I think one way to calculate idle time is to divide the TDP by half. I have a power measurement tool which confirms my PC at 95 watts under stress and 60-65 when idle, the extra 20 watts is probably due to the other components.
Not so, a 1996 game I have uses 100% on my Pentium D. I can always set the affinity to one core on task manager but what casual non-geek knows how to do that?The old games only use 100% of 1 core of a CPU. So on a 12 thread CPU this is 8% usage overall. Far from full usage.