A wireless sensor network (WSN) consists of a large number of networked sensor nodes deployed to sense and report a particular phenomenon to a base station. One of the main design considerations for WSNs is the energy consumption of the individual sensor nodes. Currently, most WSNs are radio frequency (RF) based, and communications account for a significant portion of the energy expended. The hybrid radio frequency/free space optical (RF/FSO) WSN is proposed to lower energy consumption. The RF and FSO link models used for the RF/FSO WSN are first discussed. Next, the energy expended by the sensor nodes for sensing, data processing and communications is modeled. These models are used to simulate the lifetime of the WSN. To investigate the energy efficiency of the RF/FSO WSN, an RF-only WSN is used for comparison. Simulations show that the average energy consumed by the nodes in the RF/FSOnetwork is significantly lower than that of the RF-only network, thus increasing the lifetime of traditional RF-based WSNs.