Nowadays renewable energy resources have been experiencing rapid progresses in electric power networks all over the world. As a result, it is important that these clean electrical power resources be utilized for power generation in the modern power systems in the planning and operation studies. However, the uncertainty related to these energy resources and their intermittent nature result in some challenges when integrating to the power networks. The reliability of power systems is affected when the penetration level of these intermittent renewable power generations is significant. In this regard, this paper introduces an analytical technique to study the adequacy of a power system containing significant tidal power generation. For this purpose, a multi-state reliability model of a tidal plant considering both failure rate of composed components and variation in the generated power arisen from variation in the tidal current speed is developed. At first, an equivalent reliability model considering failure effects of composed components is obtained. Then Fuzzy-C-means clustering method is applied for reducing numerous values of the output power of the plant. For determining the optimum number of states in the model, XB index is calculated. The proposed multi-state model is then used for adequacy studies of RBTS and IEEE-RTS and the related indices of these systems in presence of tidal units is evaluated.