Ok let me start by saying this is simply an interest post..

I know that the Core 2 Duo's are the best there is in terms of performance points per watt but since most PC's spend most of their time at idle I would be interested to know how processor power consumption compares at idle..

For example the old P4 socket 478 processors are always slated as being power hungry but thats under full load not idle where they are most of the time..

So let me ask you this.. Assuming two identical PC's.. One with a socket 478 P4 and one with a Core 2 Duo.. How big a difference in power consumption would there be at idle?

could be as much as 30W+. most P4's didn't have Speedstep (except the PresHOTts), and it wasn't nearly as effective as it is in the C2D. the cheapest C2D will outperform any P4, and use much less power to boot. they're a miracle chip really. the current stepping (M0) uses only 5-8W at idle; previous steppings were not as efficient (B2/L2).

Don't know total answer - but the power supply article here on SPCR has some data that might help a little.
http://www.silentpcreview.com/article28-page4.html

Unfortunately no Core2Duo rigs there, but the P4 rigs listed run from
71-141w IDLE.
(Edit - fixed numbers, originally posted DC draw, not AC).

I know it is compare apples to oranges - but one measurement found
MacMini Core2Duo runs about 35w at load (Folding).
Specs put the older MacMini CoreDuo idling about 20w.
A typical Core2Duo probably lot more power than that.

One Core2Quad (2.4GHz) system is mentioned idling at 66w.
(Data from the PPDPW discussion on the SPCR folding forum.)


[Of course this is only part of the equation - have to also consider power/resources involved in making the thing, and be sure to
turn the whole thing off when not using it. (Use WOL, timed wakeup, X-10, ...)]