Server room power backup

I’ve been tasked with creating a backup power plan for our server room. We have three tower servers and really only one of them is mission critical. We also have the router, firewall, and phone system which will need to remain on during a power outage. We need at least the one server to be able to stay up through any outage…our most recent power outage was about three hours but I suppose we should be prepared for anything. Any recommended hardware as far as battery backup, generator, etc?