My computer suddenly shutoff (abruptly), and then the APC (UPS SE 350) started to beep continuously.

I pressed the button on it and it stopped, then I pressed it again and it turned on (green light) and now my computer is working fine again.

What could be the reason for that?

4 Answers 4


With the APC ES 350 (there's no SE model) there's two audible alarms:

A continuous low beep: The UPS is now operating in battery mode. This happens if you experience a power outage and the UPS switches to the internal battery. Depending if you are operating at 100 or 200 Watts, you have between 1 and 6 minutes of battery time.

A continuous sound: (and not repetitive beeps) informs you of a power overload. You got a power spike and the UPS switched on to battery mode. The UPS will not resume from this mode until you turn it off and turn it back on. If the spike persists it will again repeat the tone.

The fact your computer turned off immediately means your battery was discharged when this happened. This model takes around 16 hours to recharge its internal battery. If it is continuously connected to a power outlet even when the computer is turned off, this may mean your UPS internal battery is not in good condition.

To test your battery condition:

  • Boot your Windows 7 up to the logon screen but don't log on to avoid file corruption.
  • Remove the UPS power cable from the wall socket.
  • The UPS should start a low beeping sound and your computer should stay on for 1 minute (if your UPS is operating at 100 watts), or 6 minutes (if at 200 watts).

For more information: APC UPS ES 350 Details
Also check your UPS manual for even more detailed information.


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-

  1. Everything works depending on the sensitivity of your components
  2. Slow damage to your PSU but it stays on
  3. High pitched eeeeee noise and slow damage to your PSU, but it stays on

Too risky in my opinion. Three solutions

  1. Replace your PSU with a less expensive unit having passive or no PFC. Not always an option for an OEM build.
  2. Buy a UPS with pure sine wave output. $400+
  3. 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.

Further resources:


There's two basic reasons for a UPS to fail to keep a PC running through a short power outage...

  1. Too much load - Remove your monitor and other non-critical items from the "battery backup" ports and compare your PC's power supply wattage against the UPS output. For a consumer UPS the model number (350 in your case) often indicates the absolute maximum wattage it can supply. For best results it should be higher than the wattage of your PC power supply.

  2. Bad battery - UPS batteries last about 18-24 months. If your UPS is older then it likely needs a new battery. A replacement battery will cost roughly 40% the cost of a new UPS. Batteries Plus is a good place to find a replacement.

** Numbers above are estimates from my experience and should not be taken literally.

  • You're welcome. I've used the above company before. They will even recycle your old battery (for a small fee IIRC) so bring it along when you go.
    – Chris Nava
    Commented Nov 24, 2009 at 16:11

My iMac was crashing several times a day for a week or two. On Sunday my APC Back-UPS XS 1200 started giving me a low-battery warning. Early Monday morning I replaced the 2 batteries, and I've experienced no crashes since.

  • 1
    Each answer should contribute something new and substantive. Your answer sugests that the problem for you was bad batteries. That was already discussed in Chris Nava's answer. Super User's purpose is to build a knowledgebase. For this model, duplicate answers aren't really useful. If your purpose is to provide confirmation that the other answer was right, the way to do that is to upvote that answer when you've gained a little more rep.
    – fixer1234
    Commented Apr 29, 2015 at 15:54

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