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Solar FAQs > Photovoltaics (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.

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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.

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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.

Other Resources: 

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Q: How 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 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.

Other Resources: 

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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.

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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.

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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.

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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.

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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.

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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.

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Q: When will I be able to buy a solar electric or PV-powered car?

A: Because most automobiles are very heavy and aren't very efficient, it would be very difficult to power one with solar cells. But if a car were built specifically for PV, it could provide suitable transportation. As we've seen in many student-built vehicles for competitions, solar cars are very light and efficient and have enough battery storage to travel for miles on a cloudy day or at night. Solar cars can travel the speed limit on normal highways, but only as long as the sun is shining or until their batteries run down.

A more realistic car would be another kind of electric vehicle, one that many companies are working on right now. These cars could be charged by solar panels during the day, for example, while people are at work. They could also be plugged in at home for charging when the sun is not shining. To see an example of this, please visit "grid-connected PV" in the PV in Use pages.

The benefits of solar cars are obvious: they don't pollute, and free sunlight is their fuel. The drawbacks are that, using today's technology, a solar car has to be very lightweight for the panels to provide enough energy to power the car at road speeds, and it has to have enough battery storage to travel long distances without sunlight (e.g., at night and on overcast days).

As part of continued research and development, many organizations are improving the systems used in solar cars to make them more efficient and cost effective and thus more widely used (the systems, anyway). Wider use of solar electric cars (or other electrics) probably depends on the availability of inexpensive, lightweight, compact energy storage methods. Car companies are making great strides in this area with the new gas/electric hybrids, and future progress is likely to be rapid.

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Q: When will solar electric systems replace coal and nuclear power plants?

A: Right now, our nuclear and fossil-fuel-based energy is quite inexpensive compared with the cost of solar energy. Oil and coal prices are low in most places, so solar energy still can't compete on a first-cost basis in many regions of the world, such as the United States. As this situation changes, we'll begin to see many more solar energy systems being built in areas that now use fossil fuels and nuclear energy for electricity generation.

Another driver in the deployment of solar systems is public demand for clean energy. Fossil-based energy pollutes the environment, and nuclear energy creates hazardous waste. If we stop to consider the environmental and health costs of fossil-fuel and nuclear energy, then solar energy already makes sense today.

However, in developing countries where there is little or no supply system for conventional energy, solar energy is being used more and more. It can be much less expensive than many other options, and the environmental benefits associated with this cleaner form of energy are significant. In developing countries, the key barriers to wider use are the need for financing and for electric distribution networks.

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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 siting for flat-plate systems. In the United States, cities and residences cover about 140 million acres 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 a competitive conversion efficiency, 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.

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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.

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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.

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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).

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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.

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Q: Why should I use photovoltaics (PV)?

A: For a growing number of users, PV is the clear choice. You should definitely consider using a PV system if it operates better and costs less than the alternatives. The cost of energy produced by PV systems continues to drop. However, kilowatt-hour for kilowatt-hour, and depending on where you live, PV energy still usually costs more than energy from your local utility. Also, the initial cost of PV equipment is higher than that of an engine generator. But there are many applications for which a PV system is the most cost-effective long-term option, such as for power in remote areas.

The number of installed PV systems increases each year because their many advantages make them the best option overall. Consider the following issues:

Site Access - A well-designed PV system will operate unattended and requires minimum periodic maintenance. The savings in labor costs and travel expenses can be significant.
Modularity - A PV system can be designed for easy expansion. If your power demand could increase in future years, the ease and cost of increasing the PV power supply should be considered.
Fuel Supply - Supplying conventional fuel to the site and storing it can be much more expensive than the fuel itself. Solar energy is delivered free of charge!
Environment - PV systems create no pollution and generate no waste products when operating.
Maintenance - Any energy system requires maintenance, but experience shows that PV systems require less maintenance than other alternatives.
Durability - Most of today's PV modules are based on a proven technology that has experienced little degradation in more than 15 years of operation.
Cost - For many applications, the advantages of PV systems offset their relatively high initial cost.

