The aim of the present study is to analyse the importance of considering the variation in the discharge coefficient (Cd) of a sluice passage and the tidal current speed (i.e., the flow speed approaching the passage) in improving the estimation of power output for a tidal power plant. The simulation results, using a commercial CFD software package, Flow 3D, have shown that the Cd value of a sluice passage varies with the upstream (i.e., on the sea side of the impounded area) water level. In addition, the results of a case study carried out in the Taedong Bay (the DPR Korea), using a zero-dimensional (0-D) model, highlight the importance of considering variations in the tidal current speed as well as Cd when estimating the power output for tidal power plant. In the present study, a detailed scheme is proposed to consider the approach speed at each stage of filling, holding, generating, and holding for the mode of ebb generation. It is hoped that the results of the present study will help scientists, engineers, and decision makers to accurately evaluate the tidal power output of tidal barrages and lagoons in many regions of the world. Highlights • The tidal current speed change must be considered in tidal power output estimation. • The tidal current speed is considered in different ways during a tidal cycle. • The variation in discharge coefficient of a sluice passage must also be considered. • The discharge coefficient gradually decreases as the upstream water level increases.