In addition to overload and low/bad battery situations, the UPS could be failing to support your computer for another reason even if the UPS is fully charged and perfectly operational. It has to do with the power output of the UPS and the input expectations of your computer's power supply unit (PSU).
Consumer oriented UPS units (everything under $400), typically output a stepped or squared sine wave output in contrast to the actual sine wave of AC. It's just cheaper to approximate a sine wave than regulate a smooth one. Many PSUs these days feature active power factor correction (active PFC). You'll certainly find them from OEMs and almost can't avoid buying one if you are a system builder yourself. They are great units. However, because of their ability to correct for power factor discrepancies, they are more sensitive to the input.
Picture a sine wave undulating up through positive and dipping again to its negative amplitude. It crosses through zero for a millisecond. A stepped or squared sine wave approximation has no smooth wave form; picture a square block in the positive, another in the negative. The problem is this wave approximation stays at zero for far longer and the active PFC PSU sees this as a zero power situation. In effect, the PSU is getting no power and it dies immediately. It is the exact situation your UPS is meant to prevent.
Okay, enough description. This is how it plays out if you have an active PFC PSU-
- Everything works depending on the sensitivity of your components
- Slow damage to your PSU but it stays on
- High pitched eeeeee noise and slow damage to your PSU, but it stays on
Too risky in my opinion. Three solutions
- Replace your PSU with a less expensive unit having passive or no PFC. Not always an option for an OEM build.
- Buy a UPS with pure sine wave output. $400+
- Buy a UPS with trapezoidal or triangular sine wave approximation output!
Option 3 is preferred, but there is a caveat here as well. There is only one company attending to this lagging gap in marketplace PSUs and UPSs. CyberPower Systems makes a CPxxxxPFCLCD product line. I have no affiliation with them. Their CPxxxAVRLCD product line has gotten many glowing reviews and positive ratings. The PFC products appear to be of the same high quality. Thankfully, the one available product in this case happens to be the best.
Update: I purchased the CyberPower CP1500PFCLCD 6 months ago, used it every day since, and I'd buy it again without hesitation. I'm really not trying to sound like a sales hawk; it's just I spent a lot of time researching this topic.