Discussion question:
In selecting a PV backfeed breaker, is there a specific feature that has to be different from a circuit load breaker consider the circuit breaker feeds the load only where as the the PV breaker feeds the load busbar? Since PV source is also AC current (via a combiner box ), I believe it also feed the combiner box also when PV is not producing power at night??? Correct?
It has to be bidirectional, but it's common that most standard breakers made today will have this rating. You do have a burden of proof to confirm the breaker is bidirectional, by looking for a lack of line and load markings. Unfortunately, this is like proving a negative, since manufacturers don't do a good job of documenting this. Generally speaking, for most standard breakers made today that don't have additional features, they are bidirectional.
There is a very small amount of current that travels back to the inverter during the nighttime, while the inverter is in standby, waiting for a viable source of sunlight to turn on. This is insignificant for a standard grid-tied inverter, that has no other destination for that power.