Choosing and Installing Solar Panels - SailNet Community
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Choosing and Installing Solar Panels

Larry has finally graduated to the status of Solar Grand Daddy.
I’ve just finished installing our fifth solar panel, and have run into a problem I didn’t anticipate. I can’t get Sue to stop calling me Solar Daddy! OK, I admit it. Lately, I have been tweaking the angles of the panels to the sun quite a bit and then rushing below to monitor the output, but that’s all part of the fun and satisfaction of having your own onboard power plant.

Solar power plays a very big part in ensuring that, we are completely energy independent on Serengeti. Sue and I made the decision long ago to forego the intrusive noise and vibrations associated with an auxiliary diesel generator and strive to obtain energy independence entirely through quiet sources. After all, one of the reasons we sail is because we enjoy the never-ending sounds of nature and each others quiet companionship.

Whether you wish to keep your batteries charged up while at dockside, or you want to strive for complete, quiet, energy independence while cruising, the addition of one or more solar panels to your boat can be a smart and rewarding move. For a cruising boat, solar panels should be viewed as only one part of an overall energy program. We’ve assembled our onboard power plant using four main components. Solar panels, a wind generator, a high output alternator, and a battery monitor. The high output alternator is reserved as a backup charging source at anchor, or to maximize charging during periods when we would be running the engine anyway. With this arrangement, we haven’t had to plug the boat into shore power for over a year now.

Types of Solar Panels    There are generally three types of solar panels available today. Panels comprised of single-crystal cells, those with multi-crystal cells, and a third type using a much different thin film technology.

The single-crystal panels were the first panels used commercially and are still the most predominant today, both on the water and on land. They provide the highest efficiency and the most power for a given size. Power output values should remain relatively constant for 10 years or more. These rigid panels have a tempered glass cover and are extremely durable, but are breakable.

Multi-crystal cell panels have similar characteristics to single-crystal panels. Their output is slightly less than single-crystal panels but they may also be less expensive.

Thin film solar panels employ a technology that results in a solar panel that is extremely durable. These flexible panels can be stepped on, banged on, or even rolled up for storage. If you require a panel for mounting on the cabinhouse that can take the punishment from a falling whisker pole or enthusiastic crew members sliding back and forth over it, this is the type to specify. The disadvantage of thin film panels is that they are 50 percent less efficient than a single-crystal panel. A thin film panel would therefore need to be twice the size of a single-crystal panel to produce the same output.

When weighing the advantages and disadvantages of different solar panels, we chose to use all single-crystal panels. This provides us the best efficiency while taking up the least amount of space.

How Many Solar Panels Will You Need?    The exact number and size of panels you will need for your boat is determined by the complexity of your onboard systems and your personal habits. Boats with 12-volt refrigeration, inverters, and power-hungry electronics will have a much greater need to replace lost power than those that use block ice for refrigeration and a sextant for navigation. Ther are several SailNet articles at the end on how to estimate your power consumption.

To help you determine how many panels you’re going to need, a rule of thumb for estimating daily panel output assumes that you will receive the full-rated panel output for a period of four hours per day. For example, an estimated daily output for a 75-watt panel would be: 75 x 4 = 300 watts/day. If your panels aren’t able to tilt and follow the sun, reduce the above figure by 50 percent.

Most sailors we know begin with one or two panels, monitor the results, then add more panels if needed, or combine a wind generator, and maybe a high-output alternator.

Simple rail mounts allow the panel to be rotated toward the sun.
Mounting Your Solar Panels    Solar panel output is directly proportional to the panel’s orientation to the sun and the amount of sunlight available. The longer a panel can maintain a 90-degree angle to the sun, the more output you’ll get. The best mounts allow the panel to be manually rotated throughout the day or moved from one side of the boat to another to follow the sun. Mounting your panels in a fixed, non-tiltable manner, will dramatically reduce power production. Keep in mind that shade across a panel, even if it’s just a small amount of shade, can also dramatically reduce the output of that panel. Search for a mounting spot that minimizes shadows from masts, booms, running or even standing rigging. On sailboats, several mounting locations seem to work best.

  • Stern Rail – When solar panels are mounted to any rail, they must first be fitted with two brackets on the back of the panel that allows the panel to clamp to the rail. This is achieved by spanning anodized aluminum bar stock across the back of the panel in at least two spots. The bar stock is bolted to the frame of the panel, and the clamp-on bracket is bolted to the bar stock. This allows the panel to clamp to a rail, and then tilt along a single axis to follow the sun. This also provides a means to completely remove the panel quickly and easily, if quick-disconnect wiring is used.

  • Davit Mount – Davits make a wonderful spot for mounting one or more solar panels. The panels are aft, up, and usually out of the way. Use the same clamp-on brackets and attach your panels to a stainless rail that spans your davits. This mounting method has a bonus of adding rigidity to your davits.

  • Radar Arch – Some cruisers add a "radar arch" overhead at their transom. These structures are often used for mounting many different pieces of equipment other than just radar, and provide an excellent site for mounting solar panels.

  • Stanchion Mount – If your stanchions don’t taper at the top, they can easily be modified with rail fittings to allow 1 inch stainless tubing to span the top of any two consecutive stanchions. This will allow you to add a panel anywhere along your port or starboard lifeline by using the above-mentioned clamp on brackets. Sometimes panels are flush mounted to the cabinhouse while underway, then moved outboard to this rail while at anchor for maximum power production.

  • Bimini Top Mount – Panels can be mounted on top of a bimini. This type of mount doesn’t usually allow the panel to be rotated, but does place the panels up and out of the way. To mount your solar panel, you drill into the bimini frame and through the canvas of two consecutive bows and bolt the aluminum frame of the solar panel to the bimini frame. This setup makes your bimini more rigid, but adds weight and takes away the flexibility of being able to fold back your top easily . Some cruisers actually sew the thin film panels into their canvas in this location.

