When we need extra power we just take robot (or car) batteries and attach a power inverter. Yes is is rather inefficient but it is also simple. And you could stretch the power by using power saving settings on the laptop.
One of the reasons that the DC-AC-DC system is used is that the inverter and the rectifier-power-bricks for the laptops or whatever supply the system with a good level of regulation and signal-grooming. Take that away, and you may just fry a laptop or two. Be warned.
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.
My freshman year (Lunacy) we tried our UPS with our 7 laptops and a switch. The inverter couldn’t handle any more than 3 laptops at a time, and the batteries instantly drained. It did NOT work for us at all. We evolved to essentially creating our own scantron. The best mix between paper and technology.