Correlation of actual efficiency of photovoltaic panels with air mass

•Real-world efficiency of photovoltaic parks is affected by atmospheric conditions.•The effect of air mass on the efficiency of photovoltaic panels is investigated.•The PV efficiency deteriorates quickly when the solar altitude is low (high Air Mass).•An improvement in prediction accuracy of overall efficiency of PV parks is attempted.

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° and the solar insolation drops below 200 W/m2. This affects the accuracy of prediction of overall efficiency of PV parks and deserves more attention and study.