A solar panel is a device that changes sunlight into electricity using several solar modules made up of photovoltaic (PV) cells. PV cells are made from materials that create excited electrons when sunlight hits them. These electrons move through a circuit and produce direct current electricity, which can power devices or be stored in batteries. Solar panels are also called solar cell panels or solar electric panels.

Solar panels are often grouped together in arrays or systems. A photovoltaic system includes one or more solar panels, an inverter that changes direct current electricity into alternating current when needed, and sometimes other parts like charge controllers, meters, or solar trackers to capture more sunlight. Most solar panels are found in solar farms or on rooftops, where they provide electricity to the power grid.

Solar panels use a clean and renewable energy source, which helps reduce greenhouse gas emissions compared to energy from hydrocarbons. However, they rely on sunlight being available and strong enough, need regular cleaning, and have high starting costs. Solar panels are used in homes, businesses, industries, and space missions, often with batteries to store energy.

History

In 1839, a French physicist named Edmond Becquerel discovered that certain materials can produce an electrical charge when exposed to light. These early solar cells were not efficient enough to power simple devices, but they were used to measure light.

Becquerel’s discovery was not repeated until 1873, when an English engineer named Willoughby Smith found that light hitting selenium could create a charge. In 1876, William Grylls Adams and Richard Evans Day published a paper titled "The action of light on selenium," describing how they repeated Smith’s findings.

In 1881, an American inventor named Charles Fritts made the first commercial solar cell. Fritts reported that this cell could generate electricity continuously from sunlight and even from dim, diffused daylight. However, these solar cells were still too inefficient to be practical for most power needs, especially compared to coal-fired power plants.

In 1939, Russell Ohl designed a solar cell that is used in many modern solar panels today. He patented this design in 1941. In 1954, Bell Labs used Ohl’s design to create the first commercially viable silicon solar cell.

Lower costs have been the main reason for the recent rapid growth of solar energy. Since 2010, the cost of solar photovoltaic electricity has dropped by 85%. Between 2008 and 2013, solar panel installers experienced significant growth. Many installers faced challenges, such as working with non-ideal rooftops, shaded areas, or poor orientation. These problems were first solved by using micro-inverters again, and later by developing power optimizers.

Solar panel manufacturers worked with micro-inverter companies to create alternating current modules. Power optimizer companies partnered with module manufacturers to create smart modules. In 2013, many solar panel manufacturers began shipping their smart module solutions.

Between 1992 and 2023, the use of photovoltaics (PV) increased rapidly. During this time, PV technology moved from being a small-scale niche market to a major source of electricity. From 2016 to 2022, PV capacity and production grew by about 26% each year, doubling every three years. By the end of 2022, the total installed PV capacity worldwide reached about 1,185 gigawatts (GW), supplying over 6% of global electricity demand, up from about 3% in 2019.

The decreasing cost of solar panels is increasing the use of solar energy in the Global South. Many countries in the Global South rely on expensive fossil fuel imports. Many homes and businesses are switching to solar energy to save money.

Theory and construction

Photovoltaic modules are made up of many solar cells that use sunlight to create electricity through the photovoltaic effect. Most modules use crystalline silicon cells made from silicon wafers or thin-film cells. The part of the module that holds weight can be either the top or the bottom layer. Solar cells must be protected from physical damage and water. Cells and modules are usually connected in a line to increase voltage and then connected side by side to increase current, forming a solar panel. Most panels are rigid, but some flexible panels made with thin-film cells are also available. The power of a solar panel, measured in watts, is calculated by multiplying voltage (in volts) by current (in amperes). This depends on how much sunlight the panel receives and the electrical devices connected to it. Specifications for solar panels are tested under standard conditions, which may not match the actual conditions where panels are used. A PV junction box is attached to the back of the panel and connects it to the rest of the system. Most modules use MC4 connectors for waterproof connections. Some panels also have a USB interface for power output.

