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.
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.
- 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.
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.
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.
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:
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.
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