Solar energy is the light and heat from the Sun that can be used with different methods, such as making electricity, heating water, or designing buildings to use sunlight. It is a key type of renewable energy. The ways to use solar energy are often divided into two types: passive solar and active solar. Passive solar uses building design to let in sunlight, choose materials that help control temperature, and arrange spaces to move air naturally. Active solar uses tools like solar panels, systems that focus sunlight to create power, and devices that use sunlight to heat water.
In 2011, the International Energy Agency stated that developing low-cost, unlimited, and clean solar energy technologies will have major long-term benefits. These benefits include helping countries rely on local, never-ending energy sources that do not require imports, improving the environment, reducing pollution, lowering costs to fight climate change, and offering advantages that apply worldwide.
Potential
The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere. About 30% of this radiation is reflected back into space, while the remaining 122 PW is absorbed by clouds, oceans, and land. At the Earth's surface, most solar light is in the visible and near-infrared ranges, with a small amount in the near-ultraviolet. Most people on Earth live in areas where insolation levels range from 150 to 300 watts per square meter, or 3.5 to 7.0 kilowatt-hours per square meter each day.
Solar radiation is absorbed by Earth's land, oceans (which cover about 71% of the planet), and atmosphere. Warm air containing water vapor from oceans rises, creating air movement called convection. When this air reaches high altitudes, where it is colder, the water vapor forms clouds, which release rain back to Earth, completing the water cycle. The heat released when water vapor turns into clouds strengthens air movement, leading to weather patterns like wind, cyclones, and anticyclones. Solar energy absorbed by oceans and land helps keep Earth's surface at an average temperature of 14°C. Through photosynthesis, green plants use solar energy to create stored chemical energy, which supports food, wood, and the biomass from which fossil fuels are made.
The total solar energy absorbed by Earth's atmosphere, oceans, and land is about 122 petawatts per year, or 3,850,000 exajoules (EJ) each year. In 2002 (2019), this amount of energy was used in just one hour and 25 minutes, compared to the world's total energy use in one year. Photosynthesis captures about 3,000 EJ of energy each year in plant life.
The amount of solar energy humans can use differs from the total solar energy near Earth's surface because factors like geography, changes over time, cloud cover, and available land limit access. In 2021, the Carbon Tracker Initiative estimated that 450,000 square kilometers of land—about the size of Sweden, Morocco, or California (0.3% of Earth's total land)—would be needed to generate all human energy from solar power alone.
Solar technologies are divided into passive and active types based on how they capture, convert, and distribute sunlight. These technologies help use solar energy globally, depending on how far a location is from the Equator. While solar energy mainly refers to using sunlight for practical purposes, all renewable energy except geothermal and tidal power comes directly or indirectly from the Sun.
Active solar methods use tools like photovoltaic panels, solar thermal collectors, pumps, and fans to convert sunlight into usable energy. Passive solar methods involve choosing materials that help control temperature, designing spaces to naturally move air, and positioning buildings to take advantage of sunlight. Active solar technologies increase energy supply and are called supply-side technologies. Passive solar technologies reduce the need for other energy sources and are called demand-side technologies.
In 2000, the United Nations Development Programme, UN Department of Economic and Social Affairs, and World Energy Council estimated the amount of solar energy humans could use each year. This estimate considered factors like insolation, cloud cover, and usable land. It stated that solar energy has a global potential of 1,600 to 49,800 exajoules (4.4 × 10¹⁷ to 1.4 × 10²⁰ kilowatt-hours) per year.
Thermal energy
Solar thermal technologies can be used for heating water, heating rooms, cooling rooms, and generating heat for industrial processes.
In 1878, at the Universal Exposition in Paris, Augustin Mouchot showed a working solar steam engine. However, he could not continue developing it because coal was cheaper and other challenges existed.
