Q: What is photovoltaics (solar electricity), or
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.
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
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)
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
Q: What's the difference between PV and other solar energy
A: There are four main types of solar
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
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.
does sunlight affect 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 hoursthe number of hours per day when the
sun's intensity is one kilowatt per square meter.
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.
Q: How much electricity does a photovoltaic (PV) system
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".
Q: What does energy conversion efficiency mean?
A: Some of the following documents are
available as Adobe Acrobat PDFs. Download
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
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
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
Building America Program site provides information about how to
reduce energy use in homes.
Links to DOE sites that provide
information about solar energy.
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,
Consumer's Guide to Buying a Solar Electric
System. September 1999. NREL (PDF 155 KB).
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
Sustainable Buildings Industries Council