In article , Meat Plow
writes

And good luck trying to clean up the generator output if you intend on
using a UPS in the circuit. My generator is 2500/3250 surge like
yours. And it is brushless. But there was nothing I could do to filter
the output well enough where my two 1000va APC UPS units would come
back off battery.

There was a recent discussion in demon.tech.pc by someone with a very
similar problem. (If you're aware of w_tom, you'll not be surprised he
featured in that thread.) We managed to establish that some APC UPSes
are specified by APC as generator-compatible and also have a button on
the back which can be used to reduce the UPS's sensitivity to poor mains
voltage.

I told the OP in that thread that his problems may be due to the lack of
a ground reference, but not being au fait with the subject had to bow
out there. Bear in mind this was a 240V (UK) installation, so I wasn't
comfortable with discussing tying one side of the output to ground.

--
Mike Tomlinson