Solar basics

Energy from the sun

The sun has been producing energy for billions of years, and it’s the original source for nearly all the energy we use. People have used the sun's rays (solar radiation) for thousands of years for warmth and for drying food. Over time, we’ve developed technologies to capture solar energy for heat and to convert it into electricity.

Solar thermal (heat) energy

A solar oven (a box for collecting and absorbing sunlight) is a simple example of a solar energy collection device. In the 1830s, British astronomer John Herschel used a solar oven to cook during an expedition to Africa. Today, we use many different technologies to capture and convert solar energy into useful energy for various purposes.

We use solar thermal energy systems to heat:

• Water for homes, buildings, or swimming pools
• Air inside homes, greenhouses, and other buildings
• Fluids in solar thermal power plants

Solar photovoltaic systems

Solar photovoltaic (PV) devices, or solar cells, convert sunlight directly into electricity. Small PV cells can power calculators, watches, and other small electronic devices. Larger solar cells are grouped to form PV panels, and PV panels are connected in arrays that can produce electricity for an entire house. Some PV power plants have large arrays covering many acres to generate electricity for thousands of homes.

Benefits and limitations of solar energy

Benefits:

• Solar energy systems do not produce air pollutants or carbon dioxide emissions while operating.
• Solar energy systems on buildings have minimal negative effects on the environment.

Limitations:

• The amount of available sunlight depends on the time of day, location, season, and weather conditions.
• A large surface area is needed to capture enough energy to be useful.

Where solar is found and used

Solar energy is sunshine

Sunshine is radiant energy from the sun. The amount of solar radiation, or solar energy, the earth receives each day is many times greater than the total amount of all energy people consume each day. However, on the earth's surface, solar energy is a variable and intermittent energy source. Nevertheless, use of solar energy, especially for electricity generation, has increased significantly in the United States and around the world in the past 30 years.

Solar energy resources vary by location

The availability and intensity of solar radiation on the earth's surface varies by time of day and location. In general, the intensity of solar radiation at any location is greatest when the sun is at its highest apparent position in the sky—at solar noon—on clear, cloudless days.

Latitude, climate, and weather patterns are major factors that affect insolation—the amount of solar radiation received on a given surface area during a specific amount of time. Locations in lower latitudes and in arid climates generally receive higher amounts of insolation than other locations. Clouds, dust, volcanic ash, and pollution in the atmosphere affect insolation levels at the surface. Buildings, trees, and mountains may shade a location during different times of the day in different months of the year. Seasonal (monthly) variations in solar resources increase with increasing distance from the earth's equator.

The type of solar collector also determines the type of solar radiation and insolation that a solar collector receives. Concentrating solar collector systems, such as those used in solar thermal-electric power plants, require direct solar radiation, which is generally greater in arid regions with few cloudy days. Flat-plate solar thermal and photovoltaic (PV) collectors can use global solar radiation, which includes diffuse (scattered) and direct solar radiation.

In general, a solar energy collector with a tracking system that keeps the solar collectors oriented toward the sun will have higher daily and annual insolation than a solar collector in a fixed position.

The two maps below show U.S. average annual solar radiation in kilowatthours (kWh) per square meter per day (kWh/m2/d) for direct normal irradiance (DNI) and global horizontal irradiance (GHI). The world map below shows average daily global solar radiation on a horizontal flat surface.

Electricity generation is the top use of solar energy in the United States

Insolation is important for the technical and economic performance of solar energy systems. The availability of financial and other incentives for solar energy are major factors that affect where solar energy systems are installed. Net metering has been especially important in encouraging PV systems on homes and businesses.

Total solar energy use in the United States increased from about 0.02 trillion British thermal units (Btu) in 1984 to about 1,438 trillion Btu (or about 1.4 quadrillion Btu) in 2025. Solar electricity generation accounted for about 92% of total solar energy use in 2025 and solar energy use for space and water heating accounted for about 8%.

