Wireless Sensor Networks (WSNs) are intrinsically energy-constrained networks. Researchers have proposed the integration of energy harvesting technologies into sensor nodes to overcome that constraint and provide a sustainable WSN. This paper presents a formal approach to model and evaluate the Green Carrier Sense Multiple Access with Collision Avoidance (GCSMA/CA) MAC protocol in Energy Harvesting WSNs. This approach exploits the Hierarchical Timed Coloured Petri Nets (HTCPNs) formal method to design the GCSMA/CA protocol with the CPN-Tools for evaluating the proposed models. In addition, both qualitative and quantitative verification of the proposed models is tackled in this research. Linear Temporal Logic is used to formalise qualitative properties, and quantitative properties are evaluated by exploiting the monitor technique in CPN-Tools. The presented approach focuses on particular properties (performance delay, waiting time, waiting time for an acknowledgement, energy consumption and throughput) that have been overlooked in most prior studies. Time and hierarchy are two crucial features of the modelling process that HTCPNs offer. The time aspect allows time constraints for the GCSMA/CA protocol to be specified. On the other hand, the hierarchy aspect simplifies the modelling process and creates a modular and manageable model.