I heat all my hot water at camp with excess PV. Small tanks tend to be as expensive as big ones. I have found you can get BOSCH ES4 4 gallon water heaters on ebay for $48 shipped. I just got one, it was brand new and just an open box someone had brought back. These have good insulation. I also have a 40 gallon tank for my laundry. These tall tanks easily stratify and the top 15 gallons heats easily and the lower section is preheat. Here is a camp setup with only 600W of panels and it has enough excess to heat water. It operates in parallel with charge controller, but uses no power from controller or batteries. They are always left fully charged. Nobody else knows how to heat water on this board.