This paper presents a global vision of the study of an IoT sensor node entirely powered by ambient Radio Frequency (RF) waves in the WiFi 2.45 GHz ISM band. The node includes a temperature sensor and an RF microcontroller to send the measurements through Bluetooth Low Energy signal. After characterizing the energy consumption of the sensor node and measuring the available WiFi surrounding power, we were able to size the RF to DC converter and the associated optimal capacitor capable of storing the converted WiFi energy necessary to supply the node. The result was a study of the energetically viable nature of such a system. According to the node’s supply voltage, varying from 1.8 V to 3.3 V, it is possible to perform a measurement and a transmission of the sensor data several times a day from a WiFi signal as low as −20 dBm.