I want to get an APC BackUPS Pro 500. It is rated for 300-315 watts. It will be powering a desktop PC with a 21" monitor. The computer will have an Enermax 350 watt power supply. Will the BackUPS 500 be enough? I think that it would work fine with more equipment connected to it than it is rated. It would just run for a shorter period right? Or do I need to get a higher rated UPS?