Solar Power on Your Boat

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A Higher Power

Is there a place for solar power onboard your boat?

Solar panel on a boat

It wasn’t too many years ago that effective solar power was the sort of technology whose practical application seemed decades away—something our kids might enjoy. But it isn’t science fiction anymore: Not only are PV (PhotoVoltaic) arrays showing up on rooftops all over the country, but just about anybody can pop over to the marine store and buy a high-wattage solar panel at a price that won’t melt the ol’ credit card. (Heck, can a transporter be far behind?) But just because you can buy a solar panel for your boat, does that mean you ought to?

Now don’t get me wrong. We’re not talking hybrid power here, not like you’ll find aboard Greenlines, Arcadias, and other yachts designed to operate under both electric and internal combustion engine power. And we’re not talking the original hybrid system, meaning sails combined with a powerful auxiliary engine. (Sailors love solar panels, by the way, because at sea they don’t have to run the engine as often to charge the batteries.) All of these yachts may be great, but few skippers are ready to go fully hybrid. Nevertheless, almost any boat can benefit from solar panels, and there are now so many sizes, shapes, and types on the market that most skippers will be able to find one that fits their needs.

solar panels

As Important as a Life Jacket

Let’s start with the basics. A solar panel is essentially a maintenance-free battery charger that generates juice whenever the sun’s out, without any operator input; there’s not even a switch. You just hook it up and forget about it. A solar panel will keep your battery fully charged without leaving your boat plugged into shore power 24/7, thereby preventing possible corrosion issues due to faulty marina wiring. If your family lives on a steady diet of electrons, a solar panel can be a godsend when you want to anchor out for a quiet weekend away from the yellow cord. Boats living on moorings; smaller, simpler boats without shore power; or guys who like to spend the day drift fishing without idling the engine can use a PV system to keep the batteries topped up, or at least delay their discharge.

And solar power is handy in emergencies. “Few powerboats have any solar onboard at all,” said Paul Landino, an electrical engineer and owner of eMarine Systems of Ft. Lauderdale, an enterprise established in 1999 which specializes in wind and water generators as well as PV systems. “But it’s as important as a life jacket.” If you’re offshore and lose power, he explained, a solar panel can generate enough juice to at least operate a radio. And after all, what’s better than hitting the starter and hearing that nice, fast cranking you get from a fully charged battery?

Control Freaks

Most solar-panel systems require a charge controller, both to regulate voltage and prevent battery discharge at night. Without voltage regulation the battery can be overcharged if the solar panel’s output, in watts, is more than 10 percent of the battery’s amp-hour rating; less than 10 percent and you can skip the controller and use an inline diode to prevent discharge. However, a controller can be used in any solar panel installation, and since a basic PWM (Pulse Width Modulation) controller is inexpensive, why not?

But while a PWM controller is fine for many low-demand applications, “it just passes the current from the panel to the battery, losing about 20 percent along the way,” said Paul Landino of eMarine Systems. Upgrading to a more-efficient MPPT (Maximum Power Point Tracking) controller can reclaim that 20 percent, at a cost: Figure on $200 or so for a basic 12-volt, 10-amp MPPT model, compared to about $50 for a PWM controller of similar spec. Costs are substantially more for higher-capacity, higher-voltage controllers of either persuasion, and/or for those with sophisticated features—digital readouts, multi-stage charging, etc. 

Landino said an MPPT controller’s black-box brain will also maximize output to match conditions, like shade, shadows falling across the panel, or simply dirt on the panel which, of course, can be addressed with an occasional swab of your solar panel with a nonabrasive cleaner. Since almost all installations onboard a boat are subject to shade and shadows, upgrading to an MPPT controller could be worth the cost if you need to wring as much juice as possible from Old Sol. “With more than a 100-watt panel, don’t even think about it,” said Landino. “Buy an MPPT controller.”

How Much Juice Can the Sun Produce?

