Ask any electronic engineer what a watt is and the answer will start off as it’s voltage times current.  This is a simple calculation which will get you into the ball park.  For many things it is this simple but with computers we need to also consider “power factor correction”.  Without the specs from your power supply manufactuere there is no way to tell but a typical power factor is about .7 or 70%.  this means that if you are trying to supply power to a 700 watt computer you should have a UPS that supplies a minimum of 1000 watts. 

Next look at the battery time at 1000 watts.  Most home UPS systems will only last a few minutes at this consumption; for home users this should be enough time to shutdown the computer, not operate the computer for an extened period.

There is another rule of thumb that does work with UPS systems.  The bigger the better.