PV cells are made from layers of semi-conducting material, usually silicon. When light shines on the cell it creates an electric field across the layers. The stronger the sunshine, the more electricity is produced. Groups of cells are mounted together in panels or modules that can be mounted on your roof or installed as a stand alone system.
The power of a PV cell is measured in kilowatts peak (kWp). That’s the rate at which it generates energy at peak performance in full direct sunlight during the summer. PV cells come in a variety of shapes and sizes and most PV systems are made up of panels that fit on top of an existing roof, but you can also fit solar roof tiles or have the panels built into the roof.
Reference: The Energy Saving Trust