At least one embodiment of the invention relates to supplying ships in a harbor with external energy in order to prevent the formation of soot and CO2. For this purpose it is already known to supply ships with power from the wharfage. One problem in such a power supply provided from the wharf is that differences in the harbor's water level must be balanced for the power supply of a ship in the harbor. Furthermore, the operational readiness of such a power supply on land may be adversely affected, such as in the case of high water. At least one embodiment of the invention proposes a buoyant harbor power supply, e.g. the components of the harbor power supply are disposed on or in a buoyant device, particularly a barge.