Classical approaches based on the Angstrom equation for expressing the solar global irradiation in terms of sunshine durations are abundant in the literature. However, all of them include mostly linear and to a lesser extent nonlinear relationships between these variables. The parameters in these relationships are determined invariably by the least squares technique leading to regression lines or curves as models. None of these models provides within year variations in thr parameters and they are all very rigid in the application yielding to a single solar global irradiation estimate for a given sunshine duration value. This paper presents a solar irradiance polygon (SIP) concept for evaluating both qualitatively and quantitatively the within year variations in the solar energy variables. On the basis of Sips, monthly, seasonal and annual parameters of the classical Angstrom equation are calculated. (C) 1999 Elsevier Science Ltd. All rights reserved.