One of my biggest peeves about the solar industry is the move to MC4 junction boxes. These boxes have the wires sealed and no access inside the j-box under the panel. In the marine environment I personally prefer a j-box with a terminal strip. It allows for easier paralleling of the panels and if installed correctly they last for a long time. I always try to find panels that do not ship with MC4 connections.

This Sun Wize panels ships with a standard j-box. Most DIY's do not own the proper crimp tool for crimping MC-4 solar connectors. If you buy a panel with MC-4's you'll either need to pay to have them terminated or pay to buy the proper crimping tool.

For the cable entering the j-box I use a waterproof cable clamp. I pick them up at my local electrical supply house for about $3.00 each.. Well worth the expense. They work best if you use a round cable. If using 4GA or larger cable each wire gets its own water tight entrance to the j-box.

Most marine wire is not UV stabilized. Despite the lack of UV inhibitors it can still last in the five to ten year range out in the sun, depending upon where you are. A UV stable SOW, STW, STOW, SEOW, or STOOW type cable can work well and can often be ordered in sizes up to 6/2. These cable types are the same or similar jacketing used in shore power cords and they hold up quite well in solar applications..

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.

A solar panel sized to actually "re-charge" a bank, not just maintain it, should ideally be routed directly to the house bank. The white jacketed wire coming into this Genasun controller is the panel feed. The connections are easy and clearly labeled on most controllers. Hooking them up is the easy part.

The black & red wires leaving the controller go to the house bank. The red wire is "switched" so the panel can be turned off if needed. Check with your controller manufacturer to see where an ON/OFF switch can be installed if you want the ability to flip the panel OFF. ON/OFF switches are normally installed on the battery/output side of a controller but this can vary from manufacturer to manufacturer.

There is also a fuse in the red wire between the battery bank and switch, that can't be seen. It is installed close to the battery + post. When connecting anything directly to a battery bank the wire should be fused. The ABYC suggestion is for the fuse to be within 7" of the + post of the house bank. While not always possible to be within 7" try to get it as close as possible.

If you have a secondary starting or reserve bank you can then use a device like an Echo Charger or an VSR type relay to charge the start or reserve bank and keep it topped up. Even without a solar panel an Echo Charger or VSR are useful devices.

Some charge controllers, such as the Morningstar SunSaver Duo PWM, can charge two banks simultaneously. This is a nice feature if you don't have an Echo Charger or VSR to charge two banks from one array.

When bringing the solar panel wires into the boat it's a good idea to enter the boat with a "drip loop". Deck and fitting leaks happen, it's just the nature of boats, and even "cable clams" can leak.

In the event of a leak the last thing you want is for the leaking water to migrate along the wire all the way to the controller, or another device near the wire that should not come in contact with water.


As always secure the wire well with wire ties on its path to the controller. This one mounted to the underside of the cockpit sole with Weld Mount tabs and wire ties.

In this photo I am showing the back side of the frame mounts. I used some SS fender washers to stiffen the aluminum frame a bit and nyloc nuts so they don't work loose. A little Tef-Gel under the washers seems to prevent stainless/aluminum interface corrosion.