Wireless Power Transfer (WPT) technology has paved the way to energize numerous sensor nodes for the Internet of Things (IoT) applications. This work theoretically and experimentally shows how a commercial multi-function sensor board can be energized by an ultra-high-frequency (UHF) multi-stage rectifier at low RF input power levels. Firstly, an analytical model characterizing multi-stage rectifiers is derived. Good accuracy of modeling results is observed in the low input power range of interest, compared with results obtained from the ADS Harmonic Balance simulator. Then, a 5-stage rectifier is employed to power the multi-function sensor board that can acquire the information of temperature, humidity, and light. This sensor board further transmits packets to an access point connected to a laptop for data visualization, making the power consumption of the sensor board relatively large. Hence, a 50-mF supercapacitor is required to drive such a power-hungry sensor board. The final demonstration shows that the multi-function sensor board works adequately in a low-duty cycle powered by a 5-stage rectifier with an input power of-10 dBm.