What is solar energy | common uses of solar energy | How Does Solar Work

solar panels
solar panels


solar energy, radiation from the Sun capable of producing heat, causing chemical reactions, or generating electricity. The total amount of solar energy incident on Earth is vastly in excess of the world’s current and anticipated energy requirements. If suitably harnessed, this highly diffused source has the potential to satisfy all future energy needs. In the 21st century solar energy is expected to become increasingly attractive as a renewable energy source because of its inexhaustible supply and its nonpolluting character, in stark contrast to the finite fossil fuels coal, petroleum, and natural gas.

The Sun is an extremely powerful energy source, and sunlight is by far the largest source of energy received by Earth, but its intensity at Earth’s surface is actually quite low. This is essentially because of the enormous radial spreading of radiation from the distant Sun. A relatively minor additional loss is due to Earth’s atmosphere and clouds, which absorb or scatter as much as 54 percent of the incoming sunlight. The sunlight that reaches the ground consists of nearly 50 percent visible light, 45 percent infrared radiation, and smaller amounts of ultraviolet and other forms of electromagnetic radiation.

Reflection and absorption of solar energy. Although some incoming sunlight is reflected by Earth's atmosphere and surface, most is absorbed by the surface, which is warmed.
Reflection and absorption of solar energy. Although some incoming sunlight is reflected by Earth's atmosphere and surface, most is absorbed by the surface, which is warmed.

Reflection and absorption of solar energy. Although some incoming sunlight is reflected by Earth's atmosphere and surface, most is absorbed by the surface, which is warmed. 

The potential for solar energy is enormous, since about 200,000 times the world’s total daily electric-generating capacity is received by Earth every day in the form of solar energy. Unfortunately, though solar energy itself is free, the high cost of its collection, conversion, and storage still limits its exploitation in many places. Solar radiation can be converted either into thermal energy (heat) or into electrical energy, though the former is easier to accomplish.

What are the common uses of solar energy?

Solar energy is commonly used for solar water heaters and house heating. The heat from solar ponds enables the production of chemicals, food, textiles, warm greenhouses, swimming pools, and livestock buildings. Cooking and providing a power source for electronic devices can also be achieved by using solar energy.

How is solar energy collected?

The most common devices used to collect solar energy and convert it to thermal energy are flat-plate collectors. Another method of thermal energy conversion is found in solar ponds, which are bodies of salt water designed to collect and store solar energy. Solar radiation may also be converted directly into electricity by solar cells, or photovoltaic cells, or harnessed to cook food in specially designed solar ovens, which typically concentrate sunlight from over a wide area to a central point.

Thermal energy

Among the most common devices used to capture solar energy and convert it to thermal energy are flat-plate collectors, which are used for solar heating applications. Because the intensity of solar radiation at Earth’s surface is so low, these collectors must be large in area. Even in sunny parts of the world’s temperate regions, for instance, a collector must have a surface area of about 40 square metres (430 square feet) to gather enough energy to serve the energy needs of one person. The most widely used flat-plate collectors consist of a blackened metal plate, covered with one or two sheets of glass, that is heated by the sunlight falling on it. This heat is then transferred to air or water, called carrier fluids, that flow past the back of the plate. The heat may be used directly, or it may be transferred to another medium for storage. Flat-plate collectors are commonly used for solar water heaters and house heating. The storage of heat for use at night or on cloudy days is commonly accomplished by using insulated tanks to store the water heated during sunny periods. Such a system can supply a home with hot water drawn from the storage tank, or, with the warmed water flowing through tubes in floors and ceilings, it can provide space heating. Flat-plate collectors typically heat carrier fluids to temperatures ranging from 66 to 93 °C (150 to 200 °F). The efficiency of such collectors (i.e., the proportion of the energy received that they convert into usable energy) ranges from 20 to 80 percent, depending on the design of the collector. 

