It is well known many models of UPS units do not like the power from motor power portable generators. Indeed it is a difficult task to keep the sine wave pristine when you don’t have the benefit of hundreds of square miles of the power network feeding you an almost infinite about of energy. Here is an example of the waveform from a truck mounted generator delivering much less power than its rated output… about 100 watts from a 5,000 watt generator.
Painful to look at eh? Amazingly the UPS this fed actually worked off this trashy waveform. When we pulled a bit more power from the generator using some simple loads like light bulbs, the waveform smoothed out and became like a sine wave. However, the UPS did not like this visibly better power and went straight to battery.
If there is a moral to the story it is UPS units are finicky about the input power they receive in different ways. Some fault on low voltage while some also fault on too high voltage. Some analyze the sine wave and some don’t. Some require 60 Hz precisely while others can handle + or – 3 Hz or so. Experimentation is about the only way you are going to gain confidence in your generator power configuration.
One style of UPS that seems to consistently win (based on reports from the Internet) are the two-stage types which always generate their power from a dc to ac converter and then just electrically OR the battery and rectified ac mains together to provide a seamless transfer.
Running a UPS off a generator is possible, but allow some time and money to get it right.