ups

Hi, when choosing a UPS What type of specifications do you look for? I’ve seen different models of ups systems and various devices connected to them. I can imagine the more you have connected, the more voltage or wattage you need. And I know u also have to consider how long the ups system will run if you lose power. What should you look for in an UPS as far as the voltage, wattage, and the length of the ups power?