We use a small solar panel that makes 5w to keep the batteries on a trickle charge. the panel just lays on the starboard cockpit seat and lounges in the sun all day. Haven't had an issue with low batteries so far. And no need to upgrade any electrical for shore power.
***Edit*** The solar panel decided to give up the ghost. I just ran an extension cord from the shore power post, up the starboard bow line, down the deck and into the cabin though the gab that is created when the companion way boards are up and the hatch is slid closed. Works great, and keeps the connections dry. We have 2 batteries total one on port, one starboard.