How Solar Energy Works

The Photovoltaic Effect

The photovoltaic effect is the physical process at the heart of every solar panel. When a photon of sunlight strikes a semiconductor material, typically crystalline silicon, it can transfer its energy to an electron in the material's atomic structure. If the photon carries enough energy (at least 1.1 electron volts for silicon), it knocks the electron free from its bond, creating a mobile charge carrier and leaving behind a positively charged hole. This process happens billions of times per second across the surface of a solar cell.

A solar cell is engineered to separate these freed electrons from the holes using an internal electric field. This field is created by doping two layers of silicon with different impurities: phosphorus atoms add extra electrons to the top layer (n-type), while boron atoms create electron vacancies in the bottom layer (p-type). At the boundary between these layers, called the p-n junction, a permanent electric field forms. When photons generate electron-hole pairs near this junction, the field pushes electrons toward the n-type layer and holes toward the p-type layer, creating a voltage difference that drives current through an external circuit.

The direct current (DC) produced by solar cells must be converted to alternating current (AC) for use in buildings and the electrical grid. Inverters perform this conversion, and modern string or microinverters also optimize the output of each panel or group of panels. Maximum power point tracking (MPPT) algorithms continuously adjust the electrical load to extract the highest possible power from the cells as sunlight conditions change throughout the day.

Not all sunlight that reaches a solar cell gets converted to electricity. The Shockley-Queisser limit establishes a theoretical maximum efficiency of 33.7% for a single-junction silicon cell. Photons with energy below the bandgap pass through without generating electron-hole pairs. Photons with energy above the bandgap generate pairs, but the excess energy dissipates as heat through a process called thermalization. Additional losses come from surface reflection, electrical resistance in the cell, and recombination of electrons with holes before they reach the external circuit.

Types of Solar Cells

Monocrystalline silicon cells, made from a single continuous crystal structure, offer the highest commercial efficiencies at 20 to 24%. Their uniform crystal lattice minimizes electron recombination losses, but the manufacturing process, growing large single crystals using the Czochralski method, is energy-intensive and produces cylindrical ingots that must be squared off, creating silicon waste. Polycrystalline (multicrystalline) cells use silicon cast into blocks and sliced into wafers, achieving 17 to 20% efficiency at lower manufacturing cost. The grain boundaries between crystal regions create recombination sites that reduce performance compared to monocrystalline cells.

Thin-film solar cells deposit extremely thin layers of photovoltaic material, often just one to three micrometers thick, onto glass, metal, or plastic substrates. Cadmium telluride (CdTe) thin films have reached 22% laboratory efficiency and dominate the thin-film market due to low manufacturing costs and a simpler production process. Copper indium gallium selenide (CIGS) cells achieve similar efficiencies with the advantage of flexibility, enabling curved and lightweight applications. Amorphous silicon thin films are less efficient (6 to 10%) but perform better than crystalline silicon in low-light and high-temperature conditions.

Perovskite solar cells represent the fastest-advancing solar technology in history, with laboratory efficiencies climbing from 3.8% in 2009 to over 26% by 2025. Perovskites are crystalline compounds with a specific ABX3 crystal structure that can be manufactured from inexpensive, abundant materials using low-temperature solution processes, including printing and spray coating. Tandem cells that stack a perovskite layer on top of a silicon cell have exceeded 33% efficiency in laboratory settings, approaching the practical limits of two-junction devices. The main challenges for commercial perovskite deployment remain long-term stability under moisture and UV exposure, and the toxicity of lead in common formulations.

Multi-junction cells, originally developed for space applications, use multiple semiconductor layers with different bandgaps to capture different portions of the solar spectrum. A triple-junction cell might combine layers of gallium indium phosphide (GaInP), gallium arsenide (GaAs), and germanium (Ge) to absorb high-energy, medium-energy, and low-energy photons respectively. These cells have reached 47.6% efficiency under concentrated sunlight in laboratory conditions, but their high manufacturing cost limits terrestrial use to concentrated photovoltaic (CPV) systems.

