In contention-based sensor networks, nodes compete to access a shared channel for data transmission and collision is a common challenge. For power conservation, periodical active/sleep dynamics is adopted in the design of medium access protocol (MAC). At the same time, the quality of service (QoS) requirements (e.g. packet delay, packet loss rate and throughput) need to be satisfied. We develop a finite queuing model for the sensor node and derive the network performance in contention-based sensor networks. Furthermore, the impact of active/sleep dynamics and node buffer size on the tradeoff between power efficiency and QoS requirements is studied based on the model. Simulation results match well with our analysis results and validate the accuracy of our model and approach. Keywords- Contention-based sensor networks, Finite queuