The standard efficiency of photovoltaics (PV) often deviates from the one achieved under real outdoor conditions. This paper presents relevant data collected using a small multi-crystalline photovoltaic array at an outdoor experimental facility located north of Athens. Measurements during summer and winter periods were used to quantify the PV performance and operating characteristics. Several correlations are provided in order to facilitate the estimation of the actual PV performance with readily available local parameters (e.g. ambient air temperature, wind speed). The PV efficiency was found to be about 18% lower than that under laboratory standard test conditions, under similar operating conditions. The mean annual PV efficiency was 8.7%. (C) 2016 Elsevier Ltd. All rights reserved.