Fuel cell
OverviewEfficiency of leading fuel cell typesHistoryTypes of fuel cells; designApplicationsMarkets and economicsResearch and developmentFurther readingThe energy efficiency of a system or device that converts energy is measured by the ratio of the amount of useful energy put out by the system ("output energy") to the total amount of energy that is put in ("input energy") or by useful output energy as a percentage of the total input energy. In the case of fuel cells, useful output energy is measured in electrical energy produced by the system. Input energy is the energy stored in the fuel. According to the U.S. Department of Ener