Power supply - someone explain this to me please.

[KagutsuchiZA]

I was comparing to power supplies - the coolermaster eXtreme power 650W and the Antec Trio 650 W power supply:

ANTEC (650W)

+3.3V 24.0A ±3?
+5V 30.0A ±3?
+12V1 19.0A ±3?
+12V2 19.0A ±3?
+12V3 19.0A ±3?
-12V 0.8A ±6?
+5VSB 3A ±3?

COOLERMASTER (650W)

+3.3V 28.0A
+5V 30.0A
+12V1 18.0A
+12V2 18.0A
-5V 0.8A
-12V 0.8A
+5VSB 2A

First the Antec. It has 3 X 12V rails @ 19A each. That is about 624W (684W minus 3% for each rail). Now for the 3.3v and the 5V it gives a maximum of 70W and 145W (3% allready deducted). Add all those together and that is about 840W, not 650W. What am I missing here?

Doing the same thing with the coolermaster PSU adds to about 670W, which is about right. So where is the error with the antec PSU?

Maybe the 3 rails don't produce that much wattage when put together, but a reduced amount? (I.E 624W * 0.7) Can someone explain this to me?

 

[Asus]

Power supplies cannot use all of their power from all the rails at the same time. Some have a graph in the manual to show you just how much they are limited to on one rail when another is in use. And if a PSU splits it's +12v rail into more than one it can't use the total from each. A 30amp +12v is better than 2 +12v rails each at 17amps (I'm not sure exactly how much is not used from multiple +12v rails). Adding them up only gives theoretical amps but not the realistic number.

For desktop systems you are not going to have more than 1 CPU chip (not talking about cores) or tons of RAM so don't worry too much about the +5v or +3.3v. Also, ignore the -v values (-12v @ .8amps).

Also, link the products.

Here is an example with 4 +12v rails. Although if I could find a graph instead it would actually be more detailed.
like these.