An autonomous Internet of Things (IoT) wireless sensor network (WSN) powered by a solar energy harvester with low power electronics is presented in this paper. The power supply is a critical challenge for IoT sensors as many of them normally have limited lifetime due to the battery with limited capacity used. The WSN system, sensor node electronics, and the energy harvesting techniques are all configured to achieve a continuous energy source and low power consumption. The IoT network system has been deployed on Monash University Clayton campus, Melbourne, for monitoring temperature, relative humidity, carbon dioxide and carbon monoxide data. The sensor network uses multiple XBee wireless modules and successfully monitors the useful data for six months. This work demonstrates that energy harvesting enables an IoT sensor platform to become always active and reliable for long-term, providing many opportunities and applications.