A solar cell cannot turn every photon into electricity
A typical silicon solar panel is built around a semiconductor with a fixed energy threshold, called its band gap. A photon—the particle of light—must carry at least that much energy to free an electron and help produce electric current. Photons with less energy generally pass through the cell or are otherwise not converted into electricity.
The mismatch works in the other direction, too. A photon with more energy than the threshold can free an electron, but the extra energy does not become extra electrical output. It is quickly transferred to the silicon as heat. Since sunlight contains photons with a range of energies, a single-junction silicon cell cannot use all of their energy equally well.
Where the rest of the sunlight goes
Some incoming light is reflected at the panel surface, though coatings and cell design can reduce that loss. Other light is absorbed without contributing useful current, or the freed electrons recombine before they can be collected. Even the electricity that does leave the cell encounters further losses in wiring and equipment.
These limits are built into the conversion process, not simply a sign that a panel is poorly made. Improving one part of the design can help—for example, trapping more light or reducing electrical losses—but the band gap still sets a basic tradeoff: a cell tuned to capture lower-energy photons tends to shed more of the energy carried by higher-energy ones as heat. That is why sunlight reaching a panel is not the same as sunlight available to power a home.
