The upper weight limit for both 20ft and 40ft shipping containers is around 28,300 kg. Since SodiumLion batteries in containers will be weight limited, it makes sense to use 20ft shipping containers. One container can probably store around 3MWh of energy. For a 2700nm trip at 15 knots with 15MW of power usage, you need 2700Mwh divided by 3MWh per container gives us 900 20 ft shipping containers (TEU) for the battery.

OTOH if we look at popular shorter range routes like within Asia or Europe the calculation looks a lot more favorable.