no... it wont over charge. I am assuming you have a G24 battery and the 5watt panel will maintain it, but if you draw the battery down very much it can take days to recharge it
with a G27 battery or larger you are barely staying ahead of the normal discharge rate of the battery itself... so it will take longer to charge it, but will maintain it in good weather.
a 5watt panel came installed on my boat when i bought it and i could run the lights and music for a weekend, then it could charge back up during the week..... but i wanted to run my laptop also and with the music radio playing softly and my computer plugged in, the voltage of the battery would fall and shut me down after awhile. the 5watt panel couldnt keep up.... there was not enough time during the week for it to fully charge back up even in the long sunny summer days.
so when i wanted to do something about my power situation, i went back and forth with the idea of going with quite a bit more power or just fixing my current situation of needing just a little bit more to keep the batteries up a bit longer while im at the boat...
after weighing out the time we spend on the boat (two people use more battery power than one does) and the years we plan to use the boat and probably wanting to add other power consuming devices in the future, the most reasonable conclusion i came to was adding a substantial amount for my needs right now, and setting it up and installing it in a way that I could add another panel later without any other modification or additional wiring on the system.... this has all been done, and I am so happy with every aspect of it that i have become a strong advocate of a bigger system rather than the bare minimum "meet your needs now" installation... if it is a quality system, it can all be taken with you if you change boats and is worth doing so...
im not sure how you use your boat or what you do when your there, but if you stay on it and/or are a few days away from the dock, or plan to be in the future, you should consider a bit more power. you will always find a use for the extra power and will never regret it.
keep in mind that a battery shouldnt be drawn down over 50% of its rated capacity to insure the longest life and best performance from it. so if you have to replace a battery every couple of years due to overworking it, you can then reflect on how that money would have been better spent on a larger charging system that would have maintained the battery properly for many years and had the power available to use as you wish....
