I would not look at power factor as an expression for efficiency. What is really happening here, is converting VA into Watts.
In a d.c. system real power is measure in watts and the concept of VA (volt-amps) does not exist. Batteries are d.c. and deliver watts.
In an a.c. system real power is measured in watts, but concept of VA now exists (due to reactive power). UPS are normally rated in VA (or kVA). To relate this VA to the battery capacity, it needs to be converted to watts.
If you had the rating of your UPS already in watts, then you would not need to apply the power factor.
We have note on Power Factor, which will explain this better.