You can Google "power factor" or "volts-amperes" and get accurate information in excruciating details (which I recommend if you understand trigonometry and its prerequisite; college level algebra). I'll assume that is not what you are looking for.
With ideal electric power, the voltage and the current are in phase. In such a situation, multiplying the current times the voltage will give you the amount of true power which is stated in watts. Sometimes just the amount of current (in amps or amperes) is only stated and you can multiply the amps times your house voltage to get what is called apparent power (since you don't know how much your current and voltage is out of phase and probably not the voltage).
Your house voltage, depending on what your utility supplies you, is not a consistent figure. I've seen it as low as 105 volts (in Hawaii) and as high as 127 volts where I live now (Illinois). So that is another unknown unless you measure it with an accurate meter. But even then, the figure varies somewhat.
So multiplying the voltage times the amperes gives you apparent power and not necessarily true power. Since manufacturers like to rate their UPS VA ratings in big letters and wattage rating in fine print, plus the fact your voltage and phase angle is probably unknown, your best bet is to use the wattage rating of the UPS when sizing it up for your equipment to be protected by the UPS. The VA ratings are high and are generally based on a worse case scenario as far as phase differences are concerned and look better for advertising purposes..
Your best bet when sizing an UPS is to look at its lower wattage rating and the wattage rating of connected equipment or where just current is given, calculate the wattage with the simple formula. The UPS is built to accomodate any phase differences which is stated in the higher VA rating.
