lehtv's response is generally correct, though PSUs do use power in a way, hence the power efficiency ratings such as 80+ for PSUs. I'm pointing this out because obviously the OP doesn't know much about PSUs, no offence, and a good quality PSU is pretty important and is worth reading up about.
As for the UPS, I would first suggest buying a load meter of some description to find out the power usage of your computer equipment (under idle and load) and pick a UPS based on the results and your requirements (do you want to continue working for some length of time when only UPS power is available, or do you want to shut down safely, for example).
I have a Corsair VX450 PSU, the maximum load I've monitored of my PC's tower is 186W IIRC (during gaming).