To propose a formula for calculating in-plane irradiation onto a tilt surface from measured data on the tilt angle, detailed examination has been made by using data obtained for every 1 min. A new model to estimate in-plane irradiation from horizontal irradiance has been developed. Especially, it is showed that new model can estimate scattered irradiance correctly rather than the existing model by using scattered component ratio and clearness index. It is considered that measuring 1-min data has favorable influences instead of hourly data. Fluctuation of 1-min data contributes to estimate the scattered component. (C) 2002 Elsevier Science B.V. All rights reserved.