Frequently Asked Questions About Solar Photovoltaic and Solar Thermal (Hot Water) Systems
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: 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.
Q: What's the difference between PV and other solar energy technologies?
A: There are four main types of solar energy technologies:
- Photovoltaic (PV) systems, which convert sunlight directly to electricity by means of PV cells made of semiconductor materials.
- 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.
- 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.
- Transpired solar collectors, or "solar walls," which use solar energy to preheat ventilation air for a building.
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
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.
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. Shading on the system can significantly reduce energy output. 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 enough sunlight to make solar energy systems useful and effective nearly everywhere in California.
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, the size of your roof, and how much you're willing to invest. Also, do you want the system to supply your complete energy usage or to supplant a portion of your higher cost energy usage? You can contact a system designer/installer to determine what type of system would suit your needs.
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.
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.
Q: Where can I find someone who designs, installs, and maintains photovoltaic (PV) systems?
A: We suggest you 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 in your local yellow pages 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.
If your house is not yet designed or built, it is important to make the building as energy efficient as possible to reduce your PV system's energy requirements.
Q: How much does a solar energy system cost, and how much will I save on utility bills?
A: Some of the following documents are available as Adobe Acrobat PDFs. Download Acrobat Reader.
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.
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.
Q: How does a solar water-heating system work?
A: Every solar water-heating system features a solar collector that faces the sun to absorb the sun's heat energy. This collector can either heat water directly or heat a "working fluid" that's then used to heat the water. In active solar water-heating systems, a pumping mechanism moves heated water through the building. In passive solar water-heating systems, the water moves by natural convection. In almost all cases, solar water-heating systems work in tandem with conventional gas or electric water-heating systems; the conventional systems operate as needed to ensure a reliable supply of heated water.
There are many types of solar water heaters. Each has strengths to recommend it for specific climates and water conditions. Solar system professionals can help you select the most appropriate system for your area and your needs.
Q: Can a solar water heater replace an electric or gas water heater?
A: Not completely. Conventional electric or gas water heating systems are still necessary as a supplement to the solar water heating system, largely because the sun might not shine in a particular area for several days at a time. However, because solar water heaters are designed provide hot water directly to the tank of a gas or electric water heater, they reduce the need for the water heater to run on conventional fuels. And this in turn reduces your gas or electric bill. Depending on where you live, solar water heaters can provide up to 80% of your home's annual water-heating needs.
Q: Can I use a solar water-heating system to heat my swimming pool?
A: Using a solar system to heat a swimming pool is the most common use for solar energy in the United States today. Solar pool-heating systems increase an unheated pool's water temperature by 10 degrees or more, and they can extend the swimming season by two to three months.
Q: What are the benefits of using solar energy to heat water in my home?
A: First, the fuel is free! Once you recover the higher initial costs of a solar system through reduced or avoided energy costs (that is, lower utility bills), your solar system will require expenditures only for maintenance. And when you include the cost of a solar water heater in a mortgage on a new home, the system often provides a positive monthly cash flow from the first day of ownership.
Second, solar water heaters and other solar technology applications do not pollute. They do not add to the carbon dioxide, nitrogen oxides, sulfur dioxide, and other air pollutants and wastes produced by most of today's power plants, even those that run on natural gas. And they allow you to burn less natural gas in your home, as well.
Q: How much does a solar water-heating system cost?
A: Unfortunately, there is no one answer to this question. The cost of a solar system depends on a number of factors, such as the size of the system and the particular system manufacturer, retailer, and installer. However, any solar rebates and other incentives available in your area will reduce that total cost.
For solar water heaters and space heaters, you will also be taking into consideration the price of the fuel used to back up the system. In most cases, you will have to add in the cost of supplemental natural gas or electricity to get a fairly accurate estimate of how much you can expect to pay for a solar system.
Q: How much money will a solar water-heating system save on my utility bill?
A: It is difficult to say how much you will save with a solar system. That depends on several factors, including how much you already pay your local utility for electricity or natural gas. You can ask your solar system professional how much heat your new system will produce on an annual basis and then subtract that number from your current annual consumption—the total amount of electricity and gas you use—to get an idea of how much you will save. Data on your current annual consumption should be available from your utility.