The prediction of solar insolation is needed to predict the photovoltaic (PV) generation output connected to power systems. This paper proposes a method for estimating the errors of solar insolation forecasts, which are unavoidable, using only the input variables employed in solar insolation prediction and the predicted solar insolation. Given that some big errors of solar insolation forecasts concentrate in the certain intervals of some variables, we use the statistics method to find those variables and the proper boundaries of the intervals and combine them as the rules to judge the categories of errors. The error estimation technique can inform the PV/power system operators what kind of error the predicted value of solar insolation is likely to have and how much the confidence of the correct estimation is. Since the big errors cause either large over expectation or under expectation of PV outputs, the results of error estimation are useful for operators to identify the extreme situations which cause the large deviation of power. From the simulation of two study cases, error estimation results of our proposed method are acceptable.
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