  • Cabin House Mount – Although inefficient from a solar collection standpoint, panels are sometimes mounted flat against a cabinhouse. For some boats, this may be a good storage location for panels while underway. When you stop, move the panels to another spot, like an outboard rail to best receive the power from the sun’s rays.

If you choose a great spot to mount your panel(s), and then scrimp on the wiring, you may be disappointed in the results down the road. To ensure high output from your solar panel(s), choose only marine grade tinned wire. Much cheaper hardware store wire may seem OK at first, but it quickly corrodes in the salty marine environment. This corrosion can result in voltage drops and will restrict your panel’s ability to charge your batteries effectively.

A charge control prevents overcharging and automates the system.
Charge Controller    For all but the smallest of solar panels, a charge controller is necessary. A charge controller senses your batteries state of charge and turns the current flowing from your solar panels on or off according to its findings. The charge controller turns on allowing current to flow from your solar panel(s) into a discharged battery. As your battery charges, its internal voltage rises. This rise in voltage is sensed by the charge controller, and eventually all current flow is halted when your batteries reach a fully charged state. Without this automatic shutoff, your batteries would overcharge and begin to gas.

The charge controller plays another important role. At night, a solar panel can actually consume power from your batteries. One way to keep this discharge from occurring is to insert a diode in the positive lead from the panel. Some charge controllers incorporate this blocking diode in their design thus eliminating the need for adding diodes externally. Other controllers have a special night disconnect feature. This accomplishes the same result as a diode without the small voltage drop associated with using diodes.

Charge controllers are rated according to the amount of amps that they can handle. If it’s possible that you’ll be adding more solar panels later, keep this in mind when you choose a controller, and buy one large enough to handle both your immediate and future needs.

Monitoring Onboard Power    Once your panels are installed, you’ll want to be able to evaluate their performance and overall effect on your batteries. The easiest way to assess your onboard power is through the use of a battery monitor. Today’s digital battery monitors can tell us at a glance how much energy our solar panels are producing. We can see the power production, digitally expressed in amps, increase as we turn the panel into the sun, then watch it drop off as we turn the panel away. This is also a helpful tool for determining solar panel placement.

Battery monitors do more than just monitor solar panels, though. A battery monitor functions as the brain of your electrical system. It gives you the ability to actually see power flowing into and out of your batteries. Today’s battery monitors tell us the voltage of our battery, the state of charge expressed in amp-hours, whether we’re currently charging or discharging our battery and by how many amps, along with an array of other helpful information designed to help us extend the life of our batteries. We think every sailor should have one onboard.

In the long run, solar panels pay for themselves in reduced engine hours.
With your new solar panels neatly in place, along with a way to regulate and monitor their performance, you’ll enjoy a whole new sense of freedom and satisfaction as you kick back in your cockpit. Every moment the sun is shining, you’ll feel confident knowing your onboard battery power is being replenished efficiently, quietly, and at no further expense.

Yesterday, I told Sue that we might need to add one more solar panel after we’ve built and installed our new freezer. She immediately told me she’d stop calling me Solar Daddy.

"Thanks", I said, a little surprised by her new, obviously more understanding attitude.

"If that happens," she continued, "I’ll be calling you Solar GRAND Daddy!"

Wiring Your Solar Panel and Charge Controller

After you’ve determined the mounting method and location for your panel, the next step is to run your wiring. The wiring from your solar panel will run first to your charge controller, then to your battery. If this route takes you through your deck or cabinhouse, and it probably will, make sure you’ve included a through-deck fitting. (See our article, "Drilling and Filling Holes in Your Boat," if you’ve never installed one of these fittings before.)

First, determine the distance from your solar panel to your battery. This distance is important, because it’s one of the deciding factors for what wire size you’ll need. The other factor is the rated amp output of your panel. See the table below, and remember to double the distance from panel to battery before plugging it into the chart.



Minimum Wire Size (AWG) Selector Table




Round-Trip Length of Conductor (Feet)


Next, find a suitable mounting location for your charge controller. It could technically be mounted anywhere along your wiring route from the panel to the batteries, but it is normally found close to batteries for the convenience of wiring the battery sense leads.

Several easy-to-make connections are required at the controller:

  • Connect the red and black battery sense leads from your controller to your battery. You’ll probably be asked by the manufacturer of the controller to add an in-line fuse to the positive (red) sense lead.
  • Next, connect the positive (red) lead from the panel to the positive input terminal on the controller.
  • The negative (black) lead from your solar panel will probably go directly to your negative battery terminal, but could wire into the controller also depending upon the model of controller you have chosen.
  • Lastly, the positive (red) output from your controller is fitted with an in-line fuse sized 1.5 times the rated output of your panel, and led directly to the positive terminal of your battery.

Before you pack up the tools, take a minute to organize and tidy up your wires. Bundle the wires with wire ties, and secure them intermittently with cable clamps so they’re tucked out of the way and look like they were professionally installed.

Installation Costs and Time

Generally, the smaller the solar panel, the greater the price per watt. Here, as an example, we’ll look at the cost of installing one 75-watt panel on a rail mount, 16 feet from a battery. This installation should take you no longer than one day.

 75-watt panel (Siemens SP 75) $499
 8-amp charge controller (one battery) $58
 Anchor Marine wire, 10 ga (20ft red, 20ft black) $25
 Helm rail mount brackets $20
 Anodized aluminum bar stock $10
 Two fuse holders and fuses$8
 Deck fitting for wiring $5

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