Modeling the amount of solar energy received by a photovoltaic panel is an essential part of Green IT research. The specific motivation of this work is the management of the energy consumption of large datacenters. We propose a new stochastic model for the solar irradiance, featuring minute-scale variations and therefore suitable for short-term control of performances. Departing from previous models, we use a weather-oriented classification of days obtained from past observations, to parameterize the solar source. We demonstrate through extensive simulations, using real workloads, that our model outperforms the existing ones in predicting performance metrics related to energy storage.