To develop a new formula for calculating irradiation onto an arbitrary orientation-inclination plane, detailed examinations have been made by using data obtained at every 1 min interval with four types of pyranometer, such as horizontal, diffuse, direct normal, and tilt global. Instantaneous irradiance values sampled with 1 min interval have been verified by clearness index, which is a parameter independent of the lapse of time. Scattered tendencies observed on direct and diffused component ratios have been studied in detail by classifying data into different classes of clearness indices. In our model, moving averages of time-series irradiance data provide the information of weather condition in order to select model parameters prepared for assumed three types of clouds conditions. Only global irradiance data monitored at 1 min intervals are used for the input of the model. The result of this work demonstrates that an algorithm associated with the Perez model can estimate the in-plane irradiation with a RMSE of about 50 W/m(2). (C) 2001 Published by Elsevier Science B.V. All rights reserved.