Turns out electric propulsion is established technology and common in even large cruise ships today. Finland is building a fleet of four corvettes (length 114 m so a bit longer than the German one JoB posted about) and they are expected to have top speed of over 26 knots. It’s more about getting enough electricity than being able ro move the ship by electricity. The Finnish corvettes will come with a gas turbine, in other words the ship carries natural gas and uses it to run what is essentially an on-board power plant.
So, we can assume that both collection efficiency and storage technology of solar power have improved considerably. It would seem plausible to have the top speed at 25 knots, but over sustained range it would be best to drive more slowly. The energy requirement grows fast with speed, and for huge cargo ships of today, minimal fuel cost is at 12-15 knots or about half of their normal speed of 24 kn or so. Soo, lets say that in good weather (hopefully your mage will be able to predict whether the sunny weather will last) I think we could comfortably say you can go about 18 knots during the day and still maintain charge to be able to go faster for a short while. At night it’s best to drop to 10 knots to avoid draining the battery completely.
If you do manage to use all you have, a couple of hours could give you some to be able to get to moving slowly, assuming of course that this doesn’t happen at night. If you are expecting trouble, drive slow to charge the batteries as full as possible before entering the danger zone.
Would the ship be fitted with an emergency motor of the Cattank type, i.e. can use anything as fuel? That could give you a few miles to clear a tricky spot, but at very low speeds.
All in all, “we are running low on power!” can plausibly happen at any time
On the other hand, you can plausibly (again allowing for advances in photovoltaics) just keep going all the time without needing to stop to charge, with the exception of extreme situations such as going over fast for longer to escape a pod of leviathans