The paper deals with extensive monitoring of photovoltaic technologies. It was carried out in the last 18 months at the outdoor development center HEnergia of HERA.S.p.A. in Forli (Italy). HEnergia has 8 different photovoltaic plants of small size (under 3 kWp) and they are differentiated by photovoltaic cell technologies (heterojunction with intrinsic thin layer, polycrystalline, cadmium telluride, amorphous silicon with microcrystalline silicon, silicon triple-junction cells, multi-junction Gallium Arsenide cells), by solar tracking systems (fixed installation, mono-axis or dual-axis tracker) and by optic devices (no concentration device, concentration through Fresnel lens or Cassegrain optic). Photovoltaic plants are remotely monitored and data on environmental conditions, module temperature and power production are continuously acquired and stored by a PC. Photovoltaic plant performance can thus be measured on-site and compared in relation to different environmental conditions. Moreover, the impact of maintenance activities (as for example the cleaning of photovoltaic modules) on photovoltaic plant performance is also assessed. On the basis of experimental data, the paper also shows an economic assessment of photovoltaic plants. (C) 2015 Elsevier Ltd. All rights reserved.