A.Martin Holladay responds: The term "solar panel" can refer to two different types of equipment: photovoltaic (PV) modules or solar thermal collectors. PV modules generate DC power to charge batteries, or when connected to an inverter, to power house loads or supply electricity to the utility grid. Solar thermal collectors are used to heat swimming pools or to supply a portion of a home's domestic hot water or space heating needs.

Cost-effectiveness calculations are complicated and site-specific. In sunny parts of the country, like Arizona or New Mexico, a PV module or solar thermal collector may have two or three times the annual energy production of the same equipment installed in northern Vermont. Cost-effectiveness calculations also depend on the cost of available...

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