This paper studies container ports' performance, particularly in relation to the time each individual berth is utilised, and by which categories of cargo ships. Such information is of relevance for both port and shipping energy management, for example planning of On-shore Power Supply (OPS) solutions. Unit of analysis is berths at container terminals, where ships are moored and loading and unloading of containers are performed. Berth utilisation ratio is the percentage of time a berth is occupied by a ship being serviced. This study develops and applies a detailed cargo ship stationary event measurement method, to investigate time and place berthed in a port area. The vessel movement data in this study is provided by the Automatic Identification System (AIS), an information system that automatically transmits a vessel’s information to nearby vessels and to port authorities. The method developed is less costly than collecting traffic and berth utilization data directly from each individual port and terminal operator. The method is suited to investigate small container ports, which may include multi-purpose terminals and mixed cargo ship traffic. A case study for the 7 principal container ports in the Oslo Fjord area is performed, measuring the actual berth utilisations. A comparison of berth utilisations with theoretical congestion factors indicates that the Oslo Fjord container terminal berth utilisations can be considered as ‘low’ for 6 of the ports, implying little queueing of vessels to get a free berth upon port arrival, to the benefit of the shipping liner operators.