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Solar FAQs — Photovoltaics — The Basics

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.

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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).

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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.

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Q: What's the difference between PV and other solar energy technologies?

A: There are four main types of solar energy technologies:
1. Photovoltaic (PV) systems, which convert sunlight directly to electricity by means of PV cells made of semiconductor materials.
2. Concentrating solar power (CSP) systems, which concentrate the sun's energy using reflective devices such as troughs or mirror panels to produce heat that is then used to generate electricity.
3. Solar water heating systems, which contain a solar collector that faces the sun and either heats water directly or heats a "working fluid" that, in turn, is used to heat water.
4. Transpired solar collectors, or "solar walls," which use solar energy to preheat ventilation air for a building.

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

To learn more about PV and solar hot water systems, please visit the Florida Solar Energy Center site.

Q: How long do photovoltaic (PV) systems last?

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.

Experience has shown that most problems occur because of poor or sloppy system 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 the electronic parts in the balance of systems (BOS): the controller, inverter, and protection components. Batteries fail quickly if they're used outside their operating specification. For most applications (uses), batteries should be fully recharged shortly after use. In many PV systems, batteries are discharged AND recharged slowly, perhaps over a period of days or weeks. Some batteries quickly fail under these conditions. Be sure the batteries specified for your system are appropriate for the application.

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Q: How does sunlight effect life on Earth?

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.

Talking about solar data requires some knowledge of terms because on any given day the solar radiation varies continuously from sunup to sundown and depends on cloud cover, sun position and content and turbidity of the atmosphere. The maximum irradiance is available at solar noon which is defined as the midpoint, in time, between sunrise and sunset. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of watts (or kilowatts) per square meter (w/m2) for irradiance. Insolation (now commonly referred as irradation) differs from irradiance because of the inclusion of time. Insolation is the amount of solar energy received on a given area over time measured in kilowatt-hours per square meter (kwh/m2) - this value is equivalent to "peak sun hours". Peak sun hours is defined as the equivalent number of hours per day, with solar irradiance equaling 1,000 w/m2, that gives the same energy received from sunrise to sundown. In other words, six peak sun hours means that the energy received during total daylight hours equals the energy that would have been received had the sun shone for six hours with an irradiance of 1,000 w/m2. Therefore, peak sun hours corresponds directly to average daily insolation given in kwh/m2. Many tables of solar data are often presented as an average daily value of peak sun hours (kwh/m2) for each month. Insolation varies seasonally because of the changing relation of the earth to the sun. This change, both daily and annually, is the reason some systems use tracking arrays to keep the array pointed at the sun. For any location on earth the sun's elevation will change about 47° from winter solstice to summer solstice. Another way to picture the sun's movement is to understand the sun moves from 23.5° north of the equator on the summer solstice to 23.5° south of the equator on the winter solstice. On the equinoxes, March 21 and September 21, the sun circumnavigates the equator. For any location the sun angle, at solar noon, will change 47° from winter to summer.

The power output of a PV array is maximized by keeping the array pointed at the sun. Single-axis tracking of the array will increase the energy production in some locations by up to 50 percent for some months and by as much as 35 percent over the course of a year. The most benefit comes in the early morning and late afternoon when the tracking array will be pointing more nearly at the sun than a fixed array. Generally, tracking is more beneficial at sites between 30° latitude North and 30° latitude South. For higher latitudes the benefit is less because the sun drops low on the horizon during winter months.

For tracking (structures that follow the sun across the sky by various mechanisms, thereby increasing the energy captured from the sun) or fixed arrays, the annual energy production is maximum when the array is tilted at the latitude angle; i.e., at 40°N latitude, the array should be tilted 40° up from horizontal. If a wintertime load is the most critical, the array tilt angle should be set at the latitude angle plus 15° degrees. To maximize summertime production, fix the array tilt angle at latitude minus 15° degrees.

Using inaccurate solar data will cause design errors, so you should try to find accurate, long-term solar data for your system location. These data are becoming more available, even for tilted and tracking surfaces. Check local sources such as solar system installers, universities, airports, or government agencies to see if they are collecting such data or know where you might obtain these values. If measured values on a tilted surface are not available, you may use the modeled data here. Data for fixed and single-axis tracking surfaces at three tilt angles (latitude and latitude ±15°) are provided. Two-axis tracking data are given also, as well as a set of world maps that show seasonal values of total insolation at the three tilt angles. All data are in units of kilowatt-hours per square meter. This is equivalent to peak sun hours—the number of hours per day when the sun's intensity is one kilowatt per square meter.

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Q: How long do PV systems last?

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.

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Q: How much electricity does a photovoltaic (PV) system generate?

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".
Download Acrobat Reader.

Q: What does energy conversion efficiency mean?

A: Some of the following documents are available as Adobe Acrobat PDFs. Download Acrobat Reader.

Energy conversion efficiency is an expression of the amount of energy produced in proportion to the amount of energy consumed, or available to a device. The sun produces a lot of energy in a wide light spectrum, but we have so far learned to capture only small portions of that spectrum and convert them to electricity using photovoltaics. So, today's commercial PV systems are about 7% to 17% efficient, which might seem low. And many PV systems degrade a little bit (lose efficiency) each year upon prolonged exposure to sunlight. For comparison, a typical fossil fuel generator has an efficiency of about 28%.

We're working on ways to convert more of the energy in sunlight to usable energy and increase the efficiency of PV systems, however. Some experimental PV cells now convert nearly 40% of the energy in light to electricity. In solar thermal systems (like solar water-heating roof panels), efficiency goes down as the solar heat is converted to a transfer medium such as water. Also, some of the heat radiates away from the system before it can be used.

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

DOE's Building America Program site provides information about how to reduce energy use in homes.

Links to DOE sites that provide information about solar energy.

NREL's High-Performance Building Research provides information about efforts to reduce energy consumption in residential and commercial buildings by integrating passive solar, energy efficiency, and renewable energy technologies.

Energy Savers: Tips on Saving Energy and Money at Home. (Brochure) August 1998. You can download copies of the brochure from this location. Hard copies of the brochure are also available through the EERE Clearinghouse, 1-800-DOE-3732.

Consumer's Guide to Buying a Solar Electric System. September 1999. NREL PDF 155 KB).

SNL's Photovoltaic Program site provides information about the various applications of PV systems.

The Solar Electric House: A Design Manual for Home-Scale Photovoltaic Power Systems by Steven Strong and William Scheller. Sustainability Press. 1993. To order contact NC Solar Center at Box 7401, NCSU, Raleigh, NC 27695, 919-515-5778. The fee for the book is $21.95.

Home Power Magazine: The Hands-On Journal of Home-Made Power

ASES's Solar Today Magazine

The New Solar Electric Home: The Photovoltaics How-To Handbook — Joel Davidson. 1990. To order contact American Solar Energy Society, 2400 Central Ave., G-1, Boulder, CO 80301, 303-443-3130. The fee for the book is $18.95 for members and $16.95 for non-members.

Florida Solar Energy Center.

Solar Energy Industry Association.

Sustainable Buildings Industries Council – 202/628-7400

U.S. Department of Energy
1000 Independence Avenue, SW
Washington, DC 20585
www.eere.energy.gov www.energy.gov