A photovoltaic energy storage system includes an energy storage device, an inverter, a photovoltaic array device, a DC/DC conversion device and a controller. When an output voltage of the photovoltaic array device is greater than or equal to a predetermined threshold value, the photovoltaic energy storage system is operated in a first working mode. When the output voltage of the photovoltaic array device is lower than the predetermined threshold value, the photovoltaic energy storage system is operated in a second working mode. The DC/DC conversion device includes a bypass switch and a DC/DC converter. When the photovoltaic energy storage system is operated in the second working mode, the bypass switch is turned on and the DC/DC converter is disabled. Consequently, the electric power of the energy storage device is transmitted to the inverter through the bypass switch.