I recently tried disabling a CPU in a multi CPU system to save power. The interesting find, it actually increases power draw on the system!
Although this is far from being a scientific test, I saw on an idling system with two Xeon 5140 dual-core CPUs a 216-219W power draw. When I disabled core 1 and 2 (on CPU #2) by doing:
echo 1 > /sys/devices/system/cpu/cpu2/online
echo 1 > /sys/devices/system/cpu/cpu3/online
the power draw increased to a steady 225 Watts.
Why?? Well, my hunch is that an idling CPU saves far more power than a CPU operating at let's say 20%. Or the other way round: that power draw grow exponential. So having one CPU doing 20% eats more than two CPUs doing 10%. Thus I switched the CPU No. 2 on again...