This paper at first describes a basic study on the prediction of hourly or daily amount of solar irradiation. The method is based on theoretical equation and observation of solar irradiance. Then, the novel method to control photovoltaic-diesel hybrid generation system using the prediction is proposed to improve the total efficiency of the system. A numerical simulation result shows the possibility of the method. (C) 2002 Elsevier Science B.V. All rights reserved.