Its not technically free for me, but I own a small business next to my full time job and we split utilities down the middle. Well, I use a laptop and 3 lights and my bill is around $600 USD every month; so I put my mining rig at my shop. If Im paying half of what their heavy, 3 phase, bindery equipment generates, they can pay half of my $50 rig 
I do something similar. My rigs are running in a business of mine, which has cheaper power than my house ($0.08) plus I don't have to deal with the heat and the noise and the business just absorbs the cost, at least for now. So I don't personally pay for it and its an extra tax write off for the business. Its a few hundred dollars a month now but if I expand it, I will end up covering part of it.