underclocking is pointless if you want to save on electricity, as everyone above has mentioned, there's already stuff like intel's speedstep & c1e & amd's cool & quiet.
now, if you want to save electricity without compromising performance/overclocking, you can undervolt your processor. i have my e8500 slightly overclocked to 3.4ghz, but undervolted to 1.088v. runs fully stable 24/7, my load temps never go past 45 degrees

thats lower than my stock temps, & imo low temps = lower power consumption heh