True, with nvidia you can't undervolt. Only way to save power is to lower TDP which actually works quite well on ethereum mining. My 5x970 can do 83MH@600w from the wall.
Excuse the ignorance. I have some 970's
What is TDP and how do I lower it ?
TDP stands for thermal design power (correct me if wrong). If you are on windows install MSI afterburner, there you have power limit slider.
I have never tried to save power with nvidia on linux so I don't know about that.