Solar panels often include metal frames with parts like brackets, reflectors, and supports to help hold the panels in place. Solar cells are connected using electrodes to form a module. Front electrodes slightly cover the surface of the cells. To improve efficiency, manufacturers design cells to maximize sunlight absorption. Techniques used include:

• Aluminum back surface field (Al-BSF): Uses aluminum on the back of the cell.
• Passivated emitter rear contact (PERC): Uses less aluminum on the back and adds a special film to capture more light.
• Tunnel oxide passivated contact (TOPCon): Uses smaller silver bars and adds a rough layer to capture more light.
• Interdigitated back contact (IBC): Places all contacts on the back to allow full sunlight exposure.
• Extended back contact (XBC): Combines features from other techniques.

Tandem solar cells use one of these connection methods and combine different materials to improve performance. A single solar panel produces limited power, so most systems use multiple panels connected to increase voltage or current. A photovoltaic system includes panels, an inverter, batteries for storage, a charge controller, wiring, switches, meters, and sometimes a tracking system that follows the sun. These components are chosen to maximize energy output, reduce losses, and convert direct current to alternating current when needed.

Smart solar panels have electronics built into the panel itself, unlike traditional panels where electronics are attached to the frame or connected through a cable. Solar electronics can help:

• Track maximum power point: Adjusts to keep energy output high even if part of the panel is shaded.
• Monitor performance: Collects data on how well the panel is working.
• Detect faults: Helps identify problems for safety.

Most solar panels today use crystalline silicon (c-Si) cells made from polycrystalline or monocrystalline silicon. In 2021, 95% of solar panels worldwide used crystalline silicon, while the rest used thin-film technologies like cadmium telluride (CdTe), copper indium gallium selenide (CIGS), or amorphous silicon (a-Si).

Bifacial solar cells collect sunlight on both sides, increasing energy output. This works best in areas with reflective surfaces like snow or overcast skies. However, if panels are placed directly on the ground with little space, the benefit may be small. Bifacial panels are now cost-competitive with traditional panels and are widely used in large solar projects.

Newer solar technologies use thin-film cells. These are lighter and cheaper than traditional silicon cells and are used in spacecraft because they generate more power per weight. Multi-junction cells, made from materials like gallium arsenide, are used in concentrator photovoltaics, which focus sunlight onto small, efficient cells.

Thin-film solar cells are made by layering photovoltaic materials on glass, plastic, or metal. They are much thinner than traditional silicon wafers and are used in flexible applications like windows or building-integrated solar panels. Thin-film cells are often made with materials like cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), or amorphous silicon (a-Si). Solar cells are classified into generations based on their materials:

• First-generation: Use single or multi-crystalline silicon (common in most solar systems).
• Second-generation: Use thin-film materials like CdTe, CIGS, or GaAs.
• Third-generation: Use newer materials like perovskite, organic compounds, or quantum dots.

Thin-film cells are lighter and more flexible than traditional cells, making them suitable for applications like windows or curved surfaces. They are often made using simpler, less expensive manufacturing methods, which reduces their environmental impact. In some cases, thin-film cells produce less pollution and toxicity than other energy sources. While early versions of third-generation cells had lower efficiency, recent advances have improved their performance.

Efficiency

Each solar module is measured based on how much power it produces under standard conditions. This means the actual power produced in real-world conditions may change. Power levels usually range from 100 to 365 Watts (W). A module's efficiency affects its size for the same power rating. For example, an 8% efficient 230 W module will need twice as much space as a 16% efficient 230 W module. Some solar modules sold today have efficiencies above 24%. As of 2025, the highest efficiency for new commercial solar modules is about 24.5%, which is lower than the efficiency of the individual cells used in them. The most efficient solar modules produced in large quantities have power density values up to 175 W/m² (16.22 W/ft²).

The current versus voltage curve of a module shows important details about how the module works electrically. Manufacturing differences can cause variations in electrical performance even among modules with the same type of cells. Only testing the I–V curve in real conditions can accurately determine a module's electrical properties. This testing is important for designing, installing, and maintaining solar systems. Usually, electrical properties are tested indoors. However, outdoor testing has benefits, such as not needing expensive artificial lights, allowing testing of many samples, and providing more even lighting for samples.

