An uninterruptible power supply provides battery power when mains electricity fails. In a security system it keeps recorders, network switches and door controllers running through short outages and allows a clean shutdown during longer ones, and its runtime depends on load and battery condition.
A UPS is the difference between a gap in the recording and a gap in the investigation. It also works only while its batteries still hold charge, which is why testing matters.
Sizing for the load
Add the wattage of the recorder, switch, cameras and any monitor, then read the runtime curve at that load. Adding cameras later reduces runtime unless battery capacity grows with them.
Clean power and signaling
Some equipment requires a pure sine wave output, and a network card can tell the recorder to shut down gracefully. Place the unit where it is ventilated and out of casual reach.