You certainly have an overkill of a PSU for that system that you are running.
Bear in mind that the 1000W is what is provided by your PSU to your PC and is in Direct Current.
Your PSU draws Alternating Current from the UPS and will not draw anywhere near the 450 Watts that the UPS is able to supply, even under a hard load.
Thanks for the detailed answer! I'm glad I'll be able to use it safely with my system.