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Although the economics of large-scale solar-energy projects for commercial or residential installations continue to be a challenge and solar-hybrid-powered boats have proved to be overly complex and expensive, many power and sailing vessels can benefit greatly from the use of solar energy to help manage their electrical power requirements. Solar panels help charge the house batteries while anchored or sitting on a mooring, reducing or eliminating the need to run the generator. Several boatbuilders now offer solar-panel packages as options or standard equipment, and owners of older boats are adding solar panels to the charging systems.

The simplest system for a small outboard-powered boat that you may have tugging on a mooring consists of connecting a removable, often flexible, solar panel directly to the boat’s battery. This can help keep the battery charged to ensure that a bilge pump will function and that the engine will start after being left unattended for an extended time. These small units cost less than $100 and typically put out less than one amp during maximum sunlight, thereby not requiring an external charger/controller to prevent overcharging. They simply keep the boat’s battery topped off and are not designed to provide energy to power the vessel’s electrical accessories.

Permanently mounted high-output solar panels can be a significant source of power on larger vessels. Before choosing any panels or associated equipment you first need to determine exactly what you’re trying to accomplish, how much of an electrical load you expect the solar system to satisfy, and whether you need to modify other components of your vessel’s electrical system. There’s a huge difference between providing enough energy to run a few DC fans and lights at night versus keeping a household-size, 110-volt refrigerator running through a DC-AC inverter fed by a large house battery bank.

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High-output solar panels are connected to a charger/controller to ensure the proper charge level of your house battery bank. The size of this bank, in ampere-hour (Ah) capacity, determines which appliances you can run for a certain amount of time. To determine your daily energy requirement, find the current draw of each appliance at 12-volt DC (or 24-volt DC if your boat is so equipped), estimate how long each will run during a 24-hour period and arrive at a daily amp-hour requirement. West Marine provides a handy “electrical budget worksheet” on its website.

Instead of amp-hour ratings, you can also determine your energy needs measured in watts by multiplying the voltage times the current. A 12-volt DC refrigerator compressor that draws four amps computes to 48 watts. Of course, remember to estimate how long each appliance runs per day. My 12-volt DC Frigoboat refrigerator, which draws 3.5 amps, is well insulated and runs about 20 minutes every hour, so its daily requirement is 28 Ah or 336 watts.