Any electricians in the house?

So far as UPS transfer, there are a couple schools of thought. Some UPS's run the load at all times, with incoming power just powering the UPS and keeping the batteries charged

Some however, must switch (like a genset) from shore power to the UPS and so far as I know, all are supposed to do this seamlessly. In other words the thing is "'synced" with line power, and when it switches, you should not "feel" a thing.