Here's some math for you.

It cost $2,400,000 to outfit an enormous car freighter with 328 solar panels.

Those solar panels will provide 40 kilowatts of energy to the ships crew, .2 per cent of the total ships energy consumption.

$2,400,000 for .2%.

Assuming you had the space, it would only cost $12 billion dollars to build a fully solar powered freighter.

Or you could spend $700k or so on a Sky Sail and save 20% of your fuel costs, a couple of thousand dollars per day of operation.

Green for the sake of green can be really dumb sometimes.