Air compressors generate pressurized air that provides a readily available power source to connect to and operate air-powered tools and equipment. They are powered by electricity or by a mounted engine. The type and size of air compressor needed is based on the air flow requirements, measured in cubic feet per minute (cfm), of the tools and equipment that the air compressor will be operating simultaneously. Air compressor pumps can replace a worn pump in an existing system of the same manufacturer. Air tanks temporarily store the compressed air for use when the compressor isn't running, or to provide additional air during periods of high demand.