System designers know that every decision made during the design of a PV system affects the cost. If the system is oversized because the design was based on unrealistic requirements, the initial cost is unnecessarily high. If less durable parts are specified, maintenance and replacement costs will increase. The overall system life-cycle cost (LCC) estimates can easily double if inappropriate choices are made during system design. Don't let unrealistic specifications or poor assumptions create unreasonable cost estimates and keep you from using this clean power source. As you size your PV system, be realistic and flexible, and select an experienced designer to assist you.

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Q: Why should I purchase a PV system?

A: People decide to buy solar energy systems for a variety of reasons. For example, some individuals buy solar products to preserve the Earth's finite fossil-fuel resources and to reduce air pollution. Others would rather spend their money on an energy-producing improvement to their property than send their money to a utility. Some people like the security of reducing the amount of electricity they buy from their utility, because it makes them less vulnerable to future increases in the price of electricity.

If it's designed correctly, a solar system might be able to provide power during a utility power outage, thereby adding power reliability to your home. Finally, some individuals live in areas where the cost of extending power lines to their home is more expensive than buying a solar energy system.

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Q: Are there disadvantages to using solar energy?

A: Solar energy technologies often have a higher "first cost." This means that a person is likely to pay more money up front to purchase and install a solar system. Still, in nearly all cases, the high initial cost is recovered through substantial fuel savings over the life of the product (15-30 years).

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Q: How much does a solar energy system cost, and how much will I save on utility bills?

A:  Unfortunately, there is no single or simple answer. But a solar rebate and other incentives can reduce the cost of a PV system. This cost depends on a number of factors, such as whether it is a stand-alone system or is integrated into the building design, the size of the system, and the particular system manufacturer, retailer, and installer. For solar water heaters and space heaters, you also have to consider the price of the fuel used to back up the system. In most cases, you would have to add the cost of natural gas or electricity to get a more accurate estimate of how much you can expect to pay for a solar energy system.

It is also difficult to say how much you will save with a solar energy system, because savings depend on how much you pay your utility for electricity or natural gas, and how much your utility will pay you for any excess power that you generate with your solar system. You can ask your solar system provider how much your new system will produce on an annual basis and compare that number to your annual electricity or hot water demand to get an idea of how much you will save.

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Q: Can I afford photovoltaics (PV)?

A: That depends. Generally, PV energy costs are higher than those of energy bought from your local utility. However, if you need power in an area not served by a utility, PV may be the most cost-effective option. All over the world, PV system installations are increasing. As more people learn about this versatile, clean power option, this trend will continue, bringing costs down and affordability up.

Most of us must consider our goals in light of our wants and needs when determining affordability. Availability is an important determinant, and it has a unique meaning for a PV system. This is because it depends not only on reliable equipment but on the level and consistency of sunshine, and the capabilities of the energy storage system, at your site.

Because the weather is unpredictable, designing a PV system to be available at all times and conditions is expensive and often unnecessary. PV systems with long-term availabilities greater than 95% are routinely achieved at half the cost or less of systems designed to be available 99.99% of the time. Designing for lower availabilities decreases the size of the PV array and batteries and saves many dollars.

Another way to resolve the cost and availability issue is to specify a hybrid system, which includes another energy source (usually one that runs on a fossil fuel such as propane). Although saving money is important, you'll want to acquire a safe system that will last 25 years or more. Quality may cost more initially, but it will save money in the long run.

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Q: How do I find out about financial incentives such as rebates or tax credits in my home state?

A: Several resources are available to help you track down this information.

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

Your U.S. Department of Energy (DOE) regional support office may also be able to provide guidance.

To find a solar energy system designer or installer in your area, please visit Find Solar for a list of sources of solar professionals.

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Q: What resources, grants, loans, and other incentives are there for installing a PV system?

A: A variety of state and local incentives are available. To learn more about energy saving tips and renewable energy technologies, please visit U. S. Department of Energy's Energy Saver's Web site

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

Your U.S. Department of Energy (DOE) regional support office may also be able to provide guidance.

To learn more about the Million Solar Roofs Initiative, an effort to install one million solar energy system by 2010.

To learn more about financing opportunities, please see The Borrower's Guide to Financing Solar Energy Systems: A Federal Overview. September 1998. (PDF 501 KB)

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Q: How can I start my own photovoltaic business?

A: We don't know of any publications that deal specifically with this topic. But you might want to contact the marketing department of the manufacturers of the types of PV products you're interested in, or search the Internet for relevant publications.

