First, there are several types, but I’ll point out that they fall into those that produce a “pure sine wave”, and those that produce a “stepped approximation of/to a sine wave.”
You pay more for the former, and it’s easier on your equipment. In fact, some of the newer power supplies (APFC) in computers will not work with the “stepped approximation” type.
If you are using it with high-end audio equipment, I strongly recommend the pure sine wave” UPS. (CyberPower makes fine units at reasonable prices.)
That said, here’s something I’ve apparently learned over the past few days. (“Apparently,” because it’s anecdotal…)
It appears to me that UPS’s which have some way of indicating the state of the battery charge, may not truly be indicating it.
One of my APC units showed a full charge, which should run my equipment for 20+ minutes. However, a practical test showed that it only ran for 4 minutes. When I completely discharged the batteries (using an incandescent bulb, which I didn’t own, and had to run out and buy one) and then recharged the unit, it (still showed) a full charge, but this time, when I tested it, said it would run for 17 minutes.
Next, I got my new UPS, which, upon removing it from the box, showed the batteries at 100% charge… and the printed caution to charge the batteries for 8 hours before using (which I’m doing now.)
Now I can’t verify that the batteries in the new unit were not fully charged, but that would be most unusual for a new, unopened device.
So I’m left with the anecdotal “evidence” that it appears that a LCD unit can provide false information about the true state of the batteries, just as older battery testers did, when they failed to run the test under some built-in load.
Why say this? Because if your UPS isn’t providing the run time you think it should, let me suggest that my experience says:
Run down the unit using a 100 watt light-bulb*, and recharge it for 8 hours (ie overnight) and see if things have not improved. I’m not swearing they will, since I don’t know the state of your unit, or its batteries, but it’s worth a go.
*Why not use something else? Because the bulb will drain the batteries to a lower level that something with higher wattage (before the self-protection on the unit cuts off all power.) For reference, my 100 watt bulb actually drew 85 watts, and took 74 minutes to drain my 1300VA unit.