Solar energy is energy derived from electromagnetic radiation emitted by the Sun. It is the primary energy source driving many processes on Earth, including atmospheric circulation, the hydrological cycle, photosynthesis, and much of the climate system. Solar radiation reaches Earth primarily as visible light, ultraviolet radiation, and infrared radiation. Technologies can capture this energy as heat or convert it into electricity. Solar thermal systems absorb radiation and convert it into useful heat, while photovoltaic cells convert light directly into electrical energy through semiconductor physics. Solar energy is renewable because the Sun continuously emits enormous quantities of energy as a consequence of nuclear fusion occurring within its interior.
The Sun produces energy through nuclear fusion, primarily through processes that convert hydrogen nuclei into helium in its extremely hot and dense core. A small amount of mass is converted into energy according to the relationship between mass and energy described by relativity. Energy generated in the core travels outward through layers of the Sun before being emitted into space as electromagnetic radiation. Only a fraction of that radiation reaches Earth, but the amount is enormous relative to human energy consumption. Solar radiation varies with latitude, season, time of day, atmospheric conditions, altitude, and surface orientation. These factors are crucial when evaluating the performance of solar energy systems.
Solar energy reaches Earth after travelling through space and interacts with the atmosphere and surface. Clouds, atmospheric gases, aerosols, and dust can absorb, reflect, or scatter incoming radiation. Direct solar radiation travels relatively straight from the Sun, while diffuse radiation reaches the surface after scattering in the atmosphere. A photovoltaic panel can use both forms under suitable conditions, although output generally changes with the available irradiance. The amount of sunlight reaching a surface depends on the angle at which radiation arrives, the duration of daylight, atmospheric conditions, and shading. Solar resource assessments therefore examine long term radiation patterns rather than relying on a single measurement.
Photovoltaic solar energy uses semiconductor materials to convert photons into electrical energy. When photons with sufficient energy are absorbed by a semiconductor, they can excite electrons into higher energy states and create charge carriers. A photovoltaic cell contains an electric field, commonly associated with a semiconductor junction, that helps separate these charges and produce a voltage. Connecting the cell to an external circuit allows electrical current to flow. Individual cells can be connected into modules, and modules can be combined into larger arrays. Modern photovoltaic systems may use silicon based cells as well as other semiconductor technologies designed to improve efficiency, reduce material use, or enable specialised applications.
Solar thermal energy captures solar radiation as heat rather than directly producing electricity through photovoltaic conversion. Low temperature solar thermal systems can heat water for domestic or commercial use. Higher temperature concentrating solar power systems use mirrors or lenses to concentrate sunlight onto a receiver, producing heat that can be used to generate steam and drive a turbine. Thermal storage can allow some concentrating solar power systems to continue supplying energy after sunlight declines. Solar thermal technologies are particularly useful where heating is a major energy demand, because using solar energy directly as heat can avoid some conversion losses associated with producing electricity first and then converting electricity back into heat.
Solar energy has advantages including a widely distributed resource, no fuel combustion during operation, modular deployment, and compatibility with buildings, infrastructure, and large ground mounted installations. Photovoltaic panels can be installed on roofs, car parks, brownfield land, utility scale sites, and other surfaces. Solar generation can be located close to electricity demand, reducing some transmission requirements. However, solar energy is variable and produces little or no electricity from direct sunlight at night. Output can also fall during cloud cover and winter periods. Energy storage, grid interconnection, demand management, complementary generation, or combinations of these measures can help address the difference between solar production and electricity demand.
Solar energy systems have environmental and material considerations. Manufacturing panels requires energy and raw materials, including silicon, aluminium, glass, copper, and other materials depending on the technology. Large solar installations can alter land use, although projects can sometimes use rooftops, degraded land, dual use agriculture, or other locations to reduce competition with natural habitats. Panel performance also changes with temperature, shading, orientation, soiling, and degradation over time. End of life recycling can recover valuable materials and reduce waste. Life cycle analysis generally provides a more meaningful assessment of environmental performance than considering only operational emissions.
Solar energy has a broad range of applications beyond grid electricity. It can provide electricity for homes, businesses, telecommunications equipment, remote infrastructure, spacecraft, water pumping, refrigeration, and many other uses. Solar water heating can reduce demand for conventional heating fuels, while passive solar design uses building orientation, glazing, insulation, thermal mass, and shading to exploit sunlight and reduce energy demand.
Solar energy can also contribute indirectly to the production of hydrogen and synthetic fuels when renewable electricity is used in industrial processes. Its importance arises from the scale and geographical distribution of solar radiation, but successful deployment depends on resource availability, system design, economics, storage, land considerations, grid conditions, and the specific energy service required.
