Hi Alex,
I am an electrical engineer, I happen to own a Honda generator with the
ECO mode switch, but I don't run off-grid or need to charge batteries with it.
I have pondered your question.
Whenever I use the generator I put it in ECO mode.
As you are aware, it adjusts the speed of the motor to produce the electrical output required.
The whole idea of ECO mode is to save fuel.
See
https://powerequipment.honda.com/gen...s/eco-throttle
The other day they replaced the power lines in the street and I had it on
all day powering the refrigerator. If I plugged in, say, the coffee pot and
drew additional load, you would hear it rev up, which you would be
intimately familiar with.
Unless there is some other specification that puts a lower limit on
the number of amps you can draw in ECO mode, then my first thought
is that whatever the load sis drawing, if that is within the rated output
spec of ECO mode, it should simply rev-up and match that demand.
I guess ECO mode is well suited when the load is lower than the maximum
supported load or when the load keeps changing. For example
when you go to make toast.
If the current draw of the load was close to the maximum rated output
and was that way essentially for hours, obviously ECO mode would become
moot.
If you learn anything more, I would be interested to know. Otherwise
I would put it in ECO mode unless it sounds like it is struggling.