A building roof with flat-plate collectors that capture solar energy to heat air or water.
A building roof with flat-plate collectors that capture solar energy to heat air or water.
Alan Mak

Another method of thermal energy conversion is found in solar ponds, which are bodies of salt water designed to collect and store solar energy. The heat extracted from such ponds enables the production of chemicals, food, textiles, and other industrial products and can also be used to warm greenhouses, swimming pools, and livestock buildings. Solar ponds are sometimes used to produce electricity through the use of the organic Rankine cycle engine, a relatively efficient and economical means of solar energy conversion, which is especially useful in remote locations. Solar ponds are fairly expensive to install and maintain and are generally limited to warm rural areas. 

On a smaller scale, the Sun’s energy can also be harnessed to cook food in specially designed solar ovens. Solar ovens typically concentrate sunlight from over a wide area to a central point, where a black-surfaced vessel converts the sunlight into heat. The ovens are typically portable and require no other fuel inputs.

A solar-powered cookstove in the Potala Palace, Lhasa, Tibet.
A solar-powered cookstove in the Potala Palace, Lhasa, Tibet.
© Elena Slepitskaya/Dreamstime.com


Electricity generation

Solar radiation may be converted directly into electricity by solar cells (photovoltaic cells). In such cells, a small electric voltage is generated when light strikes the junction between a metal and a semiconductor (such as silicon) or the junction between two different semiconductors. (See photovoltaic effect.) The power generated by a single photovoltaic cell is typically only about two watts. By connecting large numbers of individual cells together, however, as in solar-panel arrays, hundreds or even thousands of kilowatts of electric power can be generated in a solar electric plant or in a large household array. The energy efficiency of most present-day photovoltaic cells is only about 15 to 20 percent, and, since the intensity of solar radiation is low to begin with, large and costly assemblies of such cells are required to produce even moderate amounts of power. 

When sunlight strikes a solar cell, an electron is freed by the photoelectric effect. The two dissimilar semiconductors possess a natural difference in electric potential (voltage), which causes the electrons to flow through the external circuit, supplying power to the load. The flow of electricity results from the characteristics of the semiconductors and is powered entirely by light striking the cell.
When sunlight strikes a solar cell, an electron is freed by the photoelectric effect. The two dissimilar semiconductors possess a natural difference in electric potential (voltage), which causes the electrons to flow through the external circuit, supplying power to the load. The flow of electricity results from the characteristics of the semiconductors and is powered entirely by light striking the cell.

Small photovoltaic cells that operate on sunlight or artificial light have found major use in low-power applications—as power sources for calculators and watches, for example. Larger units have been used to provide power for water pumps and communications systems in remote areas and for weather and communications satellites. Classic crystalline silicon panels and emerging technologies using thin-film solar cells, including building-integrated photovoltaics, can be installed by homeowners and businesses on their rooftops to replace or augment the conventional electric supply. 

Single-family house with solar panels on the roof.
Single-family house with solar panels on the roof.
© Andreas Weber/Dreamstime.com

Concentrated solar power plants employ concentrating, or focusing, collectors to concentrate sunlight received from a wide area onto a small blackened receiver, thereby considerably increasing the light’s intensity in order to produce high temperatures. The arrays of carefully aligned mirrors or lenses can focus enough sunlight to heat a target to temperatures of 2,000 °C (3,600 °F) or more. This heat can then be used to operate a boiler, which in turn generates steam for a steam turbine electric generator power plant. For producing steam directly, the movable mirrors can be arranged so as to concentrate large amounts of solar radiation upon blackened pipes through which water is circulated and thereby heated. 

Other applications

Solar energy is also used on a small scale for purposes other than those described above. In some countries, for instance, solar energy is used to produce salt from seawater by evaporation. Similarly, solar-powered desalination units transform salt water into drinking water by converting the Sun’s energy to heat, directly or indirectly, to drive the desalination process.

Solar technology has also emerged for the clean and renewable production of hydrogen as an alternative energy source. Mimicking the process of photosynthesis, artificial leaves are silicon-based devices that use solar energy to split water into hydrogen and oxygen, leaving virtually no pollutants. Further work is needed to improve the efficiency and cost-effectiveness of these devices for industrial use.

How Does Solar Work?

The amount of sunlight that strikes the earth's surface in an hour and a half is enough to handle the entire world's energy consumption for a full year. Solar technologies convert sunlight into electrical energy either through photovoltaic (PV) panels or through mirrors that concentrate solar radiation. This energy can be used to generate electricity or be stored in batteries or thermal storage.

