The statistics of low level scattered cloud cover (cumulus or strato-cumulus types), is evolved to formulate analytical expressions for short duration mean output power and its variability, as well as power change distribution, at the output of photovoltaic generating stations. The proposed expressions are used to obtain statistical estimates of the three, which are substantiated by comparison to empirical ensemble estimates reported in available literature. The strength of concepts and formulations presented in the paper is their dependence on a single aggregated meteorological parameter, namely the optical air mass. Their applicability therefore spans over a range of PV generation plants regardless of geographical and weather conditions. (C) 2015 Elsevier Ltd. All rights reserved.