Total U.S. solar electricity generation increased from about 5 million kWh in 1984 (nearly all from utility-scale, solar thermal-electric power plants) to about 389 billion kWh in 2025. In 2025, utility-scale PV power plants accounted for about 76% of total solar electricity generation, small-scale PV systems accounted for about 24%, and utility-scale solar thermal-electric power plants accounted for about 1%. Utility-scale power plants have at least 1,000 kilowatts (kW) (or one megawatt [MW]) of electricity generation capacity. Small-scale PV systems have less than 1 MW generation capacity.

In 2025, Texas accounted for the largest percentage share of total utility-scale solar electricity generation (20%), followed by California (18%). California accounted for nearly 37% of total generation from small-scale PV systems. Most small-scale PV systems are installed on or near buildings. Residential sector small-scale PV systems accounted for 68% of total small-scale PV electricity generation, and California's percentage was 37%.

Solar energy is used throughout the world

Solar energy is used all over the world, and like the United States, global solar electricity generation has increased substantially. Total world solar electricity generation grew from 0.4 billion kWh in 1990 to about 2,090 billion kWh (2.1 trillion kWh) in 2024. China and the United States together accounted for about one-half of total world solar electricity generation in 2024. The top five producers of solar electricity and their percentage shares of world total solar electricity generation in 2024 were:

• China–40%
• United States–15%
• India–6%
• Japan–5%
• Germany–3%

Solar photovoltaic

Photovoltaic cells

A photovoltaic (PV) cell, commonly called a solar cell, is a nonmechanical device that converts sunlight directly into electricity. Some PV cells can convert artificial light into electricity.

Sunlight is composed of photons, or particles of solar energy. These photons contain varying amounts of energy that correspond to the different wavelengths of the solar spectrum.

A PV cell is made of semiconductor material. When photons strike a PV cell, they will reflect off the cell, pass through the cell, or be absorbed by the semiconductor material. Only the photons that are absorbed provide energy to generate electricity. When the semiconductor material absorbs enough sunlight (solar energy), electrons are dislodged from the material's atoms. Special treatment of the PV cell's surface during manufacturing makes the front surface of the cell more receptive to the dislodged, or free, electrons so that the electrons naturally migrate to the surface of the cell.

The flow of electricity in a solar cell

The movement of electrons, which all carry a negative charge, toward the front surface of the PV cell creates an imbalance of electrical charge between the cell's front and back surfaces. This imbalance, in turn, creates a voltage potential similar to the negative and positive terminals of a battery. Electrical conductors on the PV cell absorb the electrons. When the conductors are connected in an electrical circuit to an external load, such as a battery, electricity flows through the circuit.

Photons carry solar energy

PV cells, panels, and arrays

The PV cell is the basic building block of a PV system. Individual cells can vary from 0.5 inches to about 4.0 inches across. However, one PV cell can only produce 1 or 2 Watts, which is only enough electricity for small uses, such as powering calculators or wristwatches.

PV cells are electrically connected in a packaged, weather-tight PV panel (sometimes called a module). PV panels vary in size and in the amount of electricity they can produce. Electricity-generating capacity for PV panels increases with the number of cells in the panel or in the surface area of the panel. PV panels can be connected in groups to form a PV array. A PV array can be composed of as few as two PV panels to hundreds of PV panels. The number of PV panels connected in a PV array determines the amount of electricity the array can generate.

PV cells generate direct current (DC) electricity. DC electricity can be used to charge batteries that power devices that use DC electricity. Nearly all electricity is supplied as alternating current (AC) in electricity transmission and distribution systems. Devices called inverters are used on PV panels or in PV arrays to convert the DC electricity to AC electricity.

PV cells and panels produce the most electricity when they are directly facing the sun. PV panels and arrays can use tracking systems to keep the panels facing the sun, but these systems are expensive. Most PV systems have panels in a fixed position that are usually facing directly south in the northern hemisphere—or directly north in the southern hemisphere—at an angle that optimizes the physical and economic performance of the system.

PV system efficiency

The efficiency with which PV cells convert sunlight to electricity varies by the type of semiconductor material and PV cell technology. The efficiency of commercially available PV panels averaged less than 10% in the mid-1980s, increased to around 15% by 2015, and is now approaching 25% for state-of-the art modules. Experimental PV cells and PV cells for niche markets, such as space satellites, have achieved nearly 50% efficiency.

