Transforming the power of the sun into electricity
In bright sunlight, about 6,000 watt-hours of solar energy falls on the deck of a 30-foot boat every 15 minutes. That’s sufficient to fully charge five 12-volt, 100-amp-hour batteries from scratch.But solar panels, which generate electricity directly from the sun’s radiation, are only about 12 percent efficient, so the amount of electricity they produce is only about 25 percent of their rated wattage. That is, the number of amp-hours generated in a period of 12 hours = watts x 0.25. That means a fixed, 12-volt, rigid solar panel rated at 35 watts will give a daily output in amp-hours of approximately 35 x 0.25 = 8.75 amp-hours. If the panel is adjusted all day long so that it stays at right angles to the sun’s rays, its output may be doubled.To replace 100 amp-hours of electricity consumed on board daily, you would need solar cells covering an area of about 8 by 4 feet.The three main types of solar panel are monocrystalline, poly-crystalline, and thin-film panels. Monocrystalline are the most efficient and the most expensive. They are almost rigid, as are the polycrystalline panels. Thin-film panels are flexible, which makes
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| These are rough comparisons, taken from manufacturer’s brochures, of the electrical output of alternators—one driven by a propeller dragged through the water behind a boat, the other by a wind propeller—and solar panels |
them more convenient and adaptable on many boats, but they are far less efficient than the other two types.All types of solar panels are affected by the intensity of the sun’s radiation, and their output drops dramatically in cloudy weather or when shadows fall across them. Because the intensity of radiation decreases with increasing latitude, solar cells are less efficient farther away from the equator.The voltage of individual cells in solar panels is about 0.45. To charge a 12-volt battery, therefore, you need between 32 and 37 individual cells. Panels with 33 cells or fewer will not develop sufficient voltage to charge a 12-volt battery fully. On the other hand, they probably won’t need regulation and can be connected directly to the battery.If your panel has 36 cells or more, the higher potential requires proper regulation and a blocking diode in the circuit so that the battery doesn’t drain itself by trying to heat up the solar panel at night.
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| A solar panel generates most electricity when it is angled at 90° to the sun. This graph shows approximate efficiency at other angles |
See also
Battery Needs.
