There was an article in High Times a few months back, i think it was called "Diesel Dope" and they mentioned the number of gallons used to produce one pound of herb. I forget the exact number, but i think a factor to consider is that if you start down this road, you may be getting ready to burn thousands of gallons of diesel over the course of the grow, which means you will be either paying out the ass, or spending many many hours gathering used veggie oil, and either filtering it, replacing filters often, or convert it to diesel, forcing you to buy expensive, large equipment, as well as regularly buying processing chemicals.
If you have the money, check into wind generators or solar panels.