Wells turbines are among the most interesting power takeoff devices used in oscillating water column (OWC) systems for the conversion of ocean-wave energy into electrical energy. Several configurations have been studied during the last decades, both experimentally and numerically. Different methodologies have been proposed to estimate the efficiency of this turbine, as well as different approaches to evaluate the intermediate quantities required. Recent works have evaluated the so-called second-law efficiency of a Wells turbine, and compared it to the more often used first-law efficiency. In this study, theoretical analyses and numerical simulations have been used to demonstrate how these two efficiency measures should lead to equivalent values, given the low pressure ratio of the machine. In numerical simulations, small discrepancies can exist, but they are due to the difficulty of ensuring entropy conservation on complex three-dimensional meshes. The efficiencies of different rotor geometries are analyzed based on the proposed measures, and the main sources of loss are identified.