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Solar panel points

Jul 25, 2007 To the editor: In the recent article on solar power by John Kettlewell ("Solar voyaging," Issue 162, May/June), he gives a formula for estimating solar output. A more accurate estimate can be made by looking up the peak solar hours (PSH) per day, available at various sites (for example see http://www.sunwize.com /info_center/insolmap.htm).

In the Caribbean, 5 to 6 PSH is common, more in the summer, less in the winter. The PSH represents the number of hours of energy at 1,000 watts per square meter (the standard used to rate the modules) equivalent to the total daytime energy available, including the lower input in the morning and evening. For example, a 100-watt solar module (often called a panel) times 5 hours will make 500 watt-hours, divided by 14 (the typical charging voltage) equals 36 amp-hours per day. I think this is a more accurate estimate.

A charge controller must be used if the battery bank is small compared to the solar array, or if battery types sensitive to overcharging are used. So a large bank of "flooded" batteries with a small solar module does not require a controller. Roughly, if the solar module(s) are capable of making less than 1 percent of the amp-hour capacity of the battery (at the "20 hour rate"), a controller will not be needed when the battery is of the flooded type.

The fuse needs to be near the battery(s) or connection point (seven inches as per the ABCY). The wires should be at least large enough to carry all the short circuit current from the solar source, and short circuiting the solar modules will not harm them, but batteries are capable of melting even very large wires.

Thin film modules should be used when they are going to be partially shaded because crystalline modules lose much more of their output under those    conditions.

Since returning from the Caribbean in 1998, John Gambill and his wife Libbie Ellis own Hotwire Enterprises, the U.S. distributor of the KISS wind generator. They also sell, service, consult and install all forms of battery charging equipment and energy conserving products. They live in a solar-powered home in Tarpon Springs, Fla. Edit Module