In the event that accumulator charge drops too low (<20% by default), the backup circuit will connect, and a global alert will be sent:
If power consumption instead overwhelms both power systems, and drains the accumulators past a critical threshold (default 3%), the consumer connection will break until accumulators are somewhat recharged (>50% by default).
In general, the more accumulators there are (on the primary power circuit), the more forgiving this will be. Behavior becomes optimal once the combined power rating of accumulators reaches or exceeds the combined power rating of primary and backup power sources.
Edit: moved the non-circuit lamp so it won't annoyingly nibble on backup power overnight.