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Solar PowerFrequently Asked Questions :: Photovoltaics

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


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: 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.

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%.

Experts are 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: Where are photovoltaic (PV) systems being used?

A: PV (solar electric) systems are generating clean electric power all over the world, both here and abroad. Today, we can see PV systems at work on urban skyscrapers like 4 Times Square in New York City and in the small rural villages of Brazil . PV systems are especially well suited for places where an electrical grid cannot be accessed easily or where access is too expensive.

In many remote areas, PV is the least-cost option for meeting energy needs. However, PV is proving to be a reliable source of power in an ever-growing number of applications, even where there is easy access to power lines. Cost-effective examples of lighting powered by PV include small garden lights, street lights, lighting for recreational areas, highway signs, warning signs and signals, and lighting for businesses and homes. We can find examples of these applications in both the developed and developing world.

PV is ideal (and often used) for water pumping, because water can be pumped into a storage tank during daylight hours and then distributed by gravity whenever it is needed. These systems commonly pump water for livestock watering tanks in remote areas. In some parts of the developing world, entire village water supplies are powered by PV. Other uses include remote monitoring, refrigeration, and energy for small commercial ventures. Virtually any power need can be met with PV.

Other Resources: To learn more about applications of PV and other solar technologies, visit the Solar America Initiative.

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. For an additional aesthetic appeal, some modules resemble traditional roof shingles or can be built right into glass skylights and walls. This building-integrated PV provides a dual-use building material, reduces PV system costs by using the building as the mounting or support structure, and reduces utility bills with on-site power production.

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.

To learn more about financial incentives in your area, visit the Database of State Incentives for Renewable Energy (DSIRE) and contact the Montana Department of Environmental Quality. One caveat: any time you work with a contractor, it is wise to check references.

To find a solar energy system designer or installer in your area, see our list of Montana Renewable Energy Dealers.

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.

Today's modules can be built into glass skylights and walls. Some resemble traditional roof shingles. Architects can use building-integrated PV to design buildings that are environmentally responsive, aesthetically pleasing, and produce their own power. Building-integrated PV provides a dual-use building material, reduces PV system costs by using the building as the mounting or support structure, and reduces utility bills through on-site power production.

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 learn more about financial incentives in your area, visit the Database of State Incentives for Renewable Energy (DSIRE) and contact the Montana Department of Environmental Quality. One caveat: any time you work with a contractor, it is wise to check references.

To find a solar energy system designer or installer in your area, see our list of Montana Renewable Energy Dealers.

Q: Can I design and install a photovoltaic (PV) system myself?

A: Maybe! However, unless you are very handy or experienced in home wiring, we suggest using experienced professionals to design and install anything more than the simplest application, for the following reasons:

  • You might void the manufacturer's warranties
  • You might not have a functional system after spending your hard-earned money on the system
  • Electricity can be dangerous; you might get hurt
  • You might damage your home or appliances during installation

The goal of a stand-alone system designer is to assure customer satisfaction by providing a well-designed, durable system with a 20-year life expectancy (or more). This depends on sound design, specification and procurement of quality components, good engineering and installation practices, and a consistent preventive maintenance program.

System sizing is perhaps the easiest part of achieving a durable PV power system. To determine the correct system size, you must first analyze your electricity loads. In addition to sizing the system correctly, a thorough knowledge of the availability, performance, and cost of components is the key to good system design. Price/performance trade-offs should be made and reevaluated throughout the design process. When you start your design, obtain as much information as you can about the components you might use. After studying all the issues, you can do an initial sizing of the PV system and get some ideas about specifying system components.

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.

Other Resources: To learn more about the federal government's commitment to renewable and energy efficiency, visit the Federal Energy Management Program (FEMP) Website.

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.

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 PDF iconA Consumer's Guide: Get Your Power from the Sun.

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 PV-DesignPro (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).

Q: How is a solar electric system designed, installed, and maintained?

A: You could install a photovoltaic (PV) or solar electric system yourself. But to avoid complications or injury, you will probably want to hire a reputable professional contractor with experience in installing solar systems. PV systems have few moving parts, so they require little maintenance. The components are designed to meet strict dependability and durability standards so they can stand up to the elements. However, they are fairly sophisticated electric systems, so installation usually requires the knowledge and experience of a licensed electrical equipment contractor.

Although the initial cost for a PV system can be relatively high, by taking advantage of available financing, a complete system will pay for itself in a short time. Many PV panels have a life expectancy of 30 years or more! And many utilities are realizing that reduced utility-generated electricity demand is one of the benefits of PV systems, and some offer incentives to building owners to install PV.

