Yup. Fundamentally, the issue is whether there is any means by which locally-generated power can backfeed onto the mains. Typically cogeneration is handled by

Yup. Fundamentally, the issue is whether there is any means by which locally-generated power can backfeed onto the mains. Typically cogeneration is handled by making the local generator have no ability to create the signal, it can only come from reading the line voltage. The mains voltage dies, there's no more signal to drive it and so even in case of failure it will shut down. Any system which absolutely prevents backfeed is acceptable, your use of the mains charging the UPS and the UPS driving everything is an acceptable way to do it--even if your system goes nuts no circuit exists by which that power can backfeed. It's more typically done by a mechanical isolation switch, you can either be connected to the mains and rely upon the signal from the mains or you can run locally with no physical connection to the mains. The reason for this is repairs. If a lineman opens a breaker the wires after that point must go dead no matter what--if your system backfeeds into the wire you could kill the guy working on the wires.