The capacity factor of solar panels is mainly affected by geographic latitude and changes based on cloud cover, dust, day length, and other factors. In the United Kingdom, the seasonal capacity factor ranges from 2% (in December) to 20% (in July), with an average yearly capacity factor of 10–11%. In Spain, the capacity factor reaches 18%. Globally, the average capacity factor for large solar farms was 16.1% in 2019.

Overheating is the biggest factor affecting the efficiency of solar panels.

Depending on their design, solar modules can generate electricity from different light frequencies but usually cannot use all parts of the solar spectrum (such as ultraviolet, infrared, and low or diffused light). This means much of the sunlight energy is not used by solar modules, and they could work more efficiently with light of a single color. One design idea is to divide sunlight into six to eight different color ranges and direct each range to a cell that works best with that specific color.

Performance and degradation

Module performance is usually tested under standard conditions: sunlight intensity of 1,000 watts per square meter, a solar spectrum called AM 1.5, and a module temperature of 25 degrees Celsius. The actual voltage and current produced by the module change with lighting, temperature, and load conditions, so there is no single voltage at which the module operates. Performance depends on location, time of day, time of year, sunlight intensity, direction and angle of the module, cloud cover, shading, dirt, battery charge level, and temperature. Module performance can be measured at different times using a direct current clamp meter or shunt, and the results can be recorded, graphed, or charted with a data logger or chart recorder.

For best results, a solar panel string should include similar voltage panels placed in the same direction, facing direct sunlight. Bypass diodes help improve output by allowing current to flow around broken or shaded panels.

Electrical characteristics include nominal power (P MAX, measured in watts), open-circuit voltage (V OC), short-circuit current (I SC, measured in amperes), maximum power voltage (V MPP), maximum power current (I MPP), peak power (watt-peak, W p), and module efficiency (%).

Open-circuit voltage (V OC) is the highest voltage the module can produce when not connected to a circuit. V OC can be measured with a voltmeter on the module’s terminals or disconnected cable.

The peak power rating (W p) is the maximum output under standard test conditions, not the highest possible output. Typical modules, about 1 by 2 meters (3 ft × 7 ft) in size, are rated from 75 W to 600 W, depending on their efficiency. During testing, modules are grouped based on results, and manufacturers often rate them in 5 W steps, with ratings like +/- 3%, +/-5%, +3/-0%, or +5/-0%.

A photovoltaic (PV) module’s performance depends on environmental conditions, especially the amount of sunlight (G) hitting the module. The temperature (T) of the p–n junction also affects electrical parameters: short-circuit current (I SC), open-circuit voltage (V OC), and maximum power (P max). Generally, V OC decreases as temperature increases, while I SC increases slightly but not enough to offset the drop in V OC. As a result, P max decreases when temperature rises. This relationship depends on the semiconductor material and how temperature affects the movement of electrons and gaps inside the solar cell.

Temperature sensitivity is described using temperature coefficients, which show how each parameter changes with temperature. These values are listed in photovoltaic module data sheets:

• β: V OC variation coefficient with respect to temperature, calculated as ∂V OC /∂T.
• α: I SC variation coefficient with respect to temperature, calculated as ∂I SC /∂T.
• δ: P max variation coefficient with respect to temperature, calculated as ∂P max /∂T.

Methods to calculate these coefficients from experimental data are available in scientific literature.

Studies show that high temperatures reduce efficiency, but colder temperatures can improve performance because of lower electrical resistance. However, winter conditions, like snow and less daylight, may reduce overall output. Solar panels still generate power in winter, but output is often lower due to less sunlight and possible obstructions.

Solar modules can handle damage from rain, hail, heavy snow, and temperature changes, depending on the manufacturer. Most solar panels sold in the U.S. are UL listed, meaning they passed tests to withstand hail.

