Q: What is Photovoltaics (PV)?

A: Also known as Solar Electric, PV covers a range of technologies. The common element of these different formats is derived from the word Photovoltaic. Photo, derived from the Greek word Phos means light, while Volt, a unit of measurement is named after Alessandro Volta. Together, the literal translation/common element is Light-Energy.

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: 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.

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.

Q: What does energy conversion efficiency mean?

A: 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.

Q: How much space would be needed for photovoltaic systems to meet the entire electrical needs of the United States?

A: If PV were a primary energy source, what would the world look like? Would PV collectors cover every square inch of available land? Contrary to some popular notions, the landscape of a world relying on PV would be almost indistinguishable from the landscape we know today. The impact of PV on the landscape would be low, for three reasons. First, PV systems have siting advantages over other technologies; for example, PV can be put on roofs and can even be an integral part of a building, such as a skylight. Second, even ground-mounted PV collectors are efficient from the perspective of land use. Third, adequate sunlight is ubiquitous and often abundant, and present in predictable amounts almost everywhere. As we move away from fossil-fuel energy, PV will become important because of its land-use advantages:PV's low-impact sitting for flat-plate systems. In the United States, cities and residences cover about 140 millionacres of land. We could supply every kilowatt-hour of our nation's current energy requirements simply by applying PV to 7% of this area—on roofs, on parking lots, along highway walls, on the sides of buildings, and in other dual-use scenarios. We wouldn't have to appropriate a single acre of new land to make PV our primary energy source!  PV's efficient ratio of produced energy to land use. Even if it isn't installed on rooftops, flat-plate PV technology is the most land-efficient means to produce renewable energy.

PV has competitive conversion efficiencies, a high capacity factor, and can be "packed" densely in a given area. We still wouldn't have a land use issue, even if we didn't use roofs for PV. We would need only 10 million acres of land — only four-tenths of one percent of the area of the United States — to supply all of our nation's energy using PV.  Is that a lot of land? Not for something as important as producing electricity, and not in comparison to some of the other ways we use land.

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. Remember: an energy-efficient building requires a smaller PV system.

Q: How big a solar energy system do I need?

A: The size of solar system you need depends on several factors such as how much electricity or hot water or space heat you use, how much sunshine is available where you are, the size of your roof, and how much you're willing to invest. You can contact a system designer/installer like those listed in our PV Directory or other solar industry directories to determine what type of system would suit your needs. You can also check out DOE's Building Energy Tools Directory for energy analysis tools such as PVDesignPro (photovoltaic design, tracking systems, solar, electrical design) or RETScreen (pre-feasibility analysis, heating, renewable energy). PC-Solar 2.0 provides passive solar calculations (solar shading, external shading, internal shading, solar incidence).

(DOE, 2008)