A new data logger using the Arduino open-source electronic platform was developed to solve the current problem of monitoring photovoltaic (PV) systems at low-cost, especially in remote areas or regions in developing countries. The data logger meets all of the relevant requirements in terms of accuracy included in the International Electrotechnical Commission (IEC) standards for PV systems, with a resolution of 18-bits, including 8 analogue inputs for measuring up to 8 PV modules and/or weather sensors, 3 inputs for low-cost analogue temperature sensors and virtually unlimited inputs for digital temperature sensors. The new data logger is completely autonomous, and the prototype has achieved an initial cost of only 60 (sic). It was tested during a 6-month period under the harsh environmental conditions of the summer and winter in Southern Spain. The results using both the sensors and silicon reference cells indicate that the new system is reliable and exhibits comparable performance to commercial systems. This data logger is of special interest for both solar energy research and applications in developing countries, as it is both open-source and flexible. The data logger can be customised for the specific needs of each project at low-cost. The details of the specific design and its implementation are described. (C) 2014 Elsevier B.V. All rights reserved.