This is rechargeable and would run your IMac for a few hours between charges without damage. Or you can just build your own like I did with a couple of deep cycle batteries and a DC/AC converter.
http://www.110220volts.com/XPOWER-1500.html
My word. $250 and it weighs over 50 pounds. Wouldn't it be easier to just get a macbook? I'm pretty sure the OP was hoping for something about the size of a book.