My kayak is set up to use a 32 Watt flexible solar panel for system charging.
THIS one in fact. Got it some 10 years ago locally at a solar place that no longer exists, and it has been on every fishing kayak I've ever owned.
The main reason I'm set up for it is Baja trips. Everybody here has been saying to "just Charge every night at home" or "carry a spare" which is all fine and good for 99% of your needs. But in my past I was into expedition style trips, where I'd paddle out and not return to the truck for as long as a week (but the norm was 2-3 days). Space and weight is at a premium on these style trips, so carrying a spare meant leaving something else behind.
Haven't done one of those trips in a long while, one of my favorites was to paddle out of the Old Mill and camp way out on a little sand spit at the mouth of the bay on the point, where halibut WSB and yellowtail were within reach for sunrise hookups...
I still use it from time to time though. When my battery starts getting old it will not keep the finder running a whole day, so to stretch its full potential I plug the panel in and I'm good. Then I can spend the 30 or so bucks to replace it at my leisure.
Maybe I'll do one or two more expeditions before I get too old here, we'll see...