In the end, all those battery powered ships needs to be charged. So it is their total energy consumption split between harbours over some period. And as we are talking about trickling you could pretty much average it.

It likely will depend on patterns of harbour. Like how many ships visit, what sort of distance those go. And how much of total time is spend charging some ship.

Worst case is maximum distance trips and maximum utilization that is there being ship almost always being docked. Apart from times when docked ship change.