Doesn't me Faticus I had a PSU in my tester that showed volts but come to the crunch didn't have the amps mate. Plus I have had a few fixes that just didn't like sticks that normally would have been throttled back to the motherboard support rating..
By the way when you look at PSU's look at the rail currents (amps)
it is fairly pointless as Noel said looking at the claimed wattage it is usually the total of all rails.
Using Ohms law you can very quickly find the current rating of the different rails and today the 12v rails are all important.
Not to say the other rails are not but say for example the 12v rail was showing 25amps using Ohms law that runs out at 300 watts. For an exercise if you want look at your PSU (there will be a label on the side) find the 12v+ rail amps multiply the amps by the 12v and you get the actual wattage, therefore in my example 12 x 25 = 300!!
(same applies to the other volt rails) .