In 1897, Frank Shuman, a U.S. inventor and engineer, built a small solar engine. It used mirrors to direct sunlight onto boxes filled with ether, a liquid that boils at a lower temperature than water. The boxes had black pipes inside that powered a steam engine. In 1908, Shuman started the Sun Power Company to build larger solar power plants. With help from A.S.E. Ackermann and Sir Charles Vernon Boys, he improved the system by using mirrors to focus sunlight on collector boxes. This change allowed water to be used instead of ether, increasing the system’s heating power. By 1912, Shuman built a full-scale steam engine powered by low-pressure water and patented his solar engine system.
Between 1912 and 1913, Shuman built the world’s first solar thermal power station in Maadi, Egypt. His plant used parabolic troughs to power a 45–52 kilowatt engine. This engine pumped over 22,000 liters of water per minute from the Nile River to nearby cotton fields. The start of World War I and the discovery of cheap oil in the 1930s slowed progress in solar energy. However, Shuman’s ideas and designs were revived in the 1970s when interest in solar thermal energy grew again. In 1916, Shuman told the media that solar energy could be used to meet human needs.
Solar hot water systems use sunlight to heat water. In areas between 40 degrees north and 40 degrees south latitude, solar heating systems can provide 60–70% of the hot water needed for homes, with water temperatures up to 60°C (140°F). The most common types of solar water heaters are evacuated tube collectors (44%), glazed flat plate collectors (34%), and unglazed plastic collectors (21%). Unglazed plastic collectors are mainly used for heating swimming pools.
As of 2015, the total installed capacity of solar hot water systems was about 436 thermal gigawatts (GWth). China led the world in solar hot water systems, with 309 GWth installed, which is 71% of the global market. Israel and Cyprus had the highest use of solar hot water systems per person, with over 90% of homes using them. In the United States, Canada, and Australia, solar hot water systems are mostly used to heat swimming pools, with an installed capacity of 18 GWth as of 2005.
In the United States, heating, ventilation, and air conditioning (HVAC) systems use 30% of the energy in commercial buildings and nearly 50% of the energy in residential buildings. Solar heating, cooling, and ventilation technologies can help reduce this energy use. Solar heating systems are divided into two types: passive systems, which rely on natural processes, and active systems, which use devices like sun-tracking systems or solar concentrators.
Thermal mass refers to materials that can store heat, such as stone, cement, and water. These materials have been used in warm climates to keep buildings cool by absorbing heat during the day and releasing it at night. They can also help keep buildings warm in colder areas. The size and placement of thermal mass depend on factors like climate, sunlight, and shading. When used properly, thermal mass helps maintain comfortable indoor temperatures and reduces the need for heating or cooling equipment.
A solar chimney is a passive solar ventilation system that uses a vertical shaft to move air through a building. When the chimney heats up, the warm air rises, creating an upward flow that pulls cooler air into the building. Adding glazing and thermal mass materials can improve the system’s performance.
Deciduous trees and plants help control solar heating and cooling. When planted on the south side of buildings in the Northern Hemisphere or the north side in the Southern Hemisphere, their leaves block sunlight in the summer but allow sunlight in the winter when the trees are leafless. However, in areas where heating is needed a lot, deciduous trees should not be planted on the side of a building facing the equator, as they may block sunlight during winter. They can be planted on the east and west sides to provide summer shade without reducing winter heating.
Solar cookers use sunlight to cook food, dry items, and pasteurize liquids. They are grouped into three types: box cookers, panel cookers, and reflector cookers. The first box cooker was built by Horace de Saussure in 1767. A basic box cooker has an insulated container with a transparent lid. It works well on cloudy days and can reach temperatures of 90–150°C (194–302°F). Panel cookers use a reflective panel to direct sunlight onto an insulated container, achieving similar temperatures. Reflector cookers use mirrors or other reflective surfaces to focus sunlight on a cooking container, reaching temperatures of 315°C (599°F) or higher. These cookers require direct sunlight and must be moved to follow the sun’s position.
