Have you considered a solar panel?
I have a 10W solar panel on board and I no longer have a problem for power. The panel stays connected all the time, so even with the stereo and other things running, the drain on the battery during the day is only slight. If I recall correctly, an autopilot draws less than an ampere, which is pretty modest. By comparison, the anchor light draws almost 2A.If you need more juice, get a bigger panel, or connect two small ones in parallel. The flexible 10W panels are fairly inexpensive, mine was $150. I hope this helps.PeterS/V Raven