A generator set, also known as a genset, is the combination of an electrical generator and an engine mounted together to form a single piece of equipment that produces electrical power. In the generator, referred to as an alternator, the mechanical work of the engine generates electrical power. Generation sets are used in sites that are not connected to the power grid or to supply emergency power when the grid fails. Generator sets may also be employed to produce energy during peak usage hours when the energy costs are the highest, to decrease the amount of electricity that must be purchased from the utility company, in a power-saving application called peak shaving.