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World's first 10,800 car carrier with solar power delivered by China
(interestingengineering.com)
This is a most excellent place for technology news and articles.
The exact quote is
Based on that, the input power is a constant because they are comparing against the same motor. I would assume the power consumption is constant rather than bursty for either propulsion system, so there isn't any peak-vs-avg power to be concerned about either.
That "source" is some NASA tutorial that just states that quote without any elaboration at the bottom. I don't know how that passes as a source at all nor is it generally applicable in modern ships.
On the other hand there are multiple ships currently using these as additional propulsion system (as you can see further below in the wiki article). Flettners inital prototype used rotors as exclusive propulsion system. One could also use a kite or a sail by using the wind in favourable conditions.