Below, you can find resources and information on the basics of solar radiation, photovoltaic and concentrating solar-thermal power technologies, electrical grid systems integration, and the non-hardware aspects (soft costs) of solar energy. You can also learn more about how to go solar and the solar energy industry. In addition, you can dive deeper into solar energy and learn about how  the U.S. Department of Energy Solar Energy Technologies Office is driving innovative research and development in these areas.

Solar Energy 101

Solar radiation is light – also known as electromagnetic radiation – that is emitted by the sun. While every location on Earth receives some sunlight over a year, the amount of solar radiation that reaches any one spot on the Earth’s surface varies. Solar technologies capture this radiation and turn it into useful forms of energy.

There are two main types of solar energy technologies—photovoltaics (PV) and concentrating solar-thermal power (CSP). 

Photovoltaics Basics

You're likely most familiar with PV, which is utilized in solar panels. When the sun shines onto a solar panel, energy from the sunlight is absorbed by the PV cells in the panel. This energy creates electrical charge

Concentrating Solar-Thermal Power Basics

Concentrating solar-thermal power (CSP) systems use mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat, which can then be used to produce electricity or stored for later use. It is used primarily in very large power plants.

Systems Integration Basics

Solar energy technology doesn’t end with electricity generation by PV or CSP systems. These solar energy systems must be integrated into homes, businesses, and existing electrical grids with varying mixtures of traditional and other renewable energy sources.

Solar Technologies

There are three main ways to harness solar energy: photovoltaics, solar heating & cooling, and concentrating solar power. Photovoltaics generate electricity directly from sunlight via an electronic process and can be used to power anything from small electronics such as calculators and road signs up to homes and large commercial businesses. Solar heating & cooling (SHC) and concentrating solar power (CSP) applications both use the heat generated by the sun to provide space or water heating in the case of SHC systems, or to run traditional electricity-generating turbines in the case of CSP power plants.

How solar is used

Solar energy is a very flexible energy technology: it can be built as distributed generation (located at or near the point of use) or as a central-station, utility-scale solar power plant (similar to traditional power plants). Both of these methods can also store the energy they produce for distribution after the sun sets, using cutting edge solar + storage technologies. Solar exists within a complex and interrelated electricity system in the U.S., working alongside other technologies like wind power to transition the U.S. to a clean energy economy.

All of these applications depend on supportive policy frameworks at the local, state and federal level to ensure consumers and businesses have fair access to clean energy technologies like solar.

The solar market today

There are nearly 114 gigawatts (GW) of solar installed in the U.S., enough to power 21.8 million homes. Over the last decade, the solar market in the United States has grown at an average rate of 42% each year. There are more than 3 million individual solar installations in the U.S., ranging from small home rooftop systems to large utility-scale systems that add hundreds of megawatts of clean electricity to the power grid.

Collecting and using solar thermal (heat) energy

An example of an early solar energy collection device is the solar oven (a box for collecting and absorbing sunlight). In the 1830s, British astronomer John Herschel used a solar oven to cook food during an expedition to Africa. People now use many different technologies for collecting and converting solar radiation into useful heat energy for a variety of purposes.

We use solar thermal energy systems to heat:

Water for use in homes, buildings, or swimming pools

The inside of homes, greenhouses, and other buildings

Fluids to high temperatures in solar thermal power plants

Solar photovoltaic systems convert sunlight into electricity

Solar photovoltaic (PV) devices, or solar cells, change sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Arrangements of many solar cells in PV panels and arrangements of multiple PV panels in PV arrays can produce electricity for an entire house. Some PV power plants have large arrays that cover many acres to produce electricity for thousands of homes.

Solar energy has benefits and some limitations

Using solar energy has two main benefits:

Solar energy systems do not produce air pollutants or carbon dioxide.

Solar energy systems on buildings have minimal effects on the environment.

Solar energy also has some limitations:

The amount of sunlight that arrives at the earth's surface is not constant. The amount of sunlight varies depending on location, time of day, season of the year, and weather conditions.

The amount of sunlight reaching a square foot of the earth's surface is relatively small, so a large surface area is necessary to absorb or collect a useful amount of energy.

