An energy storage system for direct DC coupling includes a power source electrically connected to a first node, a power converter coupled to the first node, an energy storage unit coupled to the first node through direct DC coupling, and a control system. The power converter controls the output voltage of the power source under the control of the control system. The energy storage unit draws energy from the power source or the bus when charging and supplies energy to the bus when discharging. A main switch is coupled between the first node and the energy storage unit. The control system is configure to control the power converter to adjust the output voltage to the energy storage unit voltage, control the main switch to electrically connect the energy storage unit to the first node, and control the main switch to electrically disconnect the energy storage unit from the first node.