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The power rating of a UPS in watts is a function of its power factor.

Power Factor -  Power Factor is expressed as the difference between “apparent power” and “real power”.  A 1,000 VA UPS with a 0.9 power factor will be rated for a 900 watt load.  A 1,000 VA UPS with 0.7 power factor will be rated for a 700 watt load.  The amount of work that a UPS can perform is expressed in watts.  Therefore, a UPS with a higher power factor is capable of more work than a UPS with a lower one - given the same amount of voltage and amperage (VA).  This can be visualized as a glass of beer with foam on the top.  The full glass of beer + foam represents the volt-amps (VA), or apparent power. The beer itself represents the watts, the portion of the contents that can do work.   

Efficiency -  Efficiency is the difference between watts in and watts out.  For instance, a UPS that requires a 100 watt input to do 96 watts of work output would be 96% efficient.  The 4% inefficiency is energy lost primarily in the form of heat.  

When trying to figure out which UPS is the right size for a particular application look at the wattage specification. The UPS with the lowest price per watt is the best value.