My whole house has been backed up by an inverter for a few months now. My computer is still connected to a UPS so that it gets clean power - I don't trust the inverter alone to provide ultra-stable voltage.

Now whenever the computer is turned on during a power outage (and when the inverter is running), the UPS starts to "flicker" or click like mad. By flickering, what I mean is that it appears to switch several times a second between battery source and AC, making a clicking sound, anywhere between 1 to 4-5Hz, sometimes more, as if the relay is constantly switching several times a second.

Now the inverter handles all the load I try to throw at it otherwise. Having every light and fan on in the house doesn't challenge the inverter. So it's not an issue of the inverter's power capacity.

But when the UPS turns on, and it starts flickering/clicking, it also makes all the lights flicker. So I guessed (this was a pure guess, not an educated one) that perhaps this flicker was the result of a frequency mismatch between AC (as provided by the inverter) and the UPS. Perhaps I was seeing the 'beat' frequency of these two signals.

To test this - I mean if the devices were that fickle - I figured changing the impedance load of the overall circuit might affect the frequency relationship. And it did!

Turning on just the right number of lights/fans would eliminate (what I believe to be) the frequency mismatch and the UPS would keep working fine.

However this solution has proven to be fickle as well - perhaps the inverter is too wild in its frequency deviations and the impedance correction is not enough sometimes.

So here is the final question.

Is there anything I can tweak in the UPS in order to better handle this frequency mismatch?


  1. Am I right in the first place? Is this a frequency problem? If not, please explain. I do have a bit of theoretical knowledge of how electrical circuits/transformers etc should work, so please don't hold back in your analysis.

  2. Even if the inverter with its (alleged) frequency fluctuations is the root of the problem, replacing it is not an option (cost). Hence the question.

  3. Once I understand the problem theoretically, I should be able to bully an electrician into getting the necessary done to the UPS.

  4. I'm not a hardware hacker, so unfortunately I don't have a multimeter at home or see myself getting one in the next few days. I might if absolutely needed. So, sorry, I can't help with actual (experimentally measured) facts and figures on the frequency, voltage etc in the short term.

  5. Both the UPS and inverter are supposed to work at 220V/50Hz (India). If the amperage of both matters I can find out and post back. Note: As I have mentioned I don't think it's a case of the UPS somehow overloading the inverter, as I have thrown all other appliances (About 5 fluorescent lights, 4 ceiling fans, 4-5 old-fashioned 60W-ish bulbs) on at the same time, and it has stood quite well.

  6. The UPS brand is not an issue since I've tried multiple (reputed ones such as APC, local brands, etc) and they've all had issues in one form or another. So simply replacing the UPS won't suffice.

  7. Considering (1) and (4) and the absence of facts/figures is there another SE that's a better fit for this? I do need a practical solution eventually though (even if it means a delay if I HAVE to measure the frequency/voltages to be sure), so this still feels like the right place. But I'll leave it to the mods.

  • 1
    Via software, most UPS's can have their tolerances configured. You might try configuring it to be a bit less aggressive (larger tolerances) to see if that resolves it
    – Steven
    Jul 4, 2014 at 19:27
  • Unfortunately the one I do currently have does not have software control. Besides, are you talking about voltage tolerance or is there usually a setting for frequency tolerance also? I could perhaps look for one that has these controls if it would be a precise fit. Jul 7, 2014 at 19:57
  • @DevKanchen what solved your issue ? can you share ? thanks
    – user75174
    Sep 7, 2017 at 17:58

4 Answers 4


My APC UPS has a setting to be more tolerant of the input power. I found it necessary to enable this when running from a generator during power outages. Without the setting, the behavior was just as you describe -- constant switching between line and UPS power. Read the documentation for your UPS carefully -- there is probably a similar option.

Here's an excerpt from the APC BE450G UPS manual:

Voltage Sensitivity Adjustment (optional) The Back-UPS detects and reacts to line voltage distortions by transferring to battery backup power to protect connected equipment. In situations where either the Back-UPS or the connected equipment is too sensitive for the input voltage level it is necessary to adjust the transfer voltage.
1. Connect the Back-UPS to a wall outlet. The Back-UPS will be in Standby mode, no indicators will be illuminated.
2. Press and hold the ON/OFF button for 10 seconds. The OnLine LED will illuminate alternately green-amber-red, to indicate that the Back-UPS is in Program mode.
3. The Power On/Replace Battery LED will flash either green, amber, or red to indicate the current sensitivity level. Refer to the table for an explanation of the transfer voltage sensitivity levels.
4. To select LOW sensitivity, press and hold the ON/OFF button until the LED flashes green.
5. To select MEDIUM sensitivity, press and hold the ON/OFF button until the LED flashes red.
6. To select HIGH sensitivity, press and hold the ON/OFF button until the LED flashes amber.
7. To exit Program mode wait five seconds and all LED indicators will extinguish. Program mode is no longer active.

The details for other models probably will differ.

  • The fact that you had the same symptom is a good start. Thanks! But exactly what settings did you have to tweak? Is it just voltage tolerance or is there some kind of frequency tolerance setting as well? Jul 7, 2014 at 20:23
  • The setting is labelled "voltage sensitivity", but the setting for the low sensitivity setting includes this: "Use this setting with equipment that is less sensitive to fluctuations in voltage or waveform distortions."
    – TomG
    Jul 8, 2014 at 1:11
  • Wow, thanks for the details. It looks like I'd have to switch the UPS since it doesn't offer any programmability like this as far as I can see. I can't imagine being able to find better models too soon though (I'm in India - most "unsexy" equipment like UPS is sold in very few varieties with hardly any customization available. But I'll look.) Thanks for the help! Jul 15, 2014 at 16:52

What type of inverter is it? If it's a MSW (modified square wave) it's very unlikely that any UPS is going to be happy eating what it provides, since UPSes (even ones that generate MSW themselves) expect a real sine wave (or True Sine Wave - TSW) on the input side.

