In this paper the power performances of a point absorber wave energy converter (WEC) operating in a nonlinear multi-directional mixed sea have been rigorously investigated. The absorbed power of the WEC Power Take Off (PTO) system has been calculated by incorporating a second order irregular wave model into a nonlinear dynamic filter. This is a new methodology that is uniquely proposed to the ocean wave energy research community. The predicted results of the WEC power performances have been rigorously analyzed and systematically compared, and the advantages of using our proposed new methodology under extreme wave states have been convincingly substantiated. The research findings in this paper highlight that it is important to employ a nonlinear wave theory under extreme wave states for studying the power performances of wave energy converters, but that linear wave theory gives good results under normal wave states.