Solar concentrating technologies, such as parabolic dishes, troughs, and Scheffler reflectors, can provide heat for industrial and commercial uses. The first commercial system was the Solar Total Energy Project (STEP) in Shenandoah, Georgia, where 114 parabolic dishes supplied 50% of the energy needed for a clothing factory. This system produced 400 kW of electricity and thermal energy in the form of steam and chilled water. It also had a one-hour thermal storage system. Evaporation ponds are shallow pools that use sunlight to evaporate water and concentrate dissolved solids. This method has been used for centuries to extract salt from seawater. Modern uses include concentrating brine solutions in mining and removing dissolved solids from waste.
Clothes lines, clotheshorses, and clothes racks use sunlight and wind to dry clothes without electricity or gas. In some U.S. states, laws protect the right to dry clothes outdoors. Unglazed transpired collectors (UTC) are walls with holes that use sunlight to heat incoming air. These collectors can raise air temperature by up to 22°C (40°F) and deliver air temperatures of 45–60°C (113–140°F). UTCs are cost-effective, with a payback period of 3–12 years. As of 2003, over 80 UTC systems with a total area of 35,000 square meters (380,000 square feet) had been installed worldwide. Examples include a system in Costa Rica used for drying coffee beans and a system in India used for drying marigolds.
Solar distillation is a process that uses sunlight to evaporate water and collect the vapor as fresh water. This method is used in areas where access to clean water is limited.
Electricity production
Solar power, also called solar electricity, is the process of changing sunlight into electricity. This can happen directly through photovoltaics (PV) or indirectly using concentrated solar power. Solar panels use the photovoltaic effect to turn sunlight into an electric current. Concentrated solar power systems use mirrors or lenses and tracking systems to focus sunlight onto a small area, creating heat that can power a steam turbine.
Photovoltaics (PV) were first used for small and medium-sized purposes, such as powering calculators with one solar cell or providing electricity to homes not connected to the power grid. Large concentrated solar power plants were first built in the 1980s. Since then, as solar panel costs have decreased, the ability of grid-connected solar PV systems to produce electricity has doubled about every three years. Three-quarters of new electricity generation capacity comes from solar power, with millions of rooftop systems and large-scale solar power plants being built worldwide.
In 2024, solar power provided 7% of the world’s electricity and over 1% of total energy (2.7% using a specific calculation method). It added twice as much new electricity as coal did. Along with onshore wind power, large-scale solar is the cheapest source of new electricity in most countries. China produces about half of the world’s solar power. Nearly half of the solar power installed in 2022 was placed on rooftops.
More low-carbon energy is needed to reduce climate change and support electrification. The International Energy Agency stated in 2022 that more work is required to connect solar power to the grid and address challenges related to policies, rules, and funding. However, solar power can significantly lower energy costs. Solar power is important for ensuring a stable and secure energy supply.
Concentrated solar power
Concentrating Solar Power (CSP) systems use mirrors and tracking systems to focus sunlight from a large area into a small area. The heat from this focused sunlight is used to power a traditional power plant. Many different types of concentrating technologies are used, such as parabolic troughs, solar tower collectors, concentrating linear Fresnel reflectors, and Stirling dishes. Different methods are used to follow the Sun's movement and focus the light. In all of these systems, a working fluid is heated by the focused sunlight and used to generate electricity or store energy. Engineers must consider risks like dust storms, hail, or other extreme weather that can harm the glass surfaces of solar power plants. Metal grills can let most sunlight reach the mirrors and panels while reducing damage.
Architecture and urban planning
Sunlight has shaped building design throughout history. Early examples of solar architecture and city planning were used by the Greeks and Chinese, who positioned their buildings to face south to allow sunlight for warmth and lighting.
Passive solar design includes features such as building orientation toward the sun, compact shapes (which have less surface area compared to their size), shading from overhangs, and materials that store heat. When these features are adapted to the local climate, they create spaces that are well-lit and stay at comfortable temperatures. An example of this is Socrates' Megaron House. Modern solar design uses computer models to connect systems for lighting, heating, and ventilation into a complete solar plan. Active solar tools, like pumps, fans, and windows that change how much light enters, can work with passive designs to improve efficiency.