Square, modified, and sine waves

On the other and more relevant hand, you really don't need any sort of purity to feed most modern computers, many of which (check yours) are happy to take 100-250V at 50-60 Hz, (and don't care about TSW or MSW) since their own power supply is just converting that to DC with a switching converter. Only older linear-type power supplies are all that fussy about voltage and frequency. So viewing the inverter as a huge UPS is probably a valid approach, and removes a complication. I would suggest a good surge supressor power strip for the computer.

If you really need (for some reason) the cost (initally and in replacing overpriced batteries) and hassle of a separate UPS, or equivalent, consider using an adequately sized "high quality inverter" that runs on batteries all the time, and a battery charger that can handle whatever your whole-house inverter throws at it to maintain charge on that battery all the time - essentially your own "dual-conversion" UPS (most actual Dual-conversion UPSes would have the same complaints about a low-quality inverter input as a normal UPS does.)

The other obvious solution is to plug the UPS directly into the Mains supply, so that the UPS works off its batteries when the Mains go out, and the inverter works from the inverter's batteries (or whatever powers it) separately.

If it is a MSW inverter, the likely reason you can make the UPS happy with other loads is that the other loads (particularly ones with inductance and capacitance) will tend to take the "sharp edges" off the waveform and make it better approximate a sine-wave.

  • 1
    I agree. The problem is almost certainly that the inverter output has declined in quality (maybe due to a changed load). I'd look for an adjustment on it before starting fuss around with the UPS.
    – wallyk
    Jul 5, 2014 at 4:08
  • I did invest a decent amount in the power supply for the computer, so a surge suppressor might have worked. Unfortunately the invertor has a small delay (probably milliseconds) that is enough for the computer to switch off if unsupported by a UPS. This actually happened when I had a poorer quality UPS that was somehow confused by this delay, went off for a second and only then came back. This was of course too much for the computer to handle. Jul 7, 2014 at 20:27
  • Your comment on this being an MSW inverter possibly, makes sense. I haven't been able to check unfortunately, but I will get back. Connecting the UPS directly into the Mains is also not an option unfortunately as the inverter sits on some kind of bypass circuit from the mains (that's not EXACTLY what it is, I'm sorry I don't know the right terms here), so we can choose to skip the inverter COMPLETELY and supply EVERYTHING with mains, but it's either that or all the points run from the inverter. Jul 7, 2014 at 20:29
  • Sorry for the delay in checking. Unfortunately as I feared the inverter is a "locally sourced", no-name one. So I don't know if it is a Modified Square Wave or some other form. Like I said I might be able to take concrete steps in this direction only once I'm able to measure anything. Or I might replace the UPS if I can find a more tolerant one. Thanks immensely for the help, and the GREAT details. +2 if I could. Jul 15, 2014 at 16:55
  • "you really don't need any sort of purity to feed most modern computers" this is no longer strictly true. A lot of modern higher-end SMPS (e.g. mac charger) use hybrid buck-boost topology to condition the input load - it tries to make the device look as linear as possible, to minimize harmonics. These can get CRAZY hot with choppy inverter power, as all that extra harmonic energy in the square wave is dissipated by the conditioning circuit, filters, etc. Jun 18, 2020 at 14:39

The UPS type is an issue

Yes, I know you tried several brands. But you are buying at the same price-point, so you are getting similar products. The problem is there are several types of UPS's:

  • Standby and Line-interactive, which pass power straight through, and switch over during an outage, hence the "click". This does nothing to clean dirty power.
  • Online aka double-conversion, which is always on battery/inverter, and line power only keeps the battery topped up. This protects you absolutely from input power problems, but the output power quality totally depends on the quality of the inverter module in the UPS. This has no relay, so it never clicks.

You've tried several brands of standby or line-interactive UPS. Try an online.

You really want power conditioning

The purpose of a UPS is prevent power interruptions, not clean up power. Since you already have an inverter, power interruptions are not your problem. You would be better off dumping the UPS idea and focus on power conditioning for the PC.

Sometimes the real enemy is your other appliances. That's what Fractal was trying to tell you with his downvoted answer. I've seen an office stuffed with line conditioners, odd thing was, this office had its own pole transformer. The enemy wasn't "out there", it was his own fluoresecent lights!

Anyway, who's to say this PC power supply even has a problem with dirty power? Internally they just use high-frequency switching to chop up the power anyway, they don't care if the sine wave is perfect. However, they probably don't like spikes.

Or run the PC on DC

The new thing in data-centers is this: Step 1: They take 240V power, and convert to DC to feed their online UPS battery banks. OK. Step 2: they invert to 240VAC to feed servers. Step 3: the PC power supplies convert back to 5/12VDC to run the server. They are saying: Why do steps 2-3? Feed the servers direct off the battery bank.

If your whole-house inverter is an open design, where you have direct access to the DC battery bank, you may be able to obtain a power supply for your PC which does exactly that. This takes UPS's off the table entirely. And power conditioning on DC power is easy.


Try using dimmable type LED lights since they seem to handle the harmonics better. My problem was solved by using this type.

  • 1
    This doesn't answer the problem the OP is actually having May 13, 2016 at 11:44

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