Given the same vendor claims, how can I tell a "good" quality UPS from a "poor" quality UPS? E-mail
Testing, testing, testing. I can't emphasize this enough. There are many good and bad units out there that call themselves UPS's. There are many good units that are wrong for your situation. Caveat Emptor. 

Some properties you might look for are: 

Sinusoidal power output. In general, the closer the AC output of the UPS is to a sine wave, the better it is for your equipment. Many UPS units, especially the cheaper ones, deviate a great deal from a sinusoidal output. Some of them generate square waves. Waveform effects are dealt with in one of the questions below. 
Does the UPS have a manual bypass switch? If the UPS is broken or is being serviced, can you pass power through it to your equipment? The last thing you want is for a broken UPS to be the cause of extra downtime. 
The more information about a UPS's operation you can get from watching the unit itself, the better. How much power (or percentage load) the equipment is drawing, how much battery life is left and indications of the input power quality are all very useful
 
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