Back in June of 2014 I bought a CyberPower CP1500PFCLCD PFC Sinewave UPS System, 1500VA/1000W. It has worked flawlessly up until about late September of 2019 when it would not hold much of a charge. I ordered a battery for it from Amazon. I received it not long after. However, a lot was going on and it never got installed. It sat in my spare parts pile, semi forgotten, until last night.  My UPS started screeching an alarm. Apparently the UPS battery had finally had enough and wasn't going to put out any more power even though it was plugged in and energized from the wall outlet. It was late so I just unplugged it all and left it until this morning. Replacing the battery took maybe 15 minutes. Most of which involved reading the instructions and hunting up a screwdriver, that had wondered off from its normal spot in the workbench drawer.
 
So, if your UPS is running on a battery that is approaching 5 years old you really should get around to replacing it. Don't lose 12 hours of compute time on Boinc like I did.
 
 

Yes, they do screech at you when the battery has failed

Those cheap 1500VA/900W units barely can push 750W.
I had my 725W server shut down, because of a power outage of 10 seconds.
Nothing to do with battery charge. The unit still had all it's charge. It just couldn't pull the 750W.
 
The other unit ran at 680W, and was able to continue.
Last time I tested it, at 660W it almost made it to 1 minute!