A method for determining an electrical quantity in an electrical installation, by means of a measurement system comprising at least one voltage measurement module and at least one current measurement module coupled to the electrical installation, each of said measurement modules comprising a sensor, a processor, a memory and a clock, the method comprising: • by the voltage measurement module: ▪ periodically measuring an electrical voltage in the electrical installation, ▪ periodically sending a synchronisation signal to at least one of the current measurement modules, ▪ sending to said current measurement module a message comprising at least one main timestamp data indicating the instant at which the voltage measurement module transmitted the synchronisation signal, this instant being measured by the voltage measurement module with its clock, ▪ sending a message comprising at least one measured voltage value to said current measurement module, • by a current measurement module: ▪ periodically measuring an electrical current in the electrical installation, ▪ on receiving the synchronisation signal sent by the voltage measurement module, calculating, by means of the clock of said current measurement module, a local timestamp data indicating the time at which the current measurement module received said synchronisation signal, ▪ determining successive delay correction values from the main timestamp data received and the local timestamp data calculated for each synchronisation signal received from the voltage measurement module, the successive current measurements carried out by the current measurement module being time-stamped by the current measurement module, by means of its clock, taking into account the delay correction values thus determined, • by a processor, calculating at least one value of an electrical quantity from successive current and voltage values measured by the measurement modules. • by the voltage measurement module: ▪ periodically measuring an electrical voltage in the electrical installation, ▪ periodically sending a synchronisation signal to at least one of the current measurement modules, ▪ sending to said current measurement module a message comprising at least one main timestamp data indicating the instant at which the voltage measurement module transmitted the synchronisation signal, this instant being measured by the voltage measurement module with its clock, ▪ sending a message comprising at least one measured voltage value to said current measurement module, ▪ periodically measuring an electrical voltage in the electrical installation, ▪ periodically sending a synchronisation signal to at least one of the current measurement modules, ▪ sending to said current measurement module a message comprising at least one main timestamp data indicating the instant at which the voltage measurement module transmitted the synchronisation signal, this instant being measured by the voltage measurement module with its clock, ▪ sending a message comprising at least one measured voltage value to said current measurement module, • by a current measurement module: ▪ periodically measuring an electrical current in the electrical installation, ▪ on receiving the synchronisation signal sent by the voltage measurement module, calculating, by means of the clock of said current measurement module, a local timestamp data indicating the time at which the current measurement module received said synchronisation signal, ▪ determining successive delay correction values from the main timestamp data received and the local timestamp data calculated for each synchronisation signal received from the voltage measurement module, the successive current measurements carried out by the current measurement module being time-stamped by the current measurement module, by means of its clock, taking into account the delay correction values thus determined, ▪ periodically measuring an electrical current in the electrical installation, ▪ on receiving the synchronisation signal sent by the voltage measurement module, calculating, by means of the clock of said current measurement module, a local timestamp data indicating the time at which the current measurement module received said synchronisation signal, ▪ determining successive delay correction values from the main timestamp data received and the local timestamp data calculated for each synchronisation signal received from the voltage measurement module, the successive current measurements carried out by the current measurement module being time-stamped by the current measurement module, by means of its clock, taking into account the delay correction values thus determined, • by a processor, calculating at least one value of an electrical quantity from successive current and voltage values measured by the measurement modules.