In this study, the efficiency analyses of ten hydro-power plants (HPP) used for electricity generation were conducted through Data Envelopment Analysis (DEA) and window analysis (WA). Two efficiency indexes, based on production and energy unit cost performance were defined and pursued. In the calculation of operational performance, the main production factors were used as inputs, and net production was used as the output (Model 1). On the other hand, in the calculation of energy costs, the cost of generating a unit energy in terms of TL (Turkish Liras), were used as the output (Model 2). Data Envelopment Analysis (DEA) is the main instrument for measurement of relative performances of decision making units with multiple inputs and outputs. Constant returns to scale (CRS or CCR) and variable returns to scale (VRS or BCC) type DEA models were used in the analyses for this study. The relationship between efficiency scores and input/output factors was also investigated. Employing the obtained results, the hydro-power plants were evaluated with respect to both the cost of energy generation and the net production. The inputs of HPPs are not serially independent, such as the water collection at the dam reservoir through consecutive years. Thus, changes in performance relative to preceding or following years were also investigated by window analysis. (C) 2012 Elsevier Ltd. All rights reserved.