I do not know whether there is an "industry standard" for rating UPSs.
It may be that every manufacturer has their own unique way of rating
that cannot be compared directly with others. Sort of like antenna
manufacturers' gain figures. In any case, what I have seen in some UPS
ratings is that the "Power Rating" is the maximum power the inverter can
deliver, and the "Time rating" is how long it can provide 50% of the
"power rating" when the AC mains power drops out, assuming that the
batteries are in real good condition and fully charged when the AC mains
drop.
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