> No, one tonne of diesel fuel contains about 11 MWh of potential energy as determined by calorimetric methods. One tonne of fuel when consumed produces a variable amount of useful energy output depending on the efficiency of the engine.

That’s almost correct, good try.

> lights is a rounding error

Ships use electrical power for far more than lighting, and no electricity is not a rounding error compared to profit it’s a significant expense for cargo ships.