This paper concerns the prediction of the power output performances of an oscillating surge wave energy converter in irregular waves. The generated power has been calculated using a new methodology by incorporating a stochastic Lagrange wave model into a non-linear filter which takes also hydrodynamic forces into account. It is shown that the stochastic Lagrange wave model in this paper can be utilized to generate irregular waves with realistic crest-trough and front-back asymmetries, and when used in combination with the non-linear filter will produce more accurate power output predictions than the traditional Gaussian wave model can do. The research findings in this paper demonstrate that the stochastic Lagrange wave model together with a non-linear filter can be utilized as a robust tool for engineers in their design, analysis and optimization of wave energy converters. (C) 2018 Elsevier Ltd. All rights reserved.