The several error factors are included in the separations of global solar irradiance into direct and diffuse components, and the factor of short-term solar fluctuation is taken as an example. In this paper, we propose the new index for properties of short-term solar fluctuation, and indicate the characteristics how the index is occurred in actual and virtual weather conditions. We examine the round off errors of peak integrating solar irradiance, the prediction accuracy of direct-to-global irradiance, etc., with much of short-term fluctuation.
