Environmental monitoring is an important application for wireless sensing devices. Battery power requires in-the-field replacement, and the chemicals involved are environmentally harmful, so harvested energy is a useful alternative. Previous research has shown the feasibility of powering LoRaWAN sensors using high-energy ambient or wireless transfer power sources. This paper extends this work by exploring the boundaries of using low-energy RF ambient sources. Ambient RF energy harvesting is an attractive option, but it is more challenging due to the low levels of energy density typically available. Using an analytical LoRaWAN device model and RF energy data collected from around the world, a systematic investigation of the design and environmental space is performed. The main contribution of this paper is to identify the boundaries of feasibility for powering LoRaWAN sensor nodes from ambient RF energy. These boundaries include design and environmental factors.