More directly answered, most CO2 lasers are rated in optical output power. There are a lot of inefficiencies before that, so e.g. the 85% efficient power supply, the (wild-ass-guess) 40% efficient laser, then you have other electrical loads like probably at least a dozen watts of fans, substantial power for motors, lights, controllers, air pumps if you have one...

I would expect a "120w" CO2 laser to have peak power consumption of at least 500w. You can use a kill-a-watt to check out the actual consumption while cutting, frankly I'd probably go with a 750-1200 kVA ups just for substantial fudge-factor.

Curious if you've already bought the UPS and if so how it's gone?