Computational fluid dynamics (CFD) simulations have been completed to characterize the efficiency of tidal turbines. Efficiency is taken as the ratio of power production (i.e. useful electricity) to total power lost from the upstream incident flow energy. Many tidal turbine developers are considering ducted designs which accelerate the flow through the turbine, improving the power coefficient C P . It has been found that for the designs considered, this increase in C P is associated with decreasing efficiency due to increased overall drag of the entire system. It is also found that for a given turbine, the optimum operating condition for power differs from the optimum condition for efficiency. Thus a compromise between power and efficiency must be made when considering turbines in constricted waters and arrays of turbines.