I've 2 power sources in my house: One from the national power grid and another from an internal power source. I've several power sockets in my server room from these 2 sources.
Did some research and I was unable to find a UPS systems (at a reasonable cost) that use power from source 1 and, in case of failure, it would switch to source 2 until source 1 becomes available.
I was able to find expensive solutions for the entire home power grid but is there any reasonable priced consumers UPS that adds redundancy with a second power input? Like having a second input socket plug.
Note: For my user case, none of the servers contains redundant power supplies.