Solar FAQs — Photovoltaics — The Basics
A: What do we mean by photovoltaics? The word itself helps to explain how photovoltaic (PV) or solar electric technologies work. First used in about 1890, the word has two parts: photo, a stem derived from the Greek phos, which means light, and volt, a measurement unit named for Alessandro Volta (1745-1827), a pioneer in the study of electricity. So, photovoltaics could literally be translated as light-electricity. And that's just what photovoltaic materials and devices do; they convert light energy to electricity, as Edmond Becquerel and others discovered in the 18th Century.
A: When certain semiconducting materials, such as certain kinds of silicon, are exposed to sunlight, they release small amounts of electricity. This process is known as the photoelectric effect. The photoelectric effect refers to the emission, or ejection, of electrons from the surface of a metal in response to light. It is the basic physical process in which a solar electric or photovoltaic (PV) cell converts sunlight to electricity.
A: A PV system is made up of different components. These include PV modules (groups of PV cells), which are commonly called PV panels; one or more batteries; a charge regulator or controller for a stand-alone system; an inverter for a utility-grid-connected system and when alternating current (ac) rather than direct current (dc) is required; wiring; and mounting hardware or a framework.
A: There are four main types of solar energy technologies:
Other Resources: For tips on saving energy and using solar and other renewable energy technologies in your home, please visit the U.S. Department of Energy's consumer information Web pages
A: A PV system that is designed, installed, and maintained well will operate for more than 20 years. The basic PV module (interconnected, enclosed panel of PV cells) has no moving parts and can last more than 30 years. The best way to ensure and extend the life and effectiveness of your PV system is by having it installed and maintained properly.
A: All life on earth is supported by the sun, which produces an amazing amount of energy. Only a very small percentage of this energy strikes the earth but that is still enough to provide all our needs. A nearly constant 1.36 kilowatts per square meter (the solar constant) of solar radiant power impinges on the earth's outer atmosphere. Approximately 70% of this extraterrestrial radiation makes it through our atmosphere on a clear day. In the southwestern United States, the solar irradiance at ground level regularly exceeds 1,000 w/m2. In some mountain areas, readings over 1,200 w/m2 are often recorded. Average values are lower for most other areas, but maximum instantaneous values as high as 1,500 w/m2 can be received on days when puffy-clouds are present to focus the sunshine. These high levels seldom last more than a few minutes. The atmosphere is a powerful absorber and reduces the solar power reaching the earth at certain wavelengths. The part of the spectrum used by silicon PV modules is from 0.3 to 0.6 mirometers, approximately the same wavelengths to which the human eye is sensitive. These wavelengths encompass the highest energy region of the solar spectrum.
A: A well-designed and maintained PV system will operate for more than 20 years. The PV module, with no moving parts, has an expected lifetime exceeding 30 years. Experience shows most system problems occur because of poor or sloppy installation. Failed connections, insufficient wire size, components not rated for dc application, and so on, are the main culprits. The next most common cause of problems is the failure of electronic parts included in the Balance of Systems (BOS) - the controller, inverter, and protection components. Batteries will fail quickly if they are used outside their operating specification. In most applications, batteries are fully recharged shortly after use. In many PV systems the batteries are discharged AND recharged slowly, maybe over a period of days or weeks. Some batteries will fail quickly under these conditions. Be sure the batteries specified for your system are appropriate for the application.
A: A 10% efficient PV system in most areas of the United States will generate about 180 kilowatt-hours per square meter. A PV system rated at 1 kilowatt will produce about 1800 kilowatt-hours a year. Most PV panels are warranted to last 20 years or more (perhaps as many as 30 years) and to degrade (lose efficiency) at a rate of less than 1% per year. Under these conditions, a PV system could generate close to 36,000 kilowatt-hours of electricity over 20 years and close to 54,000 kilowatt-hours over 30 years. This means that a PV system generates more than $10,000 worth of electricity over 30 years.
Other Resources: Consumer's Guide to Buying a Solar Electric System. September 1999. NREL. (PDF 704 KB). Look on page 9 for a map titled, "Calculating Electricity Bill Savings for a Net-Metered PV System".
A: Some of the following documents are available as Adobe Acrobat PDFs. Download Acrobat Reader.
Other Resources: EERE's Solar Yellow Pages. A list of directories of companies that make solar products, design, install solar systems, and companies that provide training and consulting services