Battery size help

[AdrianKe]

I purchased a used bakfiets.nl cargo long bike and a bafang bbs02 48v 750w motor but am unsure of the battery size. We've got one steep hill and flat roads, will be transporting 2 kids (3 & 5 year old) and a baby. Our traveling distance will be on average 20km per trip with a max of 50km range on big excursions. What battery size do you propose we go for?

 

[Mysery]

Info on the total weight of everyone and everything would be useful. Going with what you posted though I would look for a battery of at least 720wh (15ah).