The reason I ask, is I did not understand how they had these setups and were not blowing breakers, but they did not go into detail of the power needs.
You install an outlet like this in your garage.
(or at least I am this weekend)
It is a locking L6-30R outlet (receptacle). It allows you to pull 30A @ 240V w/ a plug like this:
30A @ 240V * 0.80 = 5760W.
Also rigs don't pull as much as you think if you design them right, use high efficiency PSU, and run linux on USB stick.
My 3x 5970 rigs pull 870W at the wall.
5760W would be good for 5 4x5970 rigs or 40 GPUs.