depending on the type of the pcs you have, your ups should be between 600 and 1000 watts
your psu(on those desktops) could be bigger but rarely you use all the capacity of a psu so normally a pc uses from 170 to 350 watts(not counting monitor)
i have seen cases where a 500 watts ups can handle two crt monitors and two towers for up to 3 minutes
common brands of ups are apc, tripplite, and blazer, apc is the most popular for quality but blazer is very decent too, qpcom does some decent ups too
some models will come with 4 outlets or less, check tht before buying
some models will come with a usb or a serial port to connect them to the pc and have a app letting you verify information of the battery on the ups and also install a app to tell the system that the battery is in use(the the power outage occurs) and order the system to close apps and shut down
not sure how that works in linux
have in mind that a ups is basically power regulator, a spike suppressor and a multioutlet with something similar to a motorcycle battery that is used when the power from the electric grid is not present
the bigger the ups is, the more time you will get, as i said you could be ok with a 600 watt ups but if you can, get a 1000 watt unit, if the 600 if detects that the two pcs is excessive load for it, it will do a really annoying noise and the unit can get damaged so to be sure a 1000 watt unit
other specs as you see are similar between brands
after 2 or 3 years the battery will stop working as expected and instead of 5 minutes will give you 2 or less, the batteries on some models can be changed easily, others won't let you change the battery, afik, apc is the most user friendly in this aspect
