The present invention relates to a method and a system for calculating a system net load fluctuation reflecting renewable energy supply expansion. The method for calculating a fluctuation of a system net load comprises the steps of: accumulating electricity use load information for many years and use rate trends of new renewable power generation in a database; extracting the electricity use load information and use rate trends of new renewable power generation of a reference date matching the weather of a corresponding day by reflecting the weather of the corresponding day with regard to an analysis target load and an amount of new renewable power generation by a predetermined time period of an analysis target day in the database; and referring to the extracted trend to calculate a net load by a corresponding time period for the future including the day, and analyze a net load fluctuation so as to calculate the net load fluctuation by a load level, a net load probability and a net load maximum fluctuation.
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