The size and type of battery backup you need depends upon what you will be running off of it. I almost always recommend that you have a separate battery backup for each
computer, as if one for some reason trips off you don't want two or three people all losing their
computer at the same time!
UPS systems are one area where I really believe that brand can make a difference in quality and therefore a difference in longevity and reliability. I use CyberPower and APC brands as many other brands I have tried to use (such as Tripplite) have failed quickly. CyberPower UPS systems are usually lower cost than the APC systems.
One common misconception when purchasing a UPS is that many are branded or have their model name based on the Volt Amps, not the supported total Wattage which is normally a little less than 2/3rds of the Volt Amps. Try and look in the specifications for the Watts as this you can also compare more easily with the
computer you are going to be running on it.
What I mean is this: If you have a designer with a 24" monitor and printer and a design workstation with a 500 Watt power supply in the
computer, and you put in a UPS that only has a max of 400 Watts, it will not supply enough power to run everything in the event that the system must switch over to battery. In that case, you won't just be having a short time on battery but most likely the entire system will just shut off even though the battery is technically fully charged. Try to do a little estimation and calculation before you spend a lot of money on something that won't even work in the end for you! I usually try to figure at least 100 Watts more than what I actually need for the available power through a UPS as you just never know if you need to expand, add more to it, or if it may slowly lose some power output with age.