The best computational power vs power consumption (for the CPU) is at about 1.8Ghz but 2Ghz is much better number
![Smile :)](./images/smilies/icon_smile.gif)
The problem is not the CPU it only uses less than 4W at idle the problem is the motherboard and chipsets. The chipsets are always two or more generation behind the CPU they use old manufacturing technology for the chipset.
The most energy efficient CPUs if I exclude the manufacturing process is the one with the higher number of cores without HT. Hyper Threading is not efficient.
I'm also interested in low power topics. I will probably soon move offgrid and is easier to reduce the power consumption than to produce. Now I'm working on an electric mini tractor with front end loader and a wind turbine.