PV system applications

When the sun is shining, PV systems can generate electricity to directly power devices such as water pumps or supply electric power grids. PV systems can also charge a battery to provide electricity when the sun is not shining for individual devices, single homes, or electric power grids.

Some advantages of PV systems are:

• PV systems can supply electricity in locations where electricity distribution systems (power lines) do not exist, and they can also supply electricity to electric power grids.
• PV arrays can be installed quickly.
• The environmental effects of PV systems located on buildings are minimal.

History of PV systems

The first practical PV cell was developed in 1954 by Bell Telephone researchers. Beginning in the late 1950s, PV cells were used to power U.S. space satellites. By the late 1970s, PV panels were providing electricity in remote, or off-grid, locations that did not have electric power lines. Since 2004, most PV systems in the United States are grid-connected—they are connected to an electric power grid. These PV systems are installed on or near homes and buildings and at utility-scale power plants that have at least 1 megawatt of electric-generation capacity. Technological advances, lower costs for PV systems, and various financial incentives and government policies, especially tax credits and net metering, have helped to greatly expand PV use since the mid-1990s. Millions of grid-connected PV systems are now installed in the United States.

Electricity generation at utility-scale PV power plants increased from 6 million kilowatthours (kWh) (or 6,000 megawatthours [MWh]) in 2004 to about 292 billion kWh (or 292,817,000 MWh) in 2025. About 93 billion kWh (or 93,148,000 MWh) were generated by small-scale, grid-connected PV systems in 2025, up from 18 billion kWh (or 18,812,000 MWh) in 2016. Small-scale PV systems have less than 1,000 kilowatts of electricity-generation capacity. Most small-scale PV systems are located on buildings and are sometimes called rooftop PV systems.

Solar thermal power plants

Solar thermal-electric power systems collect and concentrate sunlight to produce the high temperatures needed to generate electricity. All solar thermal-electric power systems have solar energy collectors with two main components: reflectors (mirrors) that capture and focus sunlight and receivers, which receive the sunlight from the reflectors. In most types of systems, a heat-transfer fluid is heated and circulated in the receiver and used to produce steam. The steam is converted into mechanical energy in a turbine, which powers a generator to produce electricity. Solar thermal power systems have tracking systems that keep sunlight focused onto the receiver throughout the day as the sun changes position in the sky. Solar thermal power plants usually have a large field, or array, of collectors that supply heat to a turbine and generator. Several solar thermal power facilities in the United States have two or more solar power plants with separate arrays and generators.

Solar thermal power systems may also have a thermal energy storage system that collects heat in an energy storage system during the day, and the heat from the storage system is used to produce electricity in the evening or during cloudy weather. Solar thermal power plants may also be hybrid systems that use other fuels (usually natural gas) to supplement energy from the sun during periods of low solar radiation.

Concentrating solar thermal power plants

There are three main types of concentrating solar thermal power systems:

• Linear concentrating systems, which include parabolic troughs and linear Fresnel reflectors
• Solar power towers
• Solar dish/engine systems

Linear concentrating systems

Linear concentrating systems collect the sun's energy using long, rectangular, curved (U-shaped) mirrors. The mirrors focus sunlight onto receivers (tubes) that run the length of the mirrors. The concentrated sunlight heats a fluid flowing through the tubes. The fluid is sent to a heat exchanger to boil water in a conventional steam-turbine generator to produce electricity. There are two major types of linear concentrator systems:

• Parabolic trough systems, where receiver tubes are positioned along the focal line of each parabolic mirror
• Linear Fresnel reflector systems, where one receiver tube is positioned above several mirrors to allow the mirrors greater mobility in tracking the sun

A linear concentrating collector power plant has a large number, or field, of collectors in parallel rows that are typically aligned in a north-south orientation to maximize solar energy collection. This configuration enables the mirrors to track the sun from east to west during the day and concentrate sunlight continuously onto the receiver tubes.

