This paper describes the development of a scalable, autonomous and inexpensive monitoring system of electric fields generated by mobile phone base stations in the 850 MHz and 1900 MHz frequency bands, in Peru. The developed system is scalable because it consists of a ZigBee wireless network with electric field strength sensor and repeater nodes in a mesh topology with a dynamic routing mechanism. Sensor and repeater nodes are autonomous because they get their power from a small photovoltaic system. All electronic circuits were developed using low-cost technology. To manage the system elements and to store the information captured by the sensors, a web platform was developed. Tests in laboratory and field were performed to assess the accuracy of the sensor node. For this, a calibrated isotropic Narda SRM 3000 electric field meter was used. The results were very satisfactory for 1900 MHz frequency band but not for 850 MHz; some recommendations are given to improve the accuracy in the latter case. The performance of the sensor nodes photovoltaic system was also assessed through the monitoring of the continuity of the system operation over time. It was verified that the sensor node was kept operating continuously without failures during a three-month test.