We have a pair of UPSs here; one in the office and another in the house. In both cases, they run the network core; Ethernet switches, Wi-Fi access points, IoT hubs and the like. Our reliance on power-over-ethernet means that there are actually quite a lot of gear that’s on the UPSs.
For many years the UPS in my office was made by Belkin. It was cheap. It did the job, sustaining the network core through many a minor outage. Being fanless, it was silent…which I deeply admire.However, as with most low-cost UPSs, it was “line-interactive” design. In such a design the power provided by the batteries and invertor is connected in parallel to the utility power. When utility power falters the local circuitry tries to make up the slack. The design is simple. Any UPS under $500 new is almost certainly a line-interactive design.
More sensitive gear is better served using a more sophisticated design known as “online” or “dual-conversion.” An online UPS puts its active circuitry between the utility power and the load. The utility power is turned into DC, which feeds the batteries and the invertor, which makes brand new, pristine, stable AC power for the load.