Photovoltaic installations are already producing more than 5% of electricity in several European countries. Monitoring of photovoltaic parks produces a huge amount of data that are employed in checking the real-world efficiency of photovoltaic panels in a variety of locations, for different atmospheric and environmental conditions. The aim of this paper is to present real-world efficiency data for photovoltaic panels and photovoltaic parks, as function of air mass and environmental conditions. The PV efficiency deteriorates quickly when the solar altitude is less than 45 degrees and the solar insolation drops below 200 W/m(2). This affects the accuracy of prediction of overall efficiency of PV parks and deserves more attention and study. (C) 2014 Elsevier Ltd. All rights reserved.