To decrease fuel used in a prime mover for an auxiliary machine thereby to reduce exhaust gas by operating in an efficient region the prime mover for the auxiliary machine which drives auxiliary machine generators when a marine vessel enters a port and reaches the shore.

A marine vessel energy system includes: a power detection means 20 for detecting power consumed by an electric propulsion device 200a; a charging/discharging power storage device 21 connected to an in-vessel power bus 6; a starting switch 12 for giving a start command to a bow thruster 300; and a control circuit 22 which changes the power storage device into a charge mode when a detection signal of the power detection means becomes a threshold value or below, and changes it into a discharge mode upon the start command of the starting switch and discharges the stored power to the in-vessel power bus. When propulsion is carried out by decreasing the propulsion force of the electric propulsion device during entering the port and the power consumption thereof becomes a threshold value or below, the power storage device is charged from the auxiliary machine generators 4-1 and 4-2 by the control circuit. If the start command of the starting switch is received during the charging operation, the power storage device is operated to discharge, and the discharged power is supplied to the bow thruster.