Potential-induced degradation (PID) is a performance loss in crystalline solar modules caused by stray currents. This can reduce power by up to 30%.

Photovoltaic devices lose power over time due to sunlight and other conditions. The degradation index, which measures annual power loss, is important for predicting long-term energy production. Degradation rates are calculated by tracking changes in electrical parameters. A single panel’s degradation can affect the entire string, and not all panels in an installation degrade at the same rate.

Studies show that crystalline silicon panels degrade linearly, losing about 0.8% to 1.0% of power each year. Thin-film panels experience a sharp initial drop in performance (lasting months to years) before stabilizing, similar to crystalline silicon. Thin-film panels also show strong seasonal changes because their performance is more affected by the solar spectrum.

Amorphous silicon, micromorphic silicon, and cadmium telluride panels may lose 3% to 4% of power annually in the first years. Copper indium gallium selenide panels degrade more slowly than crystalline silicon, even early in their lifespan.

Mounting and tracking

Large solar power plants often use ground-mounted solar panels. These panels are held in place by racks or frames attached to supports on the ground. Ground-based supports include:

• Pole mounts, which are pushed directly into the ground or placed in concrete.
• Foundation mounts, such as concrete slabs or poured bases.
• Ballasted footing mounts, which use heavy bases made of concrete or steel to hold panels in place without digging into the ground. This type is useful for areas where digging is not allowed, like capped landfills, and makes it easier to remove or move panels later.

Vertical bifacial solar panels are angled toward the east and west instead of the south. This helps them collect sunlight more effectively in the morning and evening. While this setup may produce slightly less total energy, it better matches energy needs during peak times and helps reduce the "duck curve" problem. These panels are used in projects like agrivoltaics, solar fencing, highway noise barriers, and railroad shields. They work well in high-latitude areas, such as the Nordic countries, because the sun is lower in the sky there.

• Agrivoltaic vertical bifacial solar panels
• Comparison of vertical bifacial and south-facing solar arrays

Roof-mounted solar systems use panels held by racks or frames attached to supports on the roof. Roof-based supports include:

• Rail mounts, which are fixed directly to the roof and may use extra rails to attach panels.
• Ballasted footing mounts, which use heavy bases made of concrete or steel to hold panels in place without drilling into the roof. This method allows panels to be removed or moved later without damaging the roof.
• All electrical wiring connecting solar panels to energy equipment must follow local electrical rules and be protected by pipes suitable for the climate.

Solar canopies are solar panels placed on top of traditional structures like parking lot covers, carports, gazebos, pergolas, or patio shelters. These canopies help use space efficiently in cities and provide shade for vehicles. The energy they produce can power electric vehicle (EV) charging stations.

Portable solar panels generate electricity to charge devices like phones or radios through USB ports or to charge power banks. These panels are flexible, strong, and water-resistant, making them good for travel and camping.

Solar trackers increase the energy each panel produces by moving the panels to follow the sun’s path. However, they require more mechanical parts and regular maintenance.

Fixed racks keep panels in one position all day, set at a specific angle (tilt) and direction (azimuth). A common tilt angle matches the location’s latitude. Some systems adjust the tilt based on the season.

East- and west-facing solar arrays (such as those on east-west facing roofs) are often used. While these may not produce the maximum energy from each panel, they are usually cheaper than solar trackers and provide more useful power during morning and evening peak times than north- or south-facing systems.

Maintenance

Solar panel efficiency, which is usually about 20%, can decrease when dust, dirt, pollen, and other particles build up on the panels. This buildup is called soiling. Seamus Curran, a physics professor at the University of Houston, explains that dirty solar panels can lose up to 30% of their power in areas with high dust or pollen, such as deserts. In 2018, soiling caused an average loss of 3% to 4% globally.

In many places, paying for solar panel cleaning is a good choice, as shown in 2019. However, in some areas, cleaning is not cost-effective. For example, in California in 2013, soiling caused only small efficiency losses, about 0.05% per day on average.