Concentrated Solar Power

Concentrated solar power (CSP) plants use mirrors or lenses to focus sunlight onto a small area, generating intense heat that drives a conventional power cycle. Parabolic trough systems, the most widely deployed CSP technology, use curved mirrors to focus sunlight onto a tube containing heat transfer fluid running along the focal line. The fluid, typically a synthetic oil, reaches temperatures of 400 degrees Celsius and passes through heat exchangers to generate steam for a Rankine cycle turbine. The SEGS complex in California, built in the 1980s and 1990s, was the world's first large-scale CSP installation and continues operating with a combined capacity of 354 MW.

Power tower systems (also called central receiver systems) use thousands of flat, sun-tracking mirrors called heliostats to focus sunlight onto a receiver at the top of a tall tower. This design achieves higher temperatures, 565 degrees Celsius or more with molten salt receivers, and higher thermodynamic efficiency than trough systems. The Ivanpah Solar Electric Generating System in California uses three towers surrounded by over 170,000 heliostats to generate 392 MW. Dish-Stirling systems, a third CSP design, use parabolic dishes to focus sunlight onto a Stirling engine at the focal point, achieving the highest point-focus efficiencies but remaining limited to small-scale applications.

The critical advantage of CSP over photovoltaics is built-in thermal energy storage. Molten salt, a mixture of sodium nitrate and potassium nitrate, can be heated during the day to 565 degrees Celsius and stored in insulated tanks, then used to generate steam and electricity for six to ten hours after sunset. Spain's Gemasolar plant demonstrated that a CSP facility with sufficient storage can operate continuously for 24 hours, producing dispatchable solar power around the clock. This capability makes CSP valuable for grid stability in regions with strong direct solar resources, though the technology requires clear skies and is limited to desert and semi-arid climates.

Solar Energy Scale and Economics

Solar photovoltaics have become the cheapest source of new electricity generation in most of the world. Module prices have fallen from roughly $76 per watt in 1977 to under $0.20 per watt in 2025, a decline of over 99%. This cost reduction follows a remarkably consistent learning curve: every doubling of cumulative global manufacturing capacity has reduced module costs by approximately 24%. The balance-of-system costs (inverters, mounting hardware, wiring, permitting, and installation labor) have also declined significantly but now represent about 60% of total system cost for utility-scale installations.

Utility-scale solar farms in regions with strong solar resources now generate electricity at a levelized cost of $0.03 to $0.05 per kilowatt-hour, which is competitive with or cheaper than every fossil fuel alternative. Rooftop residential solar costs more, typically $0.06 to $0.12 per kilowatt-hour depending on location, system size, and local incentives. Modern panels are warrantied for 25 to 30 years and typically retain over 87% of their initial output after 25 years of operation, with a useful life extending well beyond the warranty period.

China dominates the global solar manufacturing supply chain, producing over 80% of the world's polysilicon, wafers, cells, and modules. This concentration raises supply chain resilience and energy security concerns, prompting the United States, European Union, and India to launch domestic manufacturing initiatives backed by substantial government subsidies. Despite geopolitical tensions around supply chains, global solar deployment continues to accelerate, with 2025 seeing over 510 GW of new photovoltaic capacity, more than all other electricity sources combined. At current growth rates, solar is on track to become the largest source of electricity globally before 2035.

Challenges and Limitations

Solar energy's primary limitation is intermittency: panels produce no electricity at night and reduced output during cloudy conditions. The capacity factor of solar PV, the ratio of actual output to theoretical maximum, ranges from 10 to 15% in northern climates to 25 to 30% in desert locations. This variability requires either energy storage systems, grid interconnections with other generation sources, or demand flexibility to maintain reliable electricity supply.

Land use is another consideration for large-scale solar deployment. A typical utility-scale solar farm requires 5 to 10 acres per megawatt of capacity, though this varies with panel efficiency and latitude. Agrivoltaics, the practice of co-locating solar panels with agricultural activities, and floating solar (panels installed on reservoirs and other water bodies) offer ways to reduce land-use competition. Building-integrated photovoltaics (BIPV) replace conventional building materials with solar-generating surfaces, turning roofs, facades, and windows into power generators.

End-of-life management of solar panels is an emerging concern as first-generation installations reach retirement age. Most panels contain small amounts of lead, cadmium, or other materials that require careful handling. Recycling processes can recover over 95% of the glass, aluminum, and silicon in a panel, but collection logistics and recycling economics remain challenging. Several countries and the European Union have implemented extended producer responsibility regulations requiring manufacturers to fund collection and recycling programs.