Parabolic troughs

Parabolic trough linear concentrating systems are used in one of the longest operating solar thermal power facilities in the world, the Solar Energy Generating System (SEGS) located in the Mojave Desert in California. The facility has had nine separate plants over time. The first plant in the system, SEGS I, operated from 1984 to 2015, and the second, SEGS II, operated from 1985 to 2015. SEGS III–VII each had net summer electric generation capacity of 36 megawatts (MW) and began operating between 1986 and 1988. SEGS VIII began operating in 1989 and SEGS IX in 1990; they each had a net summer electric generation capacity of 88 MW. SEGS III–VII and VIII all closed in 2021, and the SEGS IX closed in 2023.

Other parabolic trough solar thermal electric facilities operating in the United States as of December 2025 and their net summer electric generation capacity, location, and year of initial operation were:

• Solana Generating Station: a 296 MW, two-plant facility with an energy storage component in Gila Bend, Arizona, that started operating in 2013
• Mojave Solar Project: a 275 MW, two-plant facility in Barstow, California, that started operating in 2014
• Genesis Solar Energy Project: a 250 MW, two-plant facility in Blythe, California, that started operating in 2013 and 2014
• Nevada Solar One: a 69 MW plant near Boulder City, Nevada, that started operating in 2007

Linear Fresnel reflectors

Linear Fresnel reflector (LFR) systems are similar to parabolic trough systems because both have mirrors (reflectors) that concentrate sunlight onto a receiver located above the mirrors. These reflectors use the Fresnel lens effect, which has a concentrating mirror with a large aperture and short focal length. These systems can concentrate the sun's energy to approximately 30 times its normal intensity. Compact linear Fresnel reflectors (CLFR)—also referred to as concentrating linear Fresnel reflectors—are a type of LFR technology that has multiple absorbers near the mirrors. Multiple receivers allow the mirrors to change their inclination to minimize how much they block adjacent reflectors' access to sunlight. This positioning improves system efficiency and reduces material requirements and costs. A demonstration CLFR solar power plant was built near Bakersfield, California, in 2008, but it is not operational.

Solar power towers

A solar power tower system uses a large field of flat, sun-tracking mirrors called heliostats to reflect and concentrate sunlight onto a receiver on the top of a tower. Sunlight can be concentrated as much as 1,500 times. Some power towers use water as the heat-transfer fluid. Advanced designs are experimenting with molten nitrate salt because of its superior heat transfer and energy storage capabilities. The thermal energy-storage capability allows the system to produce electricity during cloudy weather or at night.

The U.S. Department of Energy, along with several electric utilities, built and operated the first demonstration solar power tower near Barstow, California, during the 1980s and 1990s. In 2025, two solar power tower facilities were operating in the United States:

• Ivanpah Solar Power Facility: a facility with three separate collector fields and towers with a combined net summer electric generation capacity of 393 MW in Ivanpah Dry Lake, California, that started operating in 2013
• Crescent Dunes Solar Energy Project: a 110 MW, one-tower facility with an energy storage component in Tonapah, Nevada, that started operating in 2015

The La Paz Solar Tower in La Paz, Arizona, is a 200 MW, one-tower facility that is set to begin operation in June 2027.

Solar dish engines

Solar dish-engine systems use a mirrored dish similar to a very large satellite dish. To reduce costs, the mirrored dish is usually made up of many smaller flat mirrors formed into a dish shape. The dish-shaped surface directs and concentrates sunlight onto a thermal receiver, which absorbs and collects the heat and transfers it to an engine generator. The most common type of heat engine used in dish-engine systems is the Stirling engine. This system uses the fluid heated by the receiver to move pistons and create mechanical power. The mechanical power runs a generator or alternator to produce electricity.

Solar dish-engine systems always point straight at the sun and concentrate the solar energy at the focal point of the dish. A solar dish's concentration ratio is much higher than linear concentrating systems; it has a working fluid temperature higher than 1,380°F. The power-generating equipment used with a solar dish can be mounted at the focal point of the dish. The energy can also be collected from a number of installations and converted into electricity at a central point.

The United States has no utility-scale solar dish-engine projects in commercial operation.

Solar thermal collectors

Heating with the sun's energy

We use the sun’s energy (solar thermal energy) for many things, including heating water, air, and the inside of buildings and generating electricity. There are two general types of solar heating systems: passive systems and active systems.