Solar panel installation and maintenance can also have risks. A UK study from 2015 to 2018 found that 80 PV-related fire incidents occurred, with over 20 being serious. Many fires were linked to poor installation or faulty equipment. The most common cause was issues with direct current isolators.

A 2021 study by kWh Analytics found that residential solar panels degrade at a median rate of 1.09% per year, while non-residential panels degrade at 0.8%. This is higher than the industry’s usual estimate of 0.5% per year, which was based on panel-level data. A 2021 study also found that 30% of manufacturers had safety issues related to junction boxes, and 26% had problems with materials used in panels.

Solar panel cleaning methods include five types: manual tools, mechanized tools (like tractor-mounted brushes), installed hydraulic systems (such as sprinklers), installed robotic systems, and deployable robots. Manual tools are most common because they are inexpensive. However, a 2014 study in Saudi Arabia suggested that installed robotic systems, mechanized systems, and hydraulic systems may be the most promising cleaning technologies.

New self-cleaning methods are being developed. In 2019, scientists used nanowires and a water-repelling coating to remove 98% of dust particles, which could help in desert areas. In March 2022, MIT researchers introduced a waterless cleaning system that uses electrostatic repulsion to remove dust. This system uses static electricity to push dust away from panels, eliminating the need for water or brushes. It can be automated with an electric motor and guide rails.

Waste and recycling

In 2021, there were 30,000 tons of PV waste. Bloomberg NEF predicted this amount would grow to more than 1 million tons by 2035 and over 10 million tons by 2050. For comparison, coal power produced 750 million tons of fly ash waste in 2022. In the United States, about 90% of old solar panels are sent to landfills as of 2023. Most parts of a solar module can be recycled, including up to 95% of certain materials like glass and metals. Some private companies and non-profit groups collect and recycle old solar panels. EU laws require manufacturers to ensure their solar panels are recycled correctly. Similar rules are being developed in Japan, India, and Australia. A 2023 Australian report found that there is a market for high-quality used panels and suggested ways to reuse them, but no rules have been put in place yet.

Recycling methods depend on the technology used in the modules:

• Silicon-based modules: Aluminum frames and junction boxes are removed by hand at the start of the process. The module is then crushed in a machine, and different parts—glass, plastics, and metals—are separated. More than 80% of the original weight can be recovered. This process is similar to recycling flat glass used in buildings and cars, so flat glass recyclers can handle it. Recovered glass is often used in glass foam and insulation.
• Non-silicon-based modules: These require special recycling methods, such as using chemical solutions to separate materials. For cadmium telluride modules, the process begins by crushing the module and separating parts. This method can recover up to 90% of the glass and 95% of semiconductor materials. Some large-scale recycling facilities have been built by private companies in recent years.

Since 2010, an annual European conference has brought together manufacturers, recyclers, and researchers to discuss the future of recycling solar panels.

Production

The production of solar power systems has followed a pattern where costs have decreased as more systems are made, and the efficiency and total output of these systems have increased.

In 2019, the number of solar power systems installed grew by more than 100% compared to the previous year. Solar panel makers increased the amount of panels they sent to customers and expanded their production abilities. This growth allowed them to become companies capable of producing billions of watts of power. According to Pulse Solar, five of the ten largest solar panel companies in 2019 increased their production of solar panels by at least 25% compared to 2019.

Most solar panels are made using silicon cells. These cells typically convert 10–20% of sunlight into electricity. Some newer models can convert more than 22% of sunlight into electricity.

In 2018, the five largest solar panel producers in the world, based on the total number of panels they shipped, were Jinko Solar, JA Solar, Trina Solar, Longi Solar, and Canadian Solar.

Making solar panels requires the use of harmful and reactive chemicals. These include cadmium telluride, copper indium selenide, cadmium gallium (di)selenide, copper indium gallium (di)selenide, hexafluoroethane, lead, and polyvinyl fluoride. Waste materials from the process include silicon tetrachloride. When silicon wafers are cut, they create silicon dust called "kerf."

