Internet of Things (IoT) has become a favored wireless communication concept in many application domains recently. Most of the IoT networks must fulfill the strict quality of service (QoS) requirements such as low latency and high reliability. Besides, low power consumption is essential in those networks due to the use of battery operated devices. Time Slotted Channel Hopping (TSCH), an operating mode of IEEE802.15.4e, has been proposed to meet the QoS requirements of IoT networks. In TSCH, scheduling is handled with dedicated or shared cells. In IoT networks with sporadic traffic, it is more appropriate to use shared cell scheduling. However, providing QoS requirements with minimum power consumption give rise to a problem in shared cell scheduling optimization. In this study, we focus on overcoming this problem. First, we create a network model that allows estimation of the network characteristics with different parameters. Then, we propose a method that finds the optimum parameters based on these characteristics in order to meet the QoS constraints while providing energy efficiency. Moreover, simulation results show that created network model is accurate and our method is able to achieve the optimum shared cell scheduling.