The amount of energy a solar panel generates each day depends on how many “sun hours” it receives, but that’s not the same as hours of sunlight. PV techs consider a sun hour (1 kW-hr/m2) to be one kilowatt of energy from direct sunlight falling for one hour on one square meter of surface. An efficient solar panel converts only 14 to 16 percent of that energy to electricity. For example, a Kyocera KCM140 solar panel measures almost exactly a square meter in area and will produce 140 watts in ideal sunlight, an efficiency of about 14 percent, right on the money for a modern solar panel.

But most of the time the Kyocera panel isn’t in ideal sunlight, so its actual output is less. How much less depends on “insolation.” Landino explained that the term is a standardized reference to estimate how much sun you get per day in any location and season. There’s an insolation map in the tutorials section of the eMarine Systems Web site, a Google search will turn up many others, or you can find detailed tables at:

I’m writing this in November in my office near New York City, where, according to the tables, I can expect just two sun hours each day, or a daily output from a 140-watt panel of 280 watt-hours. (In Miami, I could expect 3.7 hours of sun per day, or 518 watt-hours of juice.) Although the panel is rated at 17.7 volts output, Landino recommends using nominal battery voltage (12.8 volts) when calculating amperage, e.g., 280 watt-hours equals 21.9 amp-hours injected into my batteries, way more than enough to keep them fully charged while my boat sits in her winter slip. I just have to remember to sweep the snow off the solar panel. (During the boating season, I’d get, on average, three times as much sun in New York, but only about 60 percent more in Miami.)

To calculate your boat’s daily 12-volt needs, and therefore the size panel you need, multiply the average time each onboard appliance is used (in hours) times its amperage rating. (There’s a Vessel Energy Usage Estimator on the eMarine Systems Web site to help out.) Don’t get too anal with this, though: Meeting total power needs with solar is more important to skippers who never want to start their engines, or who don’t have engines, i.e., sailors or adventurers rowing across the ocean. The rest of us can fire up the powerplant when necessary and let the alternator do the work; if solar can replace half our daily needs, it’ll cut down substantially on engine or genset use. Landino suggests choosing solar capacity based on your battery amp-hour rating—for example, if you have 400 amp-hour batteries, choose a 350-watt panel.

solar panel

All Panels Are Not the Same

Once you’ve determined the solar output you need, it’s time for expert advice. There are many different types of solar panels—fixed frame, foldable, rollable, semi-flexible—and as many different ways of mounting them. “This is a real-estate problem,” said Landino: You need a place with enough room, maximum sun, and minimal shadows falling on the panel. “A hardtop or Bimini is nice real estate,” he said. A panel can be mounted across davits, on rail mounts, on a radar arch, cabin top, or upper deck. The panel should be mounted horizontally; even though it will rarely get direct sunlight, that’s the best compromise, said Landino. And panels mounted on hard surfaces usually need provision for ventilation; those on soft tops, usually not.

Most important is avoiding shadows falling across the panel. “A shadow reduces output,” said Landino, “but it’s not proportional.” A 10-percent shadow can reduce output by maybe 60 percent in some cases, he said. It depends where the shadow falls. Modern solar panels are typically broken into two circuits, so if one is shadowed the other still works. “It’s almost impossible not to have shadowing,” he said. To maximize output in shaded conditions, upgrade the standard PWM controller to a “smart” MPPT model.

Connecting the solar panel to the battery is easy—the hardest part will be fishing the wires. Simply plug the two wires from the panel into the controller, then attach the wires from the controller to the battery. If in doubt, consult with a qualified marine electrician. The solar panel won’t interfere with the alternator or 120-volt converter, and you don’t need any switches—just plug and play. Install an inline fuse near each battery, and if you’re not using a controller, install a diode to prevent the panel from discharging the battery after the sun goes down. (You don’t need a controller if the panel output, in watts, is less than 10 percent of the battery amp-hour rating; otherwise, the battery may overcharge.)

While it’s hard to beat the combination of a powerful alternator driven by a reliable engine for fast, efficient recharging of batteries, sometimes running the engine isn’t what you want to do. In that case, let the sun do it for you, with modern, reliable solar panels. You might like them so much, you decide to go fully hybrid with your next boat.

This article originally appeared in the February 2015 issue of Power & Motoryacht magazine.