It is provided a control device for controlling an energy storage system having an energy storage device, a bidirectional AC/DC converter and a grid connection to an electrical power distribution grid. The control device includes: a processor; an external memory interface for reading external instructions from an external removable memory device; a control interface for controlling the energy storage system; and an internal memory. The internal memory stores internal instructions that, when executed by the processor, cause the control device to provide access to the control interface for external instructions, read from the external removable memory device, when the external instructions are executed by the processor.