Are you asking about the PSU in your PC as a UPS is a standby power supply and not normally located inside the PC? If you are asking about the PSU inside your PC there should be a label on it or if the model number is available you can do a web search to find the output power ratings. There is no tool in the O/S that tells you what PSU you have or it's output in wattage.
It should be on th label of the UPS unit. UPS normally provides 2 information for VA rating and Watts. Go to the webpage of the maker of your UPS. Download the datasheet.