Now on to the second variable in our efficiency calculation: power.
The differences you see each step of the way relate to the voltage needed to keep our Core i7-3960X stable. We were able to use Intel's default voltage setting at up to 4.5 GHz. But once we needed an offset, the idle power started creeping up. By the time all six cores were running at 4.7 GHz, we were seeing questionable performance gains, but bigger power use.
The scale doesn't look that much more alarming under load, but the numbers speak for themselves. Power consumption increases based on frequency, and then our voltage alterations exacerbate the end result. Once you're pushing 4.7 GHz in lightly-threaded apps (and 4.5 GHz in workloads that tax the whole CPU), system power exceeds 300 W. That's not even taking into account graphics power, since only our processor is subjected to a load.