Practical solar power

As I buttoned my boat up for the winter, I noticed some crud on my solar panels. I got a wet rag and cleaned them up so my batteries will stay topped up all winter: no power cords, no worries about trashed batteries, no worries about someone unplugging things while I’m gone. It’s great to pop down to the boat and have everything electrical working perfectly, even after a long lay up.

I’ve owned one of my panels for about 20 years and it is still going strong. The only maintenance performed in all that time was a simple cleaning. How many other marine products, or land products for that matter, come with a 25-year warranty and last even longer? In those 20 years, I’ve received thousands of watts of electrical power without any noise, pollution, maintenance, or fuss. There aren’t too many man-made things that could provide more satisfaction per buck spent.

Space race

New panel mounting option

Increased interest in solar power has resulted in some new products to aid in mounting the panels. One example of this is the Solar Stik ( A rigid, stainless steel pole, the Solar Stik has adjustable, articulated arms that allow the solar panels to be pointed at the sun no matter how the boat is oriented. The product is rugged enough to allow you to use the panels while sailing, and it can also be used to mount a wind generator, GPS antenna, or other gear. The unit is set up to allow the arms to be either folded down parallel to the mounting pole, or easily removed entirely in the event of heavy weather.

John J. Kettlewell

With diesel fuel topping $5 per gallon last summer, non-fuel power generation is looking better and better. In terms of simplicity and cost it is hard to beat solar on a boat, but there are some caveats. First, very obviously, they take up a lot of space. There are many very creative mounting solutions out there, but they all involve compromises. At this time, a typical 65-watt panel by Kyocera measures about 30″ x 26″, with a large 130-watt panel coming in at 56″ x 26″. These are rigid panels, which remain the most efficient type readily available at this time. So, they have to either be mounted on a flat, or near-flat, surface, like the deck, or on some sort of device that can allow them to point towards the sun and not interfere with the boat’s operations.

Mounting sufficient panels to provide a serious boost to your energy budget can be difficult on many typical sailboats. My current 38-foot motorsailor has a rigid pilothouse over the cockpit, which provides the ideal mounting point for several panels in the 50 to 60-watt range. Many sailors mount panels on top of Bimini tops or on radar arches. The most efficient position is with the panel facing the sun directly, so many people come up with various movable mounting schemes. Of course, these large, flat, panels are also perfectly shaped to catch the wind or boarding seas, so it is important to mount them where they won’t be swept or blown overboard. Further complicating matters is the necessary wiring that must run to the panels, and the need for the panels to be secured from theft in some harbors.

Over many years of use I have tried various mounting schemes, and I have found that none are perfect. I am looking forward to the day that boats come with built-in panels and control systems as standard; however, for now you will probably be on your own in coming up with creative ways to solve this puzzle. You may want to mix and match a few ideas. After several bouts with hurricanes and offshore gales, I believe that the bulk of your solar panel array should be rigidly mounted in a position that is unlikely to be walked on (they are very slippery), where seas are not likely to catch them, where they won’t act as sails or missiles in high winds, and where they won’t be shaded much of the time.

Mounting rules

The last feature is impossible to achieve at all times of the day, under all wind directions, with rigidly mounted panels. However, there are some simple rules that help.

1. Don’t mount a panel with something directly over it, like a boom and furled sail, or in a position that is significantly off of horizontal, like on the cabin sides. The strongest sun is at noon and that is when your panels will probably be best off facing straight up, or nearly so, in order to generate maximum power.

2. It is a good idea to have some panels on both sides of a cabin top or pilothouse, so that one side or the other is getting some sun, even with the sails set.

3. It is a good idea to keep most panels well back from the bow, off the side decks, and away from places where things are likely to fall. I have dropped a winch handle onto a glass panel, only chipping it a tiny bit, but they aren’t bulletproof. You can walk on most marine panels, but they are always slippery, especially when wet.

It is ideal to supplement these hard-fastened panels with one or more portable units that can be moved around the boat in order to maximize sun exposure. This is particularly desirable if the prevailing wind direction means your fixed array is shadowed during a significant portion of the day.

