In this photo I am getting ready to install two 140W Kyocera panels for a nice 280W array capable of delivering about 70Ah's + per day to the battery bank, in Maine. They have been wired together in parallel as evidenced by the gray duplex wire running between them for the best performance, on this boat. The term "on this boat" are key to performance.
I get asked quite often about wiring multiple panels together. "Should I wire them in series or in parallel?" is the most often asked question I get. My answer to that question is almost always to wire them in parallel, on a sailboat. The real answer is that "it depends"...
Mono or polycrystalline panels are made up of a series or a daisy chain of solar cells. If you block one of these cells it acts much like a plugged sink drain stopping or greatly minimizing the "flow". Blocking just a small portion of one cell, with shade, can cut the output of both panels, if they are wired in series. The answer to this question is far more complex and really requires actual testing on YOUR BOAT to determine the best overall performance. For the most part though you can do very, very well by wiring the panels in parallel, however, in certain installations series is the clear winner. See, it depends.
By wiring the panels in parallel the shaded panels drop in output will not as drastically affect the other panels output if it is not shaded. When shading occurs across one small area of two series wired panels the output of both can be more drastically affected. When shading occurs across one of the two panels, in a parallel wired array, the other panel still pumps out its current unaffected by the shaded one.
In a perfect lab type scenario series panels with an MPPT controller work really well. Sailboats however are any thing but perfect when it comes to shade. Parallel wiring of multiple panels is often the best scenario, on a sailboat, but not in every case. If you really want to know what is best on your boat for your use then it will need to be measured over time.
Shade, the big "S" word in solar systems, is a real enemy. In this short video we can see the % output drop by shading just a small corner.
Effect of Shade On Panel Performance
As can be seen in the video even a small amount of shading drastically cuts into the output performance.