The inverter should be sized to meet the peak AC load. This is not the sum of all of the possible AC loads but the sum of the loads likely to be used at the same time. 

For example, an electric cooker will consume up to 5kW; an electric kettle 3.5kW; a fan heater 2kW; a hair dryer or a water immersion heater 1.5kW. It is unlikely that all of these will be on at the same time, so a boat fitted with all of these devices would probably need a 6 or 8kW inverter. The inverter will draw power directly from the 48V system, 8kW being 160A at 50V, which is well within the capability of the system.