Hi!

Have you looked into replacing the UPS battery with a larger capacity
one?

This does not always work, especially if the UPS manufacturer never intended
for larger batteries to be used. The charging system may behave improperly,
especially with a cheap UPS.

This is the preferred (and more elegant) way to get longer power-up time.

One also has to consider the health and well being of the inverter. It may
not be able to take the added runtime if things are already close to the
limits. Again, I'd see this as more of a problem with a cheap UPS than a
good or even moderate quality unit.

William