Passive solar heating systems

When the sun shines through the windows of a building and warms the interior, it’s called passive solar heating. Buildings designed for optimal passive solar heating usually have large windows facing south (in the Northern Hemisphere). The sun shines through the windows onto special walls or floors that absorb solar heat. These building materials then release the stored heat, warming the interior naturally. Window overhangs or shades block the sun from entering the windows during the summer to keep the building cool.

Active solar heating systems

Active solar heating systems use fans or pumps to move air or liquid through collectors. The air or liquid is heated at the collector then flows into a building or heat-storage system, where the heat can be used later. The fans or pumps recirculate the air or liquid from the building or storage system back to the collector to be reheated. Active solar water heating systems usually have a tank for storing solar-heated water.

Types of solar collectors

Non-concentrating solar collectors

Typically, buildings that use solar energy systems have non-concentrating collectors, which means they don’t amplify the sunlight with mirrors or lenses. The surface collecting the sunlight is the same surface absorbing the solar energy. Flat-plate collectors are the most common type of non-concentrating collectors and are used when temperatures lower than 200°F are sufficient, for example, the solar panels on the roof of a home or building.

Flat-plate solar collectors usually have three main components:

• A flat metal plate that collects and absorbs solar energy
• A transparent cover that allows solar energy to pass through the cover and reduces heat loss from the absorber
• A layer of insulation on the back of the absorber to reduce heat loss

Solar water heaters use a metal pipe attached to the solar collector that runs in a loop from the collector, through a water storage tank, and back to the collector. A heat-transfer fluid flows through the metal pipe. The fluid inside the pipe absorbs heat at the collector, flows down through the tube, and then heats the water inside the storage tank. The fluid continuously moves through this loop, absorbing heat at the collector, heating the water in the tank, and returning to the solar collector to be reheated.

Solar systems for heating swimming pool water usually have flat-plate collectors that do not have covers or insulation for the collector, and the pool water circulates from the pool through the collector and back to the pool.

Solar air-heating systems use fans to move air through flat-plate collectors and into buildings.

Concentrating solar collectors

These systems have a highly reflective collector that focuses, or concentrates, solar energy onto an absorber. The collector usually moves during the day with the sun to keep the sunlight focused on the absorber. Solar thermal power plants use concentrating solar collector systems because they can produce the high-temperature heat needed to generate electricity.

Solar energy & the environment

Solar energy emissions

Solar energy technologies and power plants do not produce air pollution or greenhouse gases when operating. Using solar energy can help the environment indirectly when solar energy replaces energy sources that produce harmful emissions. Although using solar energy technologies is generally environmentally friendly, producing and disposing of solar energy technologies have some effect on the environment.

Manufacturing and materials

Solar energy technologies require materials, such as metal and glass, that take a lot of energy to produce. The environmental impact related to producing these materials could be associated with the environmental impact of solar energy systems. Studies have shown that a photovoltaic (PV) system can produce energy equal to the energy used to manufacture it within 1 to 4 years. Most PV systems operate for 30 years or more, which means they continue to operate long after the initial energy investment is repaid.

Hazardous chemical use and disposal

Manufacturing PV cells and panels involves hazardous chemicals. These chemicals must be handled carefully to prevent them from being released into the environment. Some types of PV cells contain heavy metals, which require special handling when they are retired. Some solar thermal systems use potentially hazardous fluids to transfer heat, and leaks could be harmful. U.S. environmental laws regulate how these hazardous materials are used and disposed of. The U.S. Department of Energy supports various projects to address end-of-life issues for solar energy technologies, including recycling materials from old cells and panels. Several states have also enacted laws that encourage solar panel recycling.

Land use

Like any large power plant, solar power plants can affect the local environment. Clearing land for a power plant can affect the habitats of native plants and animals. However, installing solar power plants on farm land that isn’t ideal for farming or integrating them into farms may offer economic and environmental benefits to farmers.

Water use and wildlife

Some solar power plants need water to clean solar collectors and concentrators or to cool equipment. In dry environments, using large volumes of ground water or surface water for cleaning could affect the ecosystems that depend on those water sources. In addition, the intense beam of concentrated sunlight from solar power towers can harm birds and insects that fly into it.