Two solar devices on the back of my boat.
The solar mat trickle charges the batteries when the boat's at anchor. I rarely use the engine for anything but getting on or off the mooring or dock (unless the crew is impatient and the wind recalcitrant). If you use the battery to start the engine and then turn it off five minutes later, you haven't replaced the electricity used to start. Moreover, I use electricity while sailing to power the instruments. And, somewhere there's a small leak letting in either lake or rain water so the automatic bilge pump runs from time to time. All of those factors led to a dead starter battery a couple of times last year and forced me to run the engine an extra ten or fifteen minutes after mooring or on a long sail just to charge the battery. Now the batteries stay topped off with sunlight. Since I only used five gallons of diesel last year, I don't expect great savings, however.
There's a solar light with its own collector in the flagpole holder when I'm on my mooring. A real anchor light would be on the top of the mast and, technically, I don't need a light on my mooring; but the boat is moored close to the path boats take back from some popular bars on the New York side of the Lake so seemed prudent to make it more noticeable at night.
Bruiser, sitting on top of Mt. Mansfield, is wearing my Father's Day present from Mary: a pack so he can carry his own water and supplies. At 100 lbs, he can carry some of his own load.