Intel processors have had a feature called SpeedStep for quite some time. It dynamically down-clocks the CPU(s) during idle and lighter loads to conserve energy when maximum performance is not needed.
I wouldn't be too worried about the average power load of a Mac Pro. Unless you're doing something very processor intensive (like rendering video), you won't see anything close to its 1,000W maximum draw. During average use, it'll probably hover between 150-300W, depending on what video card you have, number of hard drives installed, etc. That's comparable to a medium-sized flat panel TV.