Due to low power consumption and low duty cycle, IEEE 802.15.4 standard is widely used for Low Rate Wireless Personal Area Networks (LR − W P AN s). It works in Beacon Enabled (BEn) and Non Beacon Enabled (N BEn) mode. In BEn mode, it has Contention Access Period (CAP) and an optional Contention Free Period (CFP). During CAP, nodes contend to access the medium for communication while CFP period provides Guaranteed Time Slot (GTS) for low latency and applications requiring specific bandwidth. In this paper, we evaluate the comprehensive energy consumption of slotted CSMA/CA algorithm of IEEE 802.15.4 standard during idle and backoff periods. Simulation results with acknowledgment frame are given at the end of the paper. Index Terms—Slotted CSMA/CA; GTS; IEEE 802.15.4 ; MAC Layer; LR-WPANs.