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Solar Panels

Reprinted with permission from Bob Pone, www.diybob.com/hottip.htm

What They Do

Solar panels convert the sun's incoming radiation (light) directly into electricity, which can be used to charge batteries and operate DC loads.

There are several applications for solar energy on boats:

  1. Boats that are kept on moorings need to keep their batteries topped off, and don't have access to shore power.
  2. Cruising boats may want to augment charging options. Panels can reduce engine operating time, or serve as an emergency backup.
  3. Boats without auxiliary engines may use solar power for all onboard uses.

How They Work

Energy from the sun's light strikes the solar panel crystals and manages to knock some electrons loose which creates electricity. OK, that is a little brief, but to the point. Each solar cell in a panel creates a potential of about 0.4 volts. To create sufficient voltage to charge a 12 volt battery, 30-36 cells are connected in series to produce from 14 to 17 volts.

The surface area of the solar cells determines the amperage, much like the size of a battery's cells. Roughly speaking, a cell 3" in diameter produces just under 2 amps, while a 4” cell produces 3.1 amps, and a 5" cell produces over 5 amps. There are three main types of solar panels Mono-crystalline (Siemens), Polycrystalline, and Thin Film (United Solar). Monocrystalline panels have uniform dark gray or black cells and have the highest output per area. They tend to be the most expensive as well. Polycrystalline cells look somewhat like shattered black glass and are less expensive, but less efficient for a given area. Both types are only slightly flexible. Thin Film panels are flexible, about 50% as efficient as monocrystalline, and have some advantages and disadvantages.

How Much Output Can I Expect?

Our rule of thumb is: a solar panel will contribute as much as half of its wattage, in amp-hours, each day if actively aimed at the sun, and perhaps 25% if randomly oriented. This is sort of a mixed metaphor, but means that a 48 watt panel will contribute as much as 24 amp-hours per day if aimed at the sun, assuming there is sun. Correspondingly, a 10 watt panel may contribute 3-5 amp-hours per day.

Do Solar Panels Require Regulation?

Like airlines and the S&L Industry, some form of regulation is a good idea for solar panels. Panels that are large compared to the size of the battery bank they are charging will overcharge the batteries if left in place long enough. It is generally acknowledged that you can leave a solar panel attached to a battery for extended periods of time if the rated output of the panel is less than 2% of the battery's capacity in amp hours. This means that a 2 amp panel should not be used unregulated with batteries less than 100 amp hour capacity. Regulators should be used almost anytime you have two or more large panels connected to your batteries. The regulator will shut off the panels when the battery voltage exceeds 14 volts.

If you are operating your boat with batteries that are seldom fully charged, as is common when cruising, the chance of over-charging them is much less. Remember, your solar panels will "rest" every night, and during cloudy days. This insures your batteries are not held continuously at an elevated voltage, but they may exceed a healthy voltage in the afternoon sun.

Several panels are advertised as "self-regulating" This means they can be left hooked up to a battery for an extended period of time without over-charging it. This is possible because the panel has fewer silicon cells (usually 30 instead of 33 or 36) and therefore reaches a lower voltage. Although self-regulating panels are appropriate for a long term maintenance application, like a boat on a mooring, we prefer to use higher voltage panels that are connected to a solar panel regulator.

What to Look For

When selecting a solar panel, there are several important questions you should ask yourself:

  1. Is it necessary for the panel to be flexible? The thin film panels are the only ones that flex more than a little. They can be mounted on curved surfaces like a dodger.
  2. What is the cost per watt of output of the panel? Large "hard" panels generally have the greatest output per dollar.
  3. Are you limited in the area available for solar panel coverage? The highest efficiency panels are the monocrystalline type.
  4. Will it be difficult to protect the panel from partial shading during the day? Thin film panels are more tolerant of shading.
  5. Will the panel be used in high temperatures? If so, consider a higher voltage panel, or a thin film.

The information contained herein is provided by Bob Pone. Neither ABA nor Bob Pone assume any responsibility or liability for events that occur due to actions you or others on your behalf take based on the information given. You are proceeding at your own risk.

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