I'm interested in this because I have two dedicated rigs (and the hard drives get quite hot which could mean power consumption), but I have to ask two things:
1.) How can you be sure that unplugging three hard drives alone dropped your bill fifteen dollars? That sounds like quite an overstatement considering they don't draw that much power
2.) Did you have windows set to power them down after 5 minutes? I have that set on mine, and I can't actually tell if it's doing anything because with all the gpu fans at 100% and the box fan running full blast I can't hear it or feel it over the noise and vibrations