Originally Posted by PaulTa
Here's a basic PSU rundown.
You have three different power rails. the 3.3V, the 5V, and the 12V. A good modern PSU will have most of it's power going to the 12V rail, although the 12V isn't the only place your PSU draws power. It's the place your graphics card and your CPU draw power though, which are typically the most power hungry components in your machine.
So let's say you have two 12V rails (labeled 12V1 and 12V2) and they are both rated at 18A. Well unfortunately you don't have 36A on the 12V, since you lose a little power overall from having multiple rails. Your total Amps on the 12V would be about 30-32ish.
Now a lot of people will recommend a single rail PSU, and for a good reason. Let's say that your graphics card needs 20A by itself to run at full load. Well each of your rails only provide a max of 18A, which isn't enough. Since your PSU will divide your CPU (typically here) on one rail and the GFX on another, the extra (let's say) 5A that your CPU isn't using wont go to your graphics card... you lose that extra power because that rail is dedicated to something else.
PCP&C PSUs are single rail monsters that are rated to run while being shot at by a tank. That's why you pay a pretty penny for them.
This is also invalid. The issue is not EFFICENCY
. It is about design. Most multiple rail PSUs utilize one transformer to produce the 12v voltage. To follow ATX specs of less than 250w per wire (which is email@example.comA), PSU manufactors put in current limiting circuitry. So a PSU transformer may put out 32A but that is too much to follow ATX specs. Therefore, they split the rail. Therefore, the have two circuits which draw from the 32A pool but each rail will shut the PSU down if they draw over the 18A by design. There are dual 12v rails at 18A and have a total of 36A.
Single rails PSU allows user to utilize more of their PSU power since users do not have to worry about load balancing components on different rails.