[klmx30302]

um... didn't read the part about 7 laptops. So lets see 1100wh/12v= 91ish amp hrs so that's 2 normal car batteries now take into account 80% inverter efficiency 91/.8= 113.5ah now say the laptop power bricks efficiency is around 80% 113.5/.8=142.5ah that means you would probably need 3 fully charged car batteries hooked up in parallel to get the appropriate amount of power which at this point would be 142.5x12=1700wh.
Unless of course you already took this into account in which case you should be fine with 2 car batteries.