Price

The cost of solar electricity has kept decreasing, so in many countries, it is now cheaper than electricity from fossil fuels connected to the power grid since 2012. This situation is called grid parity. As people around the world have become more aware of energy issues, organizations like the IRS have created tax credits that return part of the cost for private solar panel systems. The price of solar panels keeps getting lower over time.

Solar pricing is divided into three main groups: buyers who purchase small amounts (modules of all sizes in the kilowatt range each year), mid-sized buyers (usually up to 10 MWp each year), and large buyers (who get the lowest prices). Over time, the cost of solar cells and panels has steadily decreased. For example, in 2012, the cost per watt was about US$0.60, which was 250 times less than the cost in 1970, which was US$150. A 2015 study showed that the cost per kilowatt-hour dropped by 10% each year since 1980. It also predicted that solar energy could supply 20% of all electricity by 2030. The International Energy Agency estimated that solar could supply 16% of all electricity by 2050.

The actual cost of producing energy from solar panels depends heavily on local weather conditions. In a cloudy country like the United Kingdom, the cost per kilowatt-hour produced is higher than in sunnier countries like Spain.

According to RMI, Balance-of-System (BoS) costs, which include expenses for non-module parts such as wiring, converters, racking systems, and other components, make up about half of the total cost for solar installations.

For solar power stations that sell electricity to the power grid, the cost of solar energy must match the wholesale price of electricity. This point is sometimes called "wholesale grid parity" or "busbar parity."

Standards

Common standards used for photovoltaic modules include:

• IEC 61215 (crystalline silicon modules), IEC 61646 (thin film modules), IEC 61730 (safety requirements for all types of modules), and IEC 61853 (performance testing and energy rating of photovoltaic modules)
• ISO 9488 (Solar energy vocabulary)
• UL 1703, UL 1741, and UL 2703 from Underwriters Laboratories
• CE mark certification
• Electrical Safety Tester (EST) Series, including models EST-460, EST-22V, EST-22H, and EST-110.

Applications

Solar panels, also called photovoltaics, have many useful purposes. They can be used in farming to power water pumps for irrigation. In healthcare, they help keep medicines and vaccines cold by providing electricity for refrigeration. They are also used in infrastructure, such as buildings and roads. Solar panels are part of systems that use sunlight to make electricity and can power many different electrical devices.

Limitations

As more homes install rooftop solar panels, electricity moves in two directions—both to and from the power grid. When homes produce more electricity than they use, the extra power is sent back to the grid. However, traditional power networks were not built to handle electricity moving in both directions, which can cause problems. For example, in Queensland, Australia, more than 30% of homes had rooftop solar panels by the end of 2017. Starting in 2015, a pattern called the "duck curve" became common in many communities. This can lead to over-voltage issues when electricity flows from homes with solar panels back into the grid. Solutions to manage over-voltage include adjusting the power factor of solar inverters, using new voltage and energy control equipment, replacing or upgrading electrical wires, and managing energy use on the consumer side. These solutions often have limits and costs. To provide backup power during a power outage, a battery is usually needed.

Quality assurance

Solar module quality assurance includes testing and checking solar cells and panels to make sure they meet quality requirements. Solar modules, also called panels, are designed to last between 20 and 40 years. They must provide the power they are expected to give reliably over time. Testing can involve physical tests, lab experiments, and numerical analysis. Solar modules must also be checked at each stage of their life cycle. Companies such as Southern Research Energy & Environment, SGS Consumer Testing Services, TÜV Rheinland, Sinovoltaics, Clean Energy Associates (CEA), CSA Solar International, and Enertis offer quality assurance services for solar modules. Consistent, traceable, and stable manufacturing processes are required to ensure the quality of photovoltaic (PV) modules. Certification follows standards such as ANSI/UL1703, IEC 17025, IEC 61215, IEC 61701, and IEC 61730-1/-2.