Let's say your 600 Watt PC needs to run for one hour after power loss (not surge!) You need at least the 600 Watt Hour UPS to get to about one hour. As the unit ages a year they don't tell you to expect up to 50% less time. So I'd size that to 1200 Watt Hours.
I have delayed purchasing a UPS because I have been confused by all the options out there. There is every price range and power rating on these units and I am not sure what to look for. Does it relate to what my power supply on my computer is? I have a 600 Watt power supply for my desktop. I might also like to keep my VCR and TV cable box on the unit, as we have to reset these every time there is a power surge. Thanks for any advice.