Not all installations need a UPS. But, if a power loss (even brief) would lead to lost operating time, lost data or unsafe conditions the return on investment in a UPS could be significantly positive. In those cases, how to make the best selection?
Standby is the least expensive class of battery backup. These units provide little in power conditioning, run connected equipment off the utility power until there's a blackout and sometimes have a square wave, rather than sine wave output. Many consumer grade UPSs are standby units. Pick this for budget constrained, non critical installations.
Line Interactive provides voltage regulation to boost sagging power and reduce reliance on batteries. A blackout causes the UPS to switch to battery. Although some of these UPSs offer surge protection and filtration, small power transients and fluctuations can get through. Pick this if electric power quality is generally good and a blackout is the primary concern.
On Line is the premium UPS architecture. These units power connected equipment from the battery 100% of the time, providing complete isolation of line and neutral between the electric utility and your equipment. Pick this if electric power quality is poor or if other circumstances call for the best possible solution.
Most UPS models include a number that refers to the volt-amp ("VA") rating of the unit (e.g. KIN-1000). VA is the product of the operating voltage and the amperage (current) rating of the UPS. Wattage is the amount of real power output available from a UPS. A UPS wattage rating will be lower than the VA (60% to 90%). To right size a UPS, look at its wattage rating and compare that to the maximum total wattage of the connected equipment.
What would you want to be able to do after the lights go out? How long would you need to do it? Run time is a function of the size of the power load and size of the batteries. Carefully consider how much run time you really need.
Most UPSs are designed to run for only a few minutes under full load, but are still very good for bridging short, transient blackouts. Run time can be extended by getting a larger UPS or purchasing an external battery module. Compare the cost of those two options. (Note: not all UPS models allow additional battery connection.)
You need not spend a lot to get a great deal. By the same token, inexpensive, consumer grade models may not last long or have quality power output - and end up being a waste of the purchase price. Higher quality models will have a sine wave output. Avoid less expensive square wave models.
Once you know what you need, shop around. There are big differences in prices between UPS sources in the commercial AV market.
To get the best product at the best price, do your homework.