Ship designers hardly ever receive feedback from the actual operation of their designs apart from sea acceptance trials. This results in decisions based on assumptions that lack accounting for the diversity of actual operational conditions. Similarly, crews operating the vessels do not receive a clear picture on the energy efficiency, effectiveness and environmental footprint of different options. This paper proposes an energy assessment method using operational data from continuous monitoring, in order to provide insight on the impact of design and operational decisions and assist in taking better advised and weighted ones.