hmmm.. I don't know if "RMS" (Root mean square of signal) is the proper term for this here - wrt to the peak wattage and the sustainable wattage. Even tho, I've been guilty of using it now and then
. For a constant amplitude sine wave, the RMS value and peak value are governed by a constant relationship as we all know. So in that context, knowing one implies the other.
In this case, saying RMS=450 W and saying the peak voltage is 550 W is not really accurate as both WOULD relay the same information if
it was the case IMO. I think what it really means is that the PSU can maintain a sustainable power output of 450 W for all time (either RMS or peak - whatever the components are measured at also, i.e. RMS or peak). Whereas, the 550 W is not really related to 450 W, but is simply the "peak" output that can be provided in an "emergency" so to speak.
While the power supply is 450 W, some people "can" have components that can go over this. e.g. 510 W. How often will a person be playing Doom 3, doing signal processing, writing to all the hard disks, running some crap with all the USB devices, AND reading some junk from the floppy disk, AND the CD Rom, while surfing the net :s.. Chances are not often. So the 510 W will not be used often. So 450 W supply would be sufficient. In the event all these events happen at once for a "little" while, the 550 W max PSU can handle it.
That's what I think.
Correction are welcome