Other Resources: For tips on saving energy and using solar and other renewable energy technologies in your home, visit the U.S. Department of Energy's consumer information Web pages

Other recommended sources include:
The North Carolina Solar Center
Home Power Magazine
Solar Today Magazine
Florida Solar Energy Center

Q: Where can I find someone who designs, installs, and maintains PV systems?

A: Check out the list of Montana Renewable Energy Dealers. You can also look for a PV installer or equipment provider in the telephone directory under "Solar Energy Equipment and Systems Dealers."

It is a good idea select a designer or installer of solar energy systems from the list by first asking for information from several of them about their experience with PV systems as well as how much their services and products cost. With a system designer, you can discuss power requirements or hot water needs for your building, sunlight availability, and other important factors, and determine the type of system that's needed to meet your needs. System designers and installers should be able to provide you with cost estimates and other pertinent information.

Q: Will a PV system on my house reduce my power bills?

A: Yes, but the amount of savings depends on the size of the system and the amount of solar potential at the site, especially if efficiency measures are included. Adding insulation, changing to compact fluorescent lights, and replacing old appliances with high-efficiency models can dramatically lower your power bills. Many customers are able to watch their electricity meters run backwards as their homes deliver electricity back to the utility system at times when their own energy needs are low.

For more information on ways to increase your home's energy efficiency, see:

Q: Are hybrid systems that use both PV and wind available?

A: Yes. Hybrid systems are configured on the type and size of the individual applications. The theory behind hybrid systems is to combine resources and to provide power during time periods when either the wind isn't blowing or the sun isn't shining. On a cost basis in a net-metered application, hybrid systems aren't as practical as in a battery storage application. For more information, see DOE's Small Hybrid Solar and Wind Electric Systems.

Q: Can I both net meter a system and use batteries?

A: Yes. Some customers who want security and a "back-up" in the case of possible emergencies and outages can install additional components that provide power to batteries. The extra components include batteries, a battery charge controller, and separate subpanel(s) for critical load circuits. The number of appliances the homeowner wants to power in the event of a blackout dictates the additional expenses of the system. In the case of NorthWestern Energy, additional components and safeguards to prevent transfer of power to the utility grid are required and will be validated prior to hook-up.

Q: How do I perform a cost analysis for a system?

A: There is no finite answer for how to perform a cost analysis that is 100% accurate. However, customers should either base their analysis on present values or on predicted values (on both sides of the equation).

For simplicity purposes, if calculating payback on a system using present values, the equation would include the cost of your current electricity bill (adjusted at 3% inflation per year). In this case, if the system was being financed, interest also would have to be included.

However, as customers realize, the production costs of electric energy have undergone several large increases in recent history. This is not a controllable variable for NorthWestern Energy, and the future price for residential electricity is uncertain. With a home renewable energy system, cost spikes would be of less importance because, although the electricity that would need to be purchased by the homeowner would be indicative of the current market price, the power transferred back to the utility would also be accounted for at the current wholesale market price.

Q: With all the ratings, capacity factors, and differing inputs regarding the amount of output I can expect, what is a realistic, attainable production number?

A: Although all of the variables do become confusing, and sometimes different opinions originate from different perspectives, the majority of research and data suggest the 7-18% of rated capacity amount as a reasonable figure for Montana PV and small wind applications. This is a "bottom line" number and assumes that you are purchasing and installing a reputable generator and system components.

Q: Are there any disadvantages to using solar energy?

A: The energy in sunlight can be used for many purposes, including heating water for a building or swimming pool. And using solar energy has many environmental and life-cycle economic benefits. However, solar energy heating or solar electric products often have higher first costs than other, similar products do. This means it will probably cost more initially to purchase and install a solar system than it will to purchase and install another kind of heating or electric system. Still, in nearly all cases, you will recover your initial costs through substantial fuel savings (as shown in lower utility bills) over the life of the product. Many solar systems last 15 to 30 years.

Q: What is the energy payback for PV?

A: Producing electricity with photovoltaics (PV) emits no pollution, produces no greenhouse gases, and uses no finite fossilfuel resources. The environmental benefits of PV are great. But just as we say that it takes money to make money, it
also takes energy to save energy. The term “energy payback” captures this idea. How long does a PV system have to
operate to recover the energy—and associated generation of pollution and CO2—that went into making the system,
in the first place?

Energy payback estimates for rooftop PV systems are 4, 3, 2, and 1 years: 4 years for systems using current multicrystalline-silicon PV modules, 3 years for current thin-film modules, 2 years for anticipated multicrystalline modules, and 1 year for anticipated thin-film modules. With energy paybacks of 1 to 4 years and assumed life
expectancies of 30 years, 87% to 97% of the energy that PV systems generate won’t be plagued by pollution, greenhouse
gases, and depletion of resources. Read NREL's full fact sheet.


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