Providing your Mac with a reliable filtered AC supply is very important to protect it from sustaining damage during electrical storms, when there’s utility maintenance underway, or some idiot trips a circuit-breaker or pulls the mains plug.
Most Macs are well-protected if the UPS keeps them going long enough to allow an orderly shutdown – a minute or two at most. It’s far better for a Mac to be given that chance than to have no UPS at all.
When that’s connected to your Mac, the Energy Saver pane should recognise it and offer both display options and Shutdown Options, which determine how quickly your Mac shuts down when mains power is lost and it’s running off the battery in the UPS.
There’s an important fact which can sometimes be forgotten: the USB interface on a UPS can only be connected to one Mac. If you’ve got two Macs to protect, they each need their own UPS, as one UPS can’t tell two Macs to shut down, except by SNMP, which is considerably more complex to use.
I used APC’s Back-UPS LS for many years with success, but eventually the units (not just the batteries, which I’d replaced many times) failed, and APC discontinued the design that I liked. Lately, I’ve been using a Tripp Lite, which is great except that starting with Catalina the Mac always thinks it’s out of power and needs to shut down. Now the USB connection is useless, and it’s no longer a good solution for a Mac that will be running unattended.
Some other things to consider when getting a UPS: