weights between a Seasonic 500W and a Raidmax 500W - almost a 2 pound difference in weight.
For the more technical people all you need to do is look at the 12V specs and see how many Amps the PSU can output.
Your explanation made me think of Jurassic Park when they were sitting in the trucks by the Tyrannosaurus paddock... the power has gone out and the kids start digging through stuff...Tim pops up wearing night vision goggles.
Gennaro - "Hey, where'd you find that?"
Timmy - "In a box under my seat."
Gennaro - "Are they heavy?"
Timmy - "Yeah"
Gennaro - "Then they're expensive. Put them back."