Urban heat islands are city areas that are warmer than the surrounding areas. This happens because materials like asphalt and concrete in cities absorb more sunlight and hold more heat than natural materials. One simple way to reduce this effect is to paint buildings and roads white and plant trees. A proposed "cool communities" program in Los Angeles predicted that these steps could lower city temperatures by about 3 degrees Celsius at a cost of $1 billion. This could save about $530 million yearly through lower air-conditioning costs and healthcare expenses.
Agriculture and horticulture
Agriculture and horticulture work to use sunlight as efficiently as possible to help plants grow better. Methods like planting at certain times of the year, arranging rows in specific directions, spacing rows at different heights, and mixing different types of plants can increase how much crops are produced. Even though sunlight is usually plentiful, there are times when it is not enough, showing how important solar energy is for farming. During the short growing seasons of the Little Ice Age, farmers in France and England used fruit walls to collect more solar energy. These walls helped store heat and kept plants warm, which helped them ripen faster. Early fruit walls were built straight up from the ground and faced south, but later, walls with slopes were made to use sunlight more effectively. In 1699, Nicolas Fatio de Duillier proposed using a system that could move to follow the Sun’s path. Solar energy is used in farming in ways other than growing crops, such as pumping water, drying crops, warming areas for baby chickens, and drying chicken waste. Recently, winemakers have started using solar panels to power equipment that presses grapes.
Greenhouses change sunlight into heat, allowing plants to grow all year and in enclosed spaces, even if the local climate is not suitable for them. The earliest greenhouses were used in Roman times to grow cucumbers for Emperor Tiberius throughout the year. The first modern greenhouses were built in Europe during the 16th century to care for unusual plants brought back from explorations. Greenhouses are still important in horticulture today. Transparent plastic materials are also used in similar ways in structures like polytunnels and row covers.
Transport
The development of a solar-powered car has been a goal for engineers since the 1980s. The World Solar Challenge is a race held every two years, where teams from universities and companies compete to travel 3,021 kilometers (1,877 miles) across central Australia from Darwin to Adelaide. When the race began in 1987, the winning car had an average speed of 67 kilometers per hour (42 mph). By 2007, the winning car’s average speed had increased to 90.87 kilometers per hour (56.46 mph). Similar races, such as the North American Solar Challenge and the planned South African Solar Challenge, show that people around the world are interested in building solar-powered vehicles.
Some vehicles use solar panels to provide extra power, such as for air conditioning, which helps keep the inside of the vehicle cool and reduces the need for fuel.
In 1975, the first working solar boat was built in England. By 1995, passenger boats with solar panels began to appear and are now used widely. In 1996, Kenichi Horie completed the first solar-powered trip across the Pacific Ocean. In 2006–2007, the Sun21 catamaran made the first solar-powered trip across the Atlantic Ocean. There were plans to sail around the world using solar power in 2010.
In 1974, the unmanned AstroFlight Sunrise airplane made the first flight powered by solar energy. On April 29, 1979, the Solar Riser became the first solar-powered, fully controlled flying machine that could carry a person, reaching an altitude of 40 feet (12 meters). In 1980, the Gossamer Penguin made the first flights powered only by solar panels. Soon after, the Solar Challenger flew across the English Channel in July 1981. In 1990, Eric Scott Raymond flew from California to North Carolina using solar power in 21 trips. Later, attention turned to unmanned aircraft, such as the Pathfinder (1997) and the Helios, which reached an altitude of 29,524 meters (96,864 feet) in 2001. The Zephyr, developed by BAE Systems, is a solar-powered aircraft that flew for 54 hours in 2007. By 2010, it was planned to fly for a month. From March 2015 to July 2016, the Solar Impulse, an electric plane powered by solar cells, completed a trip around the world. This single-seat plane can take off on its own and stay in the air for several days.
A solar balloon is a black balloon filled with regular air. When sunlight hits the balloon, the air inside warms up and expands, creating an upward force that lifts the balloon, similar to a hot air balloon. Some solar balloons are large enough for people to fly in, but they are mostly used as toys because they are not strong enough to carry heavy loads.
