I may be completely wrong here as I do not know this particular item but I fear you are misunderstanding what it is.
As I see it this Sterling unit is just a battery charger. A clever one in that it will work from any voltage around the world but all it does is charge the batteries. Your 13A sockets will only work when you are plugged into the mains supply. Without an inverter you cannot have "mains" supply on a boat unless it is plugged in or you have a generator.