Wiring:


The wiring of your panel to the boats battery bank is critically important. Most decent 12V nominal panels will output a voltage of around 16.0 to 18.0 volts +/-. Giving up any of this voltage to voltage drop can affect how quickly your bank is recharged. If you're using an MPPT controller voltage drop should ideally be almost nothing to maximize the performance of the MPPT controller. I like to size panel wiring for a 1-2% voltage drop max. With 12V nominal panels the wire from panel to controller will need to be bigger than with 24V or 48V arrays.


Controller:


Any panel sized to actually "recharge" your bank will need a solar controller. There are vast quality ranges amongst controllers and a bad controller can be a waste of money and diminish the efficiency of your system.


There are essentially three types of controllers;


MPPT = Multi-Point Power Tracking - In a nut shell MPPT controllers take excess voltage and turn it into current rather than wasting it. Your batteries only need around 14.4V to charge but you have panels putting out roughly 18+V. Without an MPPT controller this excess voltage is just wasted and lost. An MPPT controller converts excess voltage, above the 14.4V or what ever you set it at, into usable charging current. The claims are 10 - 30% more output. I must admit I have never seen a 30% boost but I have seen 6-14% increases over the max panel amperage output.
Most MPPT controllers will work best when your array gets over 150-180 watts in size. The Genasun MPPT controllers are specifically engineered for small panels below 150W and are the only MPPT controllers I know of specifically designed as such. They extract the max current they can out of small panels and they work quite well.


PWM = Pulse Width Modulated - These are the best alternative to an MPPT and they charge quite efficiently but can not turn excess voltage into current.


Shunting Controllers - These controllers shunt or switch the panel current/voltage OFF when a preset voltage is reached and then turn back on when the battery bank voltage drops/falls to a second preset ON voltage. These are the least desirable controllers. If a controller does not specifically say PWM or MPPT is is probably a cheap shunting controller. I can not suggest spending your hard earned money on shunting controllers for a marine solar application. (SEE VIDEOS BELOW FOR ACTUAL BEHAVIOR)