Training programs for solar installers are offered by several groups. A Sandia National Laboratories Web site lists many of them www.sandia.gov/pv/training.htm.

For more information about the tax credits, grants, and other incentives available in your area, check the national Database of State Incentives for Renewable Energy (DSIRE). DSIRE is a searchable online renewable energy database that includes state financial incentives, programs and regulatory policies, utility programs and incentives, solar programs in school, local government and community incentives, programs and policies that support renewable energy.

For more information about opportunities to work with DOE and its partners. See also your U.S. Department of Energy (DOE) regional support office . Please visit our education page to learn about adult education and career opportunities in solar energy.

To learn more about the Million Solar Roofs Initiative, an effort to install one million solar energy system by 2010.

To learn more about partnership opportunities with the National Renewable Energy Laboratory, please visit Applying Technologies to learn about partnership opportunities.

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Q: What is net metering? Is net metering available where I live and work?

A: Net metering is a policy that allows homeowners to receive the full retail value for the electricity that their solar energy system produces. The term net metering refers to the method of accounting for the photovoltaic (PV) system's electricity production. Net metering allows homeowners with PV systems to use any excess electricity they produce to offset their electric bill. As the homeowner's PV system produces electricity, the kilowatts are first used for any electric appliances in the home. If the PV system produces more electricity than the homeowner needs, the extra kilowatts are fed into the utility grid.

Currently, more than 30 states have metering programs across the United States. To find out more about the programs in specific states, please check our National Database of State Incentives for Renewable Energy (DSIRE).

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Q: What is the energy payback period for photovoltaics (PV)?

A:  Typically, the energy payback time (i.e., the time it takes for a PV system to generate the same amount of energy that it took to manufacture the system) for PV systems is 2 to 5 years. Since a well-designed and maintained PV system will operate for more than 20 years, and a system without moving parts will operate for close to 30 years, PV systems produce far more energy over their useful life than we use to manufacture them.

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Q: What is an average break-even point for a solar energy system?

A:  It depends. The break-even point for a system depends on financing and incentives, which vary from place to place, and it depends on your solar resources and what you would pay for another source of energy. A system designer that has information about your location, the amount of energy you typically use, how much land or roof area you have for the system, etc., could give you a more accurate answer.

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Q: How can federal facilities make the economics of solar work for them, given the long payback period? Are there financial incentives available to help them pay for solar roofs?

A: Solar energy systems meet federal life-cycle cost requirements in situations where they are providing high-value power or where the cost of alternatives is high. All costs must be considered in comparing solar to other options, however. For remote systems, the government considers the cost of transporting fuel to a site, and the risk and cost of fuel spills during transit or storage. Solar energy is often the least-cost alternative when a project involves building an electricity distribution line. The Federal Energy Management Program (FEMP) has many examples of cost-effective solar installations on its Web site.

Federal facilities may also qualify for state or utility incentive programs for using renewable energy. In Hawaii, the Coast Guard and the Navy have used a utility rebate to buy down the cost of thousands of solar hot water systems. Check the DSIRE Web site for more information about state incentives for renewable energy.

Direct financial support for federal renewable energy projects depends on appropriations. When grants are available, FEMP usually announces them on its Web site or in its newsletter for federal energy managers.

A wide variety of resources are available through DOE FEMP to assist federal agencies in financing solar energy projects. Funding options available for federal projects include the following:

Direct Appropriations – These allow the federal government to retain all of the savings from cost-effective renovations and to avoid incurring interest charges. This method also enables agencies to implement energy efficient projects with minimal contractual obligations.

Energy Saving Performance Contracts (ESPCs) – Here, federal facilities enter into contracts with energy service companies that assume the capital costs of installing energy and water conservation equipment and renewable energy systems in federal facilities; companies are repaid out of the agency's guaranteed energy cost savings over the term of the contract.

Super Energy Saving Performance Contracts (Super ESPC's) – These were developed to make the ESPC process easier. Super ESPC contracts are based on the indefinite delivery, indefinite quantity provision of the Federal Acquisition Regulation.

Utility Incentives – These allow federal facilities to participate in utility incentive programs to finance projects.

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Note: the information in this page comes from U.S Department of Energy EERE website.