A general and widely applicable methodology to assess and present the performance of wave energy converters (WEC) based on sea trials is presented. It is meant to encourage WEC developers to present the performance of their WEC prototypes, on a transparent and equitable way while taking care of possible discrepancy in the observed performance of the WEC. Due to the harsh uncontrollable conditions of the sea that is encountered by WECs during sea trials, some of the performance of the WECs might be sub optimal and the data sets not fully complete. The methodology enables to filter the data by applying a selection criterion on the performance data that was obtained for a certain range of wave conditions. This selection criteria result in a subset of performance data representing the performance of the WEC for specific wave conditions, from which an average value an appreciation of the related uncertainty can be derived. This can lead to the estimation of the annual energy output of the WEC at its test location, while it also provides a method to estimate its annual energy output for another location of interest and possibly also at another scaling ratio. The same methodology can also be used to perform parametric studies with environmental or device dependent parameters and to analyse the power conversion chain from wave to wire, which both could lead to an enhanced understanding of the performance and behaviour of the WEC. The same methodology is also applicable to tidal devices or any other developing technologies that are used in an uncontrollable environment. (C) 2012 Elsevier Ltd. All rights reserved.