Can anyone explain this electrical/generator mystery for me?

Your terminology confuses me. An inverter, as I understand the term simply changes DC to AC so that the appliances you have that run on AC power can still be powered… by a DC battery, which in this case happened to be your UPS (which is essentially a large battery). So, given this understanding (and if your inverter was what I think it was), then it is quite understandable and necessary for your UPS to have been discharging, since it was providing the power you needed.

I don’t know what generator you had used in the past, but a fuel-powered mechanical / electrical generator generally runs a motor to produce AC power (or DC, if it also includes its own inverter or you plug one in). If you can specify any models that you’ve used, perhaps someone who knows more about it than I do can answer more fully. Electricity is not my forte!

@CWOTUS the hire company called it an inverter generator, I can’t tell you any more about it, sorry. I imagined the power coming from the electricity generator would run through the UPS, as mains power does normally, so the UPS would not need to use up its own charge.

Well, okay. I just learned a little bit. An inverter generator does produce power. In fact, it produces a “cleaner” AC power than the generators that most of us are probably used to. And you needed “clean” AC power for the devices you use, I expect.

So the only thing I can guess is that your generator wasn’t producing quite as much power as you needed, and the UPS was having to make up the difference. I’d still like to hear from others who know more about this than I do. I’m guessing.

It was a 240 V battery, exactly the same as mains power in NZ, and it was not running anything more than I would have plugged in to a multi-plug board normally, there was nothing different about the arrangement to the way things are usually, that’s partly why it is all a mystery to me.

I think the problem here is assuming that things were performing at their rated capacities.

I would hazard a guess that the UPS sensed an under-voltage condition (maybe it only saw 215V) and acted accordingly. Not all inverters are created equally, not all of them are in perfect working order, and UPSs tend to be quite sensitive to what they are fed, so it’s possible that there was a difference between the two generators, even if the only difference was condition.

Trust me, I’ve seen many things where just a tiny, non-obvious difference yielded vastly different results. You’d probably need to take a multimeter (or maybe even a oscilloscope) to see what the difference between the two generators was, but suffice it to say that generator wasn’t doing it’s job.

As an aside, I have seen storage banks with batteries also called “inverter generators”, so which are you referring to; was the DC-to-AC inverter fed by an engine-driven DC alternator, or a battery? Both can go awry in different ways.

@jerv your theory sounds completely plausible, and I can understand it what’s more. In answer to your question, the generator was petrol-driven. And who’s to know how well maintained hire company equipment is…