The Squad Solar is a small electric car for neighborhoods. It has a solar roof and can be charged using a standard 120-volt outlet.
Fuel production
Solar chemical processes use sunlight to help chemical reactions happen. These processes reduce the need for energy from fossil fuels and can change sunlight into fuels that can be stored and moved. These reactions are divided into two types: thermochemical, which uses heat, and photochemical, which uses light. Artificial photosynthesis can create many types of fuels. Making carbon-based fuels, like methanol, from carbon dioxide is difficult because it involves complex chemical reactions. A better option is producing hydrogen from protons. However, using water as the source of electrons, like plants do, requires splitting two water molecules into oxygen, which is a complex process. Some people think solar fuel plants near coasts might be built by 2050. These plants could split seawater to make hydrogen, which would power nearby electric plants, and the clean water made could go directly into city water systems. Another way to store solar energy is through chemical energy storage.
Since the 1970s, making hydrogen has been an important area of research for solar chemical processes. Besides using electricity from solar panels or light-based cells to split water, other methods use heat from the sun. One method uses concentrators to split water into oxygen and hydrogen at very high temperatures (2,300–2,600 °C or 4,200–4,700 °F). Another method uses heat from solar concentrators to help change natural gas into hydrogen, which produces more hydrogen than traditional methods. Some processes involve breaking down and rebuilding chemicals to make hydrogen. For example, the Solzinc process, being developed at the Weizmann Institute of Science, uses a 1 MW solar furnace to break down zinc oxide (ZnO) at temperatures above 1,200 °C (2,200 °F). This creates pure zinc, which can then react with water to make hydrogen.
Energy storage methods
Thermal mass systems can store solar energy as heat at temperatures useful for home use, for short periods or even across seasons. These systems often use common materials that can absorb and release heat easily, such as water, soil, and stone. Well-designed systems can reduce the highest energy needs, move energy use to times when demand is lower, and decrease the need for heating and cooling overall.
Phase change materials, like paraffin wax and Glauber's salt, are another way to store heat. These materials are affordable, easy to find, and can provide heat at temperatures around 64 °C (147 °F). The "Dover House" in Dover, Massachusetts, was the first home to use a Glauber's salt heating system in 1948. Solar energy can also be stored at very high temperatures using molten salts. Salts are good for storage because they are low-cost, have strong heat-absorbing abilities, and can provide heat at temperatures suitable for power systems. The Solar Two project used this method, storing 1.44 terajoules (400,000 kWh) in a 68-meter storage tank with about 99% efficiency each year.
Off-grid solar systems have usually used rechargeable batteries to save extra electricity. For grid-connected systems, extra electricity can be sent to the power grid, while regular grid electricity can be used when needed. Net metering programs allow homes to earn credit for electricity they send to the grid. This is done by moving the meter backward when a home produces more electricity than it uses. If a home uses less electricity than it produces in a month, the credit is carried over to the next month. Some systems use two meters to track electricity used and produced separately, but this is rare because it costs more. Most standard meters can measure electricity flow in both directions, so a second meter is not needed.
Pumped-storage hydroelectricity saves energy by pumping water from a lower reservoir to a higher one when extra energy is available. Energy is released later by letting the water flow back down, turning the pump into a power generator.
Development, deployment and economics
Beginning with the increase in coal use during the Industrial Revolution, energy consumption changed from wood and biomass to fossil fuels. Early solar technology development started in the 1860s because people expected coal to become scarce. However, solar technology progress slowed in the early 1900s due to the growing availability, cost-effectiveness, and usefulness of coal and petroleum.
The 1973 oil embargo and 1979 energy crisis led to changes in energy policies worldwide. These events increased interest in solar technology. Programs to support solar development included the Federal Photovoltaic Utilization Program in the United States and the Sunshine Program in Japan. Research centers were also created in the United States (SERI, now NREL), Japan (NEDO), and Germany (Fraunhofer Institute for Solar Energy Systems ISE).
