摘要： The performance analysis of photovoltaic (PV) plants plays an important role in making investment decisions, creating more efficient designs, and in the operation and maintenance decisions in new plants. In addition, performance analysis provides a way to identify real-time operational problems and allows comparisons among different PV plants. Performance of PV plants can be indexed with many parameters, but service providers are giving energy guarantees according to their performance ratio (PR). There are many factors that affect PR, such as irradiation, temperature, wind speed, humidity and soiling. But, the most important factors affecting PR are irradiation and temperature. Hence, both the irradiation and the temperature must be measured correctly. Solar irradiation is usually measured with a pyranometer or reference cell. However, there are differences between the obtained data from these two sensors, which affects the PR significantly. In this study, the PR of an MW PV plant installed in Turkey was investigated. Two types of irradiation sensors (pyranometer and reference cells) and PVGIS program were used. The data were collected every 5 min during 1 year. The data passed through quality control and filtered process and some improper values were removed from the row data set. In addition, new data were written instead of the missing data. PR was calculated by using conventional and corrected method. As a result, the average annual PR is determined to be 81.05%, 79.04% and 78.96% and the corrected PR is determined to be 83.69%, 82.48% and 80.87% using the reference cell, pyranometer and PVGIS program, respectively.