Solar panels, also called solar modules, are flat sheets that turn sunlight into direct current (DC) electricity. A solar panel is a single such sheet. Most panels are made of silicon cells, thin wafers that give out electricity when light falls on them. Some panels use a very thin layer of another material laid on glass, which is called thin film.
A panel is rated in watts. A rating of 400 watts means the panel gives 400 W in a standard test. The test shines light of 1,000 watts on each square metre, with the cells at 25 °C. On a roof, the panel usually gives less. A typical panel is about 2 metres by 1 metre and turns roughly one-fifth of the sunlight on it into electricity. Panels have no moving parts, so they last 25 years or more, though their output falls slowly each year.
A panel gives out DC, which moves in one direction. A home runs on alternating current (AC). So a panel is always joined to a solar inverter. The difference between the two is explained on the page about AC and DC.
Consider a rooftop in Pune with ten panels of 400 watts each. In full noon sun they give about 4,000 watts. In the evening they give almost nothing. The output also falls when a panel is dirty or in shade. Over a day, the roof makes roughly 16 to 20 units (kilowatt-hours) in Indian conditions, which is what the electricity bill counts.
Panels are bought in three ways. A home owner buys a few, using solar design software to plan the roof. A power producer buys lakhs of them for a solar park, often with a tracker under each row. A group of households may share one large array, which is called community solar. A panel's electricity can also run an electrolyser that makes hydrogen. Most panels sold in India are now assembled in Indian factories. But many of the solar cells inside them are still imported.