Commercial solar water heaters first appeared in the United States in the 1890s. Their use grew until the 1920s but declined as cheaper and more reliable heating fuels became available. Like photovoltaics, solar water heating gained attention again after the 1970s oil crises but lost interest in the 1980s due to lower petroleum prices. Solar water heating development continued steadily in the 1990s, with average annual growth rates of 20% since 1999. Solar water heating and cooling is the most widely used solar technology, with an estimated capacity of 154 gigawatts as of 2007.
The International Energy Agency has stated that solar energy can help solve many important global problems. A 2011 report by the International Energy Agency found that solar technologies, such as photovoltaics, solar hot water, and concentrated solar power, could supply one-third of the world’s energy by 2060 if governments commit to reducing climate change and switching to renewable energy. Solar energy could play a major role in reducing carbon emissions, along with improving energy efficiency and charging costs for greenhouse gas emitters. "The strength of solar is the incredible variety and flexibility of applications, from small scale to big scale." — Frank Shuman, The New York Times, 2 July 1916.
In 2021, Lazard estimated the cost of new, unsubsidized utility-scale solar electricity to be less than $37 per megawatt-hour, while existing coal-fired power was more expensive. The 2021 report also said new solar was cheaper than new gas-fired power but not always cheaper than existing gas power.
Concentrated photovoltaics (CPV) systems use sunlight focused onto photovoltaic surfaces to generate electricity. Thermoelectric, or "thermovoltaic" devices convert heat differences between materials into electricity.
Floating solar, or floating photovoltaics (FPV), are solar panels mounted on floating structures. These structures often use plastic buoys and cables and are placed on water, such as reservoirs, quarry lakes, irrigation canals, or remediation and tailing ponds.
Floating solar systems can have advantages over land-based solar panels. Water surfaces may be less expensive than land, and fewer rules apply to structures on water not used for recreation. Studies show foam-based FPV has the shortest time to recover energy (1.3 years) and the lowest greenhouse gas emissions (11 kg CO2 per megawatt-hour) among crystalline silicon solar technologies. FPV can generate electricity for any use and produce green hydrogen through electrolysis on the same water.
Floating solar arrays can be more efficient than land-based panels because water cools the panels. Panels may have special coatings to prevent rust or corrosion. Floating solar also provides shade, reduces water evaporation, and limits algae growth.
The market for floating solar has grown rapidly since 2016. The first 20 plants, with capacities of a few dozen kilowatts, were built between 2007 and 2013. Installed power increased from 3 gigawatts in 2020 to 13 gigawatts in 2022, surpassing a prediction of 10 gigawatts by 2025. The World Bank estimated there are 6,600 large water bodies suitable for floating solar, with a technical capacity of over 4,000 gigawatts if 10% of their surfaces were covered with panels.
The United States has the most floating solar potential of any country. Suitable water bodies are spread across the United States, with reservoirs in the southeast and southern plains states having the largest capacities.
A heat pump is a device that moves heat energy from a source to a destination called a "heat sink." Heat pumps move thermal energy against the natural direction of heat flow by absorbing heat from a cold space and releasing it to a warmer one. A solar-assisted heat pump combines a heat pump and solar thermal panels into one system. Usually, these technologies are used separately or placed in parallel to produce hot water. In this system, solar thermal panels act as the low-temperature heat source, and the heat is used to feed the heat pump’s evaporator. The goal is to achieve high efficiency and produce energy in a more cost-effective way.
Any type of solar thermal panel (sheet and tubes, roll-bond, heat pipe, thermal plates) or hybrid panels (mono/polycrystalline, thin film) can be used with a heat pump. Hybrid panels are preferred because they reduce the heat pump’s electricity demand, lowering power consumption and system costs.
An electric aircraft uses electric motors instead of internal combustion engines, with electricity from fuel cells, solar cells, ultracapacitors, power beaming, or batteries. Currently, most electric aircraft that carry people are experimental, though many small unmanned aerial vehicles use batteries. Electric model aircraft have been flown since the 1970s, with one report in 1957. The first electric flight carrying a person was in 1973. Between 201