This paper evaluates the impact of hydraulic losses on the overall performance of oscillating-water-column wave power converters. A review on energy loss in OWC’s is firstly presented, within the scope of shoreline systems. In order to assess the efficiency of the power conversion process a series of wave-tank experiments were conducted with a physical model, in regular waves. Different wave conditions and power take-off constraints were imposed on the model, for which the amplification factor, the energy absorption efficiency, the internal efficiency and the overall conversion performance of the system were determined. In parallel, a sequence of video-frames of the flow in the water column helped us to uncover the physics of energy dissipation in the system.