Electrical – How to fix flickering and continuous-clicking with a UPS on an inverter-supplied circuit

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?

P.S:

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.