But you asked, how to calculate your "consumption"? You don't "calculate" consumption, you "measure" it. Is that what you wanted to do? If so, then many UPS do have a status display panel that will tell you how much the connected devices are currently consuming. Many UPS come with software, like APC's
PowerChute, that will give you that information too - if you have connected the APC UPS to your computer via USB.
You can also use a
kill-a-watt meter to measure what is being pulled through the wall outlet.
Oh, and FTR, many sites and PSU makers, like
Seasonic, use a trimmed down version of the OuterVision PSU calc.
Edit comment: fixed typo and added comment about Seasonic's PSU calc.