Using the same density as they achieved in an open field where land is cheap, they could cover 2% of their cost.

Using the latest solar tech in a space-conscious deployment I would assume you could probably 2x that at least. So we’re in the realm of 5% of energy cost.

Give the technology another 10 years and where would we be? Unfortunately panels are already astoundingly efficient going from about 15% in the 60s to 20-25% today. In the lab we can approach 50% so there’s probably one more doubling.

To cover 10% of the energy cost of moving that much cargo just from the sun is mind boggling to me.

It makes me wonder if a different ship design which would scale down to have more surface area per volume, driving the solar power factor up, could actually work economically.

Since the trend has just been bigger and bigger super-freighters this would be an interesting shift.

Is a Tesla cargo ship in our future?

If you cover your ship in solar panels and can save 2% of energy costs, it would mean a huge win nevertheless, as oil is expensive.

A fair amount of waste comes from idling in port. There are still issues with loading and unloading containers if you cover the ship with panels.

If a ship's in port, there is no need for using the on-ship engines at all. You could just connect it to the land based grid, which, optimally, uses renewables. Hamburg harbour has project to do this precisely, and it seems Cuxhaven has one too.