Not sure about an actual efficiency curve for PPD vs power consumption, but from what I remember I was able to undervolt my R9 Furys back when I did 24/7 folding on those without a really large PPD drop and dropped temps by quite a bit. I know that's the least quantitative answer I could have given though.Does under clocking and under volting make more efficient way of folding?
I am not sure if completed work units vs kilowatts hour used is better for the electric bill.
I know it is about getting the max output for some but A/C plus folding is a nogo for me
Also keep in mind there are some variables in there, as some projects/WUs don't push your hardware quite as much as others, so power consumption would be affected. At the end of the day though, I say if undervolting allows you to make the power bill hit acceptable, any folding contribution helps.