The UPS does not have to be located inside the enclosure in order to feed UPS protected AC into the enclosure. But it is more complicated to house the UPS elsewhere and run the correct AC wiring from UPS into the cabinet. Possible though.

Often the question of how long is long enough, in terms of the UPS being able to keep the load powered up without AC. I now configure my UPS size to run the telecom gear for several hours after the AC feed fails. It is so much easier to cope with an extended power outage when the WiFi and home telephone continues to work.

Yes, we have cellular voice and data, but there is some hassle to get the computers, iPads, etc all switched to smartphone tethering. And then switch them all back to WiFi after power is restored.

I am about to install one of these along with an external battery box. It should run my entire computer desk (four big displays) for multiple hours, and be loafing.

Will be modifying the cooling fans to minimize fan noise, then putting it into service.