> A typical solar panel is 100w in peak conditions

That doesn't mean anything. A standard panel (1m×1.9m) has a peak power of 300W.

What we must compare is the solar radiance times the efficiency of the panel, and multiplied by the surface, would give you your available power

And 1hp is actually 745.7w. It makes very little difference with regards to the big picture of whether something is feasible. Whether you tell someone the ferry is going to need 20,000 solar panels or 5000 it's still way more than reasonably be put onboard.

The solar factor usually employed in the UK is about 0.1 to get from nameplate capacity to actual production (accounting for hours of darkness and suboptimal atmospheric conditions) so it would be reasonable to assume your 300w is actually more like 30w.

At least some percentage of solar panels on our hypothetical solar power ferry are likely to be in shade or not pointing directly at the sun. Then you have various conversions and charging losses to get the power into a usable form.

I was talking about the assertion, not about if it was feasible or not.

I was talking in broad strokes about how to encourage someone to have an intuition of the power requirements for such an endeavour. I was using easy round numbers not asserting exact numbers.

Note that the solar panels are not necessarily the primary energy source, they can do trickle-charging, range extension and/or emergency recharge / supply (e.g. when you've run out).

These are roughly the use cases lightyear one tends to cover, under a somewhat ideal 12km/h WLTP (that is in good sun the panels can charge 12km worth of WLTP range per hour, which can extend the range during a drive, recharge a parked car or let you drive at 10~12km/h when the batteries have run out).

Yeah, that’s a good way of looking at it. Note that a typical modern solar panel is around 285-340 watt these days.