Since the early 1980s I’ve been equipping my voyaging sailboats with solar panels. At first I just had a small 10-watt panel to keep the electronics running in case of engine failure. With nothing but VHF, a loran and a depthsounder, that wasn’t much of a task. As I moved up in boat size, my electrical demands also grew – as did my solar array.
My last boat was a very simple 32-foot catamaran with ocean-crossing capabilities. Auxiliary power was provided by a 9.9-hp outboard that could provide 10 amps of charging current, so we mainly relied upon several solar panels and an auxiliary wind generator. That combination is the most common out here in the Caribbean. There’s plenty of wind and sun most of the time, and a lot of boats can sit at anchor forever without running a generator or the main engine.
Over many years of use, I’ve discovered some interesting things about solar power. First, you have to have a big array to put out much power, even in the Caribbean. Panels are generally rated by their peak wattage output. In other words, you purchase a 55-watt panel or a 150-watt panel. But that rating is under full direct sunlight with a low temperature under ideal conditions. Clouds, high temperatures, partial shading, etc., reduce peak output.
As a crude rule of thumb I’ve developed the following formula to approximate the amp hours provided by a solar panel or a solar array based on my experience. Take the peak output in watts, multiply that by 12 hours, divide that by two (efficiency), then divide that by 13 or 14 (volts). We can abbreviate like this: A = (12W/2)/13.
Let’s try an example: A 100-watt panel could theoretically produce 1,200 watts over the course of 12 hours, but we divide that by our efficiency factor of two to get 600 watts, then we divide that by the voltage factor of 13 to get about 46 amp hours of power for the day. I consider that an optimistic figure suitable for areas with abundant sunshine. If you spend most of your time in foggy Maine, you’ll get lower performance.
There are now available “maximum power point” charge controllers that can theoretically increase the output from any solar array. They work by adjusting the output of your panels to a point that is most efficient. Typical panels put out (no load) around 17 volts or so, which is not necessary to push a battery at 12 volts toward a proper charge. However, if the voltage was adjusted to only 14 volts, more watts would be produced and therefore more amp hours. In other words, 50 watts at 17 volts is about 2.9 amps, but 50 watts at 13 volts is 3.8 amps, or almost an amp more charging going into your batteries. However, there are some losses in the power conversion. Manufacturers claim increases in amp hours of up to 25 percent. I’ve installed one of these controllers on my current boat in order to maximize the output of three panels.
However, in the past I’ve successfully used solar panels wired directly to the batteries through switches. By frequently monitoring battery voltage I could tell the rough state of charge. I had an expanded scale voltmeter that would provide very accurate readings at a glance. This system works well if your solar panel output is close to your average daily electrical demand. However, you have to be sure to disconnect most of the panels if you are leaving the boat for long periods with little or no electrical drain on the system. I used this system for many years. I fine-tuned my solar output so it closely matched my electrical draw, and I had no problems with overcharging.
Most boaters will be better off using a charge controller that will automatically adjust panel output to match battery state of charge. However, even with a charge controller it is handy to be able to switch to a direct connection to your batteries, providing higher than normal voltage and an effective means of equalizing the batteries.
Connections are critical
Solar panels are generally quite durable. I have used some for more than ten years, but there was a gradual decline in output after that point. One thing to be careful about is proper fusing of the panels. Even if the panel only puts out an amp or two, a short in the wiring can dump the entire battery capacity down the wiring. When wiring up multiple panels, be sure the connecting wire and then the wire to the batteries is heavy enough for the load of the entire array. It is also a good idea to work out a system that allows the main electrical system to be shut down using the main battery switch while still allowing solar power to keep the batteries up to charge when the boat is left unattended. Just be wary of inadvertantly powering up some boat circuits with the solar panels alone. When troubleshooting electrical problems I frequently disconnect the panels to prevent misleading currents in odd places.
Panel mounting strategy
I prefer to permanently mount my solar panels. I’ve experimented with various pivoting and tilting arrangements, but I feel they leave the panels vulnerable to damage in heavy weather. And a loose solar panel could be a real hazard to the crew. There are flexible panels that can be temporarily lashed to the deck or on top of the boom, but they produce much lower power for the same surface area as hard panels. I do like to keep at least one smallish hard panel loose so I can move it around the boat for best sun exposure when at anchor. I shock cord it down and take care to never leave it unsecured when the boat is unattended. The glass-topped panels are quite strong, but dropping a hard object on the glass can break them. They are also extremely slippery, particularly when wet, so they should not be mounted in areas where the crew is likely to walk. On my current boat, a Finnsailer 38 motorsailor, I’ve through-bolted the panels to the top of the hard cockpit dodger.
Solar power is one of the simplest and most reliable additions that can be made to any cruising sailboat. At the very least, they can provide power for the GPS and the radio, no matter what’s happening with the rest of your charging system.
John Kettlewell lives aboard with his family: Leslie, Ian and Heather.