I used to keep a 55-watt panel on deck most of the time with a long coiled up wire attached. I used a cigarette plug to attach the panel to the electrical system when in use. Since I had several cigarette outlets in various places, I could easily hook up the panel anywhere I wanted. Several shock cords would allow me to attach the panel to the deck in various ways depending on the sun direction. I found it was good to have a secure parking spot on deck for the panel where I could lash it down when the weather picked up or we headed offshore. It is important to lash loose panels down securely when you leave the boat in case of a sudden thunderstorm, etc.

I have tried various rail mounting schemes, and I think they have some potential, especially on the stern. I have seen folks with panels mounted along the side lifelines, but I would consider that location suspect on an offshore boat due to the likelihood of encountering boarding seas at some point. The top of a solid and wide radar arch is probably as good a place as any, as long as your other gear up there doesn’t create too many shadows on the panels.

Many rigid solar panels have extruded aluminum frames that raise the panel an inch or two off the deck. I have used lengths of aluminum angle iron, purchased at an ordinary hardware store, to create small, right-angle brackets that can first be attached to the deck (through-bolting preferred) then machine screwed into pre-drilled holes in the solar panel’s aluminum side pieces. Four of these angles per panel have held 65-watt units through winds up to hurricane force.

Sun in, watts out
Further exacerbating the space and mounting issue is the relative inefficiency of today’s crop of panels. Based on almost 30 years of experience using solar everywhere from Canada to the Caribbean, I’ve developed the following crude formula to approximate the amp hours provided by a solar panel or a solar array. I take the peak output of a panel in watts, multiply that by 12 hours (typical daylight), divide that by two to account for various inefficiences, then divide that by 13 volts (the approximate voltage of a charged battery).

For example, one of those Kyocera 65-watt panels would generate a maximum of 780 watts over 12 hours, which I divide by two to get 390 watts, which is divided by 13 volts to get approximate amp hours of 30 for the day. And, those 30 amp hours come from a sunny climate with the panels pointing pretty much at the sun most of the day. If the mounting isn’t ideal, or the sun is hazy, you’ll get less.

In the tropics I’ve found that my electric refrigeration alone draws around 120 amp hours each day, meaning I would need a minimum of four 65-watt panels just to keep my beer cold. I would need to add more for the rest of my electrical load. By they way, if you depend on an electric autopilot, I have found that most units use at least twice the power claimed by the manufacturer, so I find I need even more power offshore. In addition to everything I use in harbor I need the autopilot, running lights, GPS, depth sounders, radios, etc. Plan accordingly.

You can purchase “maximum power point” charge controllers that increase your solar output by adjusting the panel output voltage to just enough to charge the batteries. Your solar array needs to put out a higher voltage than the batteries are at in order to push the power into the battery. With a maximum power point controller, you get more charging amps when the battery voltage is low, and as the battery voltage increases, the solar panel output voltage will be increased to provide the maximum amps possible.

For example, a 65-watt panel at 12 volts puts out nearly 5.4 amps vs. 4.6 amps at 14 volts. There is some loss in the power conversion, but manufacturers claim an overall gain of around 25 percent in charging amps. I’ve installed one of these controllers on my current boat in order to maximize the output of three panels and it does seem to work as advertised. In addition, the controller has a neat feature that lets excess power spill over into the engine cranking battery, keeping that seldom-used item (on my boat I use the house batteries to start up) always in tip-top shape.

Charge controller
Some sort of charge controller is almost a requirement, particularly when you add more panels or if you want to leave your boat unattended. I have used a crude system of switches going to different panels, while I kept a close eye on battery voltage. As the voltage hit around 14 volts of direct current, I would turn off various panels to avoid overcharging.

Many voyaging boats burn up so much juice each day that it is often the case that every solar panel can simply be left online all the time as the boat’s batteries never get up to snuff on solar alone. In that situation, be sure to have some sort of foolproof system to cut off your solar panels when you leave the boat for extended periods, or else your batteries will get cooked. I used to wire my panels through the main battery selector switch so when the switch was in the off position the panels were disconnected from the batteries. Put the incoming solar amps on a single, devoted circuit, or else you will find yourself with live wires leading throughout the boat even when the batteries are offline. This can be the cause of mystery currents that turn on lights or electronics. On my previous boat I used position one of the selector switch for the house batteries, with solar on position two. When the switch was on “both” the solar was hooked up. Similarly, you could add one big additional switch that turns off the entire solar system, though keep in mind that the wires from the panels will still be charged up when the sun is shining.

