In a single-phase ac circuit, instantaneous power to a load is of a pulsating nature. Even at unity power factor (i.e., when the voltage and the current are in phase with respect to each other), the instantaneous power is less than unity (i.e., when the voltage and the current are not in phase). The instantaneous power is not only zero four times in each cycle but it is also negative twice in each cycle. Therefore, because of economy and performance, almost all electrical power is produced by polyphase sources (i.e., by those generating voltages with more than one phase*).