The standard photovoltaic effect is directly related to the photoelectric effect, though they are different processes. When the sunlight or any other light is incident upon a material surface, the electrons present in the valence band absorb energy and, being excited, jump to the conduction band and become free. These highly excited, non-thermal electrons diffuse, and some reach a junction where they are accelerated into a different material by a built-in potential. This generates an electromotive force, and thus some of the light energy is converted into electric energy.
In most photovoltaic applications the radiation is sunlight, which as solar cells is why the devices are known. In the case of a p-n junction solar cell, illuminating the material creates an electric current as excited electrons and the remaining holes are swept in different directions by the built-in electric field of the depletion region.
A small PV system may provide energy to a single consumer, or to an isolated device like a lamp or a weather instrument.