The problem with not using a charge controller is that when I left the boat I turned off the solar power too, so my batteries weren’t getting topped up. I now have a direct-to-battery connection that is fused at the battery and fused at a charge controller, so the circuit is protected and the solar is always connected, and the batteries are always getting the charge they need. The charge controller keeps the batteries from getting overcharged.

Proper fusing at various points is crucial. I had some solar panel wires short out once and there was a nice little set of hot wires and burnt insulation &mdash luckily these were on the stern pulpit where they did no harm. It would be a wise idea to incorporate fuses right at the solar panel, though I have not seen this done by manufacturers. This inevitably leads to long runs of wire that can get powered up by the solar panels if there was to be a short. You will only be getting 4 or 5 amps with one small panel, but some larger systems might be generating more than 20 peak amps, which is plenty to do some damage or start a fire. For this reason, it is a good idea to put a fuse someplace in the wire (protected from the weather) fairly close to the panel, in addition to one between the battery and the charging circuit.

Prior to mounting the panels, seal all electrical junctions on the back of the solar panel with silicone sealant. I take apart any junction boxes and reseal them with the gaskets and silicone goop. A sure fire way to make a good waterproof electrical connection or splice is to use crimp on connectors, then generously smear the junction area with silicone, then wrap with high quality electrical tape. I like to use white silicone, white tape, and white wire for on-deck purposes as it seems to last a lot longer in the tropical sun than typical black tape and wire. If you want to make the connection really bullet proof, wrap the joint with silicone, self-amalgamating tape, available at hardware stores for a lot less than in marine stores. Then make the final wraps with electrical or rigging tape. This type of waterproof junction is cheap, easy to take apart, and completely waterproof.

Over many years I have experimented with various types of through-deck electrical connectors, and they all suffer from one problem or another. Simply running a wire through a drilled hole and then filling that hole with silicone goop works great.

Be sure to use hefty wires for the long runs to the controller and batteries. In general, the thicker the wire, the better in order to minimize power loss. For example, a 130-watt array located 20 feet from the batteries will require a 10-gauge wire to keep the loss less than 3 percent. There is a great online wire-size calculator at

Pro choice
At this time, there is much talk of the next generation of solar panels that will be cheaper, more efficient, and easier to mount in various configurations. Unfortunately, for now this remains just talk, though some advanced commercial-sized units are now shipping.

The real choice at the moment is between traditional, silicon wafer-type panels and thin film units. The older silicon wafer type is generally more expensive, bulkier due to the glass cover and heavy frame required for protection, and they are more efficient. Due to the many connections between the various cells and wires, plus the required protective structure, these panels are difficult to mass produce in sufficient quantity to bring the price down. Thin film types have been developed that reduce manufacturing costs. These units can be lashed to a soft Bimini top or a furled sail. On a cost-per-watt basis, and on a watts-per-square foot basis, silicon wafer types win at this time. With the space constraints on most boats, I recommend paying more for panels that produce more in less space.

A company called Nanosolar ( is working on third generation solar technology that utilizes a printed out thin layer of semiconductor using special materials. This produces more solar generating area at a lower cost, while improving efficiency and reducing the difficulty of manufacturing. However, I do not know of production units for small-scale marine use, and it remains to be seen what the watts-per-square foot results will be using boat sizes. Other companies are working on solar collectors that focus light on each cell, improving output, though heat constraints may be a problem. As is frequently the case with boat gear, the tried and true is the way to go.

John Kettlewell and his wife, Leslie, have co-authored The Intracoastal Waterway Chartbook: Norfolk to Miami (McGraw-Hill) and The Intracoastal Waterway Chartbook: Miami to Mobile (International Marine).

By Ocean Navigator