Solar Electric Photovoltaic PV System Frequently Asked Questions

(source: U.S. DOE, Office of Energy Efficiency and Renewable Energy)

Photovoltaics

• What is photovoltaics (solar electricity), or "PV"?
• How can we get electricity from the sun?
• What are the components of a photovoltaic (PV) system?
• Can I use photovoltaics (PV) to power my home?
• Can I use photovoltaics (PV) to power my business?
• Are photovoltaic (PV) systems used in government buildings?
• How do I know if I have enough sunlight for PV?

Q:   What is photovoltaics (solar electricity), or "PV" ?
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.

Q:   How can we get electricity from the sun?
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.

Sunlight is made up of photons , or particles of solar energy. Photons contain various amounts of energy, corresponding to the different wavelengths of the solar spectrum. When photons strike a PV cell , they may be reflected or absorbed, or they may pass right through. Only the absorbed photons generate electricity. When this happens, the energy of the photon is transferred to an electron in an atom of the PV cell (which is actually a semiconductor ).

With its newfound energy, the electron escapes from its normal position in an atom of the semiconductor material and becomes part of the current in an electrical circuit. By leaving its position, the electron causes a hole to form. Special electrical properties of the PV cell—a built-in electric field—provide the voltage needed to drive the current through an external load (such as a light bulb).

Q:   What are the components of a photovoltaic (PV) system?
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 .

Q:   Can I use photovoltaics (PV) to power my home?
A:  PV can be used to power your entire home's electrical systems, including lights, cooling systems, and appliances. PV systems today can be blended easily into both traditional and nontraditional homes. The most common practice is to mount modules onto a south-facing roof or wall.

Other Resources:  To learn more about energy saving tips and renewable energy technologies, visit U. S. Department of Energy's Energy Saver's Web site

Q:   Can I use photovoltaics (PV) to power my business?
A:  PV systems can be blended into virtually every conceivable structure for commercial buildings. You will find PV being used outdoors for security lighting as well as in structures that serve as covers for parking lots and bus shelters, generating power at the same time. Indoors , PV systems are used to offset and operate all kinds of electrical systems, including lights, cooling systems, and appliances.

To learn more about energy saving tips and renewable energy technologies, visit U. S. Department of Energy's Energy Saver's Website.

Q:   Are photovoltaic (PV) systems used in government buildings?
A:  With its 500,000 buildings, the federal sector represents about one-half of one percent of the entire U.S. building inventory, but this is still considerable. Each year, U.S. taxpayers spend more than $3 billion to heat, cool, light, and power those buildings.

During the past 20 years, this energy bill has been reduced by investing in energy efficiency and using renewable energy (including solar) systems in new and existing federal buildings. The federal government is committed to installing solar electric and solar thermal energy systems on 20,000 federal buildings by 2010. In fact, the government exceeded its preliminary commitment of installing 2,000 systems on federal buildings by the year 2000.

Q:   How do I know if I have enough sunlight for PV?
A:  A photovoltaic (PV) system needs unobstructed access to the sun's rays for most or all of the day. Climate is not really a concern, because PV systems are relatively unaffected by severe weather. In fact, some PV modules actually work better in colder weather. Most PV modules are angled to catch the sun's rays, so any snow that collects on them usually melts quickly. There is thus enough sunlight to make solar energy systems useful and effective nearly everywhere in the United States .

Even hail won't harm most PV systems. Most homes have adequate roof space for a PV system, but you will have to size your system first to discover how much space is required. If you don't have adequate roof space, look at other options such as integrating the system into a wall or putting the system in the backyard. You could also use the system to cover a porch or patio in the backyard or mount the system on the roof or wall of a garage.

Other Resources:  To learn more about energy saving tips and renewable energy technologies, visit U. S. Department of Energy's Energy Saver's Website .

To obtain information about solar resources in your area, see Solar Radiation Basics.

To view some helpful decision-making tools, visit DOE's Building Energy Software Tools Directory.

To learn more about the basics of PV, see Sandia National Laboratories (SNL) PV website to request a copy of Stand-Alone Photovoltaic Systems: A Handbook, which presents recommended design practices for stand-alone PV systems.

See also NREL's A Consumer's Guide: Get Your Power from the Sun.