ENEL (Italian Electricity Board) has developed a data monitoring system D.M.S. specially suited to isolated PV (Photovoltaic) power plants and which provides a highly reliable and low-cost global monitoring of plant operation and performance. This D.M.S. combines several characteristics particularly suitable in isolated installations, and contains innovative dc energy measuring counters as compared with similar equipment. This paper describes the main features of this D.M.S., shows how it is installed in small PV plants, and what quantities can be acquired and calculated.