How does solar energy work? What is solar power used for?

There are many ways to use energy from the sun. The two main ways to use energy from the sun are photovoltaics and solar thermal capture. Photovoltaics are much more common for smaller-scale electricity projects (like residential solar panel installations), and solar thermal capture is typically only used for electricity production on massive scales in utility solar installations. In addition to producing electricity, lower temperature variations of solar thermal projects can be used for heating and cooling.

Solar is one of the fastest-growing and cheapest sources of power in the world and will continue to spread rapidly in the coming years. With solar panel technology improving each year, the economic benefits of solar improve, adding to the environmental perks of choosing a clean, renewable energy source.

Photovoltaic solar energy

A common way for property owners to take advantage of solar energy is with a photovoltaic (PV) solar system. With a solar PV system, solar panels convert sunlight right into electricity that can be used immediately, stored in a solar battery, or sent to the electric grid for credits on your electric bill.

Solar panels convert solar energy into usable electricity through a process known as the photovoltaic effect. Incoming sunlight strikes a semiconductor material (typically silicon) and knocks electrons loose, setting them in motion and generating an electric current that can be captured with wiring. This current is known as direct current (DC) electricity and must be converted to alternating current (AC) electricity using a solar inverter. This conversion is necessary because the U.S. electric grid operates using AC electricity, as do most household electric appliances.

Solar energy can be captured at many scales using photovoltaics, and installing solar panels is a smart way to save money on your electric bill while reducing your dependence on nonrenewable fossil fuels. Large companies and electric utilities can also benefit from photovoltaic solar energy generation by installing large solar arrays that can power company operations or supply energy to the electric grid.

Solar thermal

A second way to use solar energy is to capture the heat from solar radiation directly and use that heat in a variety of ways. Solar thermal energy has a broader range of uses than a photovoltaic system, but using solar thermal energy for electricity generation at small scales is not as practical as using photovoltaics.

There are three general types of solar thermal energy used: low-temperature, used for heating and cooling; mid-temperature, used for heating water; and high-temperature, used for electrical power generation.

Low-temperature solar thermal energy systems involve heating and cooling air as a means of climate control. An example of this type of solar energy usage is in passive solar building design. In properties built for passive solar energy use, the sun’s rays are allowed into a living space to heat an area and blocked when the area needs to be cooled.

Mid-temperature solar thermal energy systems include solar hot water heating systems. In a solar hot water setup, heat from the sun is captured by collectors on your rooftop. This heat is then transferred to the water running through your home’s piping so you don’t have to rely on traditional water heating methods, such as water heaters powered with oil or gas.

High-temperature solar thermal energy systems are used for generating electricity on a larger scale. In a solar thermal electricity plant, mirrors focus the sun’s rays on tubes containing a liquid that can hold heat energy well. This heated fluid can then be used to turn water into steam, which then can turn a turbine and generate electricity. This type of technology is often referred to as concentrated solar power.

Take advantage of solar energy on your property

The best way for individual property owners to save money with solar energy is to install a home solar photovoltaic system. To find the right system for the right price, you should shop on the Energy Sage Solar Marketplace. After signing up, you will receive free solar quotes from qualified, pre-vetted solar installers near you. Looking at quotes in our apples-to-apples setup is a great way to understand offers and compare key metrics such as energy needs met and cost per watt.

Frequently asked questions about solar power

There’s a lot to solar energy – here are a few quick answers to common questions about what solar energy is:

How does solar energy work?

Solar power generation starts when solar panels absorb photons, or particles of light, with photovoltaic cells, generating this direct current (DC) energy and then converting it to usable alternating current (AC) energy with the help of inverter technology. AC energy then flows through the home’s electrical panel and is distributed accordingly.

What can solar power be used for?

Solar power is used in two main ways: generating electricity (like with rooftop solar panels) or generating thermal energy (like with concentrated solar power plants). For most homeowners, solar panels that convert solar energy to electricity is the best use of solar energy – you can save on your electric bills with a home solar installation.

How long do solar panels last?

Solar panels usually last for 25 to 30 years before they start to see steeper drops in performance than is useful for many homes. For those almost three decades, however, you’ll be generating free solar electricity from the sun.

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