the switch should be on. think of it as a disconnect switch for your batteries which it is. when you have it off and you are plugged in to 120V power, you won't know the difference until you try use a high amperage draw item that uses the battery bank as surge capacity. so things like power jacks, slides and so on. the rest of the time your converter will be sending 12V power out, but it will not be charging your batteries if that switch is off. so if you run it like that all the time be prepared to replace batteries more frequently