An energy storage system, connectable to a power supply source for the supply of electric power and comprising: - a storage assembly for storing at least part of the power supplied by said power supply source and comprising at least one electric charge storage unit and - a conversion assembly, interposed between said power supply source and said storage assembly, for the conversion of an electric current (i) in transit between said power supply source and said storage assembly, characterized in that said conversion assembly comprises: - a variable-frequency three-phase vector machine consisting of a driver for vector control of sensorless field-oriented motors which comprises a transistor bridge connected to capacitors, said variable-frequency three-phase vector machine being configured to convert bidirectionally the electric current (i) that circulates between said power supply source and said storage assembly, delivering or absorbing selectively active power, inductive reactive power and capacitive reactive power; - a filter assembly, arranged between the three-phase vector machine and the power supply source, for harmonics management, said filter assembly comprising in series, from the power supply source toward the three-phase vector machine: a three-phase low-pass filter of the EMI type, a line choke and a sinusoidal filter, said sinusoidal filter being configured to resonate at the switching frequency of the three-phase vector machine; - an electronic control system, configured to control at least said three-phase vector machine in order to vary one or more electrical parameters of said electric current (i), as a function of one or more electrical parameters detected by a plurality of detection devices (71, 72) functionally connected to said electronic control system; at least said storage assembly, said conversion assembly and at least one of said detection devices being functionally connected by means of a bidirectional connection apparatus for the passage of electric current (i) and of data. - a storage assembly for storing at least part of the power supplied by said power supply source and comprising at least one electric charge storage unit and - a conversion assembly, interposed between said power supply source and said storage assembly, for the conversion of an electric current (i) in transit between said power supply source and said storage assembly, - a variable-frequency three-phase vector machine consisting of a driver for vector control of sensorless field-oriented motors which comprises a transistor bridge connected to capacitors, said variable-frequency three-phase vector machine being configured to convert bidirectionally the electric current (i) that circulates between said power supply source and said storage assembly, delivering or absorbing selectively active power, inductive reactive power and capacitive reactive power; - a filter assembly, arranged between the three-phase vector machine and the power supply source, for harmonics management, said filter assembly comprising in series, from the power supply source toward the three-phase vector machine: a three-phase low-pass filter of the EMI type, a line choke and a sinusoidal filter, said sinusoidal filter being configured to resonate at the switching frequency of the three-phase vector machine; - an electronic control system, configured to control at least said three-phase vector machine in order to vary one or more electrical parameters of said electric current (i), as a function of one or more electrical parameters detected by a plurality of detection devices (71, 72) functionally connected to said electronic control system;