Some places have a peak usage fee, so not only do you pay for the KW hours you use, but if you have too much on at once and the amp draw to the house goes over a limit you get hit by that as well.

Its been a while since I actualy bothered to read my bill. The heating bill in the winter has been more then the power bill in the summer even with the central air on all the time. I have run a few machines at once before, generally 2 were on (whatever I was working on and my server) and sometimes a few more.