How Solar Panels Work: From Sunlight to Usable Electricity

A solar panel does not convert heat into electricity the way a solar thermal plant does, and it does not store sunlight. It uses a quantum physics phenomenon called the photovoltaic effect to directly knock electrons loose from semiconductor atoms whenever light strikes them. The result is an electric current that can power your home or flow back into the grid.

Step by Step: From Photon to Power Outlet

1. Photons arrive from the sun. Sunlight is composed of particles of light called photons. Each photon carries an amount of energy determined by its wavelength: ultraviolet photons carry more energy than visible light photons, which in turn carry more than infrared ones. Not all of them will be usable.
2. The photon enters a silicon solar cell. Most commercial panels use silicon, the second most abundant element in Earth's crust. A solar cell is made of two thin layers of silicon processed to have different electrical properties. The n-type layer has been doped with phosphorus atoms, giving it an excess of free electrons. The p-type layer has been doped with boron atoms, creating an excess of positively charged holes where electrons are absent.
3. The p-n junction creates an electric field. Where the two layers meet is the p-n junction. Free electrons from the n-type side and holes from the p-type side diffuse across this boundary until an equilibrium forms, producing a built-in electric field pointing from the n-type side toward the p-type side.
4. A photon knocks an electron loose. When a photon with enough energy strikes a silicon atom near the p-n junction, it transfers its energy to a valence electron, breaking it free from its bond. The photon is absorbed in the process. This is the photovoltaic effect.
5. The electric field separates the charges. The built-in field at the junction pushes the freed electron toward the n-type side and the corresponding hole toward the p-type side, preventing them from immediately recombining. This charge separation creates a voltage difference across the cell, typically around 0.5 to 0.6 volts for silicon.
6. Metal contacts collect a direct current. Thin metal fingers printed on the front surface and a metal sheet on the back collect the flowing electrons. Connect an external circuit and electrons flow from the n-type contact through the load and back to the p-type contact. This constitutes direct current (DC). Dozens of individual cells wired together inside a panel accumulate both voltage and current to a useful level.
7. The inverter converts DC to AC. Household appliances and the electrical grid operate on alternating current, which reverses direction 50 or 60 times per second depending on the country. A power electronic device called an inverter takes the panel's DC output and rapidly switches its direction using transistors, producing AC output that matches the grid's frequency and voltage.
8. Net metering returns surplus to the grid. When the panels produce more power than the household is consuming at that moment, the surplus flows backward through the utility meter. Under net metering policies, the utility credits the homeowner for this exported electricity, effectively using the grid as a giant battery during sunny hours and drawing from it at night.

Comparing the Three Main Panel Types

Type	Typical efficiency	Relative cost	Best application
Monocrystalline silicon	20–24%	Highest	Space-constrained rooftops where maximum output matters
Polycrystalline silicon	15–18%	Medium	Larger rooftops where cost per watt outweighs space
Thin-film (CdTe, CIGS)	10–13%	Lowest	Curved surfaces, building-integrated installations, large ground arrays

Why Efficiency Has a Hard Ceiling

Photons with energy below the silicon bandgap pass straight through the cell without being absorbed. Photons with energy far above the bandgap free an electron, but the excess energy simply becomes heat rather than electrical output. These two losses alone account for more than half of all incoming solar energy. Physicists William Shockley and Hans Queisser calculated in 1961 that a single-junction silicon solar cell under standard sunlight conditions can convert at most about 33 percent of incoming photon energy into electricity, regardless of manufacturing quality. This is called the Shockley-Queisser limit.

The best commercial monocrystalline panels reach around 23 percent under real-world conditions, approaching but not exceeding this boundary. Laboratory cells using multiple junctions of different semiconductor materials, each tuned to absorb a different portion of the solar spectrum, have exceeded 40 percent efficiency. These multi-junction cells are currently too expensive for mainstream rooftop use but are standard in satellites and concentrated solar systems where cost per square centimeter is less important than maximum output.