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Solar Panels / What they do

Solar panels convert the sun's 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 - Maintenance charging for boats on moorings.
2 - Augmenting engine-based charging on cruising boats.
3 - Sole-source charging for boats without auxiliary engines.

How they work
Energy from the sun's light strikes the solar panel crystals knocking some electrons loose and creating electricity. (OK, that is a little brief, but to the point.) Each solar cell in a panel creates a potential of about 0.45 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. Amperage (current) is proportional to the area of each cell: larger cells produce more current. 3" cells = 2 amps, 4" cells = 3.1 amps, 5" cells = 5 amps.

There are three main types of solar panels: Monocrystalline (Siemens), SunPower, Polycrystalline, and Thin Film (United Solar). Monocrystalline panels have uniform dark grey 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 as well as less efficient. Both types are inflexible. Thin Film panels are flexible and about 50% as efficient as monocrystalline.

How much output can I expect?
A design rule is that a solar panel will contribute as much as half of its wattage, in amp-hours, each day if aimed at the sun and perhaps 25% if randomly oriented. This means that a 48-watt panel will contribute as much as 24 amp-hours per day if aimed at the sun and perhaps 12 amp-hours if randomly oriented.


Do solar panels require regulation?
As a general design rule, panels that produce less than 1%-2% of the battery's capacity in Ah don't require regulation. This means that a 2-amp panel is the largest you should use without a regulator (Such as the Outback MX60) on a 100-amp-hour battery. 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's charge exceeds 14.3 volts.

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

Self-regulating panels produce less voltage (fewer cells), and theoretically will not overcharge your batteries. Frankly, we don't think this is the right solution. We would rather use higher voltage panels with regulators, or smaller panels which are incapable of overcharging. Self-regulating panels are much less effective in high temperatures, since their voltage can drop too low to be effective in charging.


Mad Dog Voyager is outfitted with 2 SunPower SPR-90 solar panels