The modeling of energy storage devices such as supercapacitors for wireless sensor networks is important for assessing performance of harvesting-aware routing protocols that could be key to a green energy future. In this paper, we present a circuit-based model (CBM), which is a good fit to the empirical data available in the existing literature. We compare it with a linear energy model (LEM) that is often used in literature, and show that the choice of models implies a significant difference in the throughput per node.