I have 4 or 5 computers running in my house at any given time, and 2 air conditioners that run now and then. I'm very surprised, but my last electric bill was around $90. right now, about 300 watts of cfl power being used in the grow room. half on 12/12 and the other half on 18/6. I haven't noticed any change at all in the electric bill. I don't expect much change when I get a 600 watt hps either.