Never, unless you have a very big boat with lots of space for a lot of Wp solar power and a boat with low power requirements. That space can be improved on smaller boats by using small flexible panels on the boom and mast and solar cloth sails, as the Arcona 380Z does. This all assumes a sailing yacht of course.
An example. At my latitude in summer we typically get on average 2.5 times the array rating per day, so say if you had 100 Wp of solar you'd harvest with a good MPPT 250 Wh of stored energy, but that's over a day not in an instant. On my boat that stored energy from that panel size would be enough for about a mile at 4 knots.
So you can see you need quite a lot of panels if say your boat needs 1,000 Watts of instant power at a slow cruise speed. At solar noon to drive that 1,000 W cruise speed boat on solar alone you'd need a 1,000Wp of solar which is what panel STC ratings are related to. Panel output can exceed it. It's all down to temperature. Hot panels not good, cold good. Solar panel area required depends on panel efficiency. More efficient panels means less area required but at more cost. An 11% efficient panel will have twice the area of a 22% one.
Panels subject to latitude might be at my latitude a peak of 5 times their rating over a day and a worse case of 1 times in Summer, worse in winter. All depends on lat / long and the season. At my latitude of 56N and because a boat moves it is best to have flat panels. Little advantage to angled panels in my case.Tracking panels really only any good on a long constant track.
The real question one should ask is how much solar energy can I store in my batteries whilst at anchor for the next day's sailing, which includes house loads as well as propulsion. At best you might have enough to get in and out of port and keep house loads going for the day.
John R.
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