For a cheap solution, we use a Marine Battery. Holds a long charge, is 'relatively' portable, and doesn't take up too much room. I'll add a link to an example if I can get one. We also have a charging station in our pit structure as well.
EDIT: We just got a new one, but here is what our older Marine Battery looked like:
http://www.autozone.com/autozone/acc...ier=95824_0_0_. We can run about 10 laptops off of it.