An energy storage system , connectable to a power supply source (R) 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 (R) and comprising at least one electric charge storage unit and - a conversion assembly , interposed between said power supply source (R) and said storage assembly , for the conversion of an electric current (i) in transit between said power supply source (R) 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 (R) 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 (R), for harmonics management, said filter assembly comprising in series, from the power supply source (R) 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 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 (R) and comprising at least one electric charge storage unit and - a conversion assembly , interposed between said power supply source (R) and said storage assembly , for the conversion of an electric current (i) in transit between said power supply source (R) 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 (R) 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 (R), for harmonics management, said filter assembly comprising in series, from the power supply source (R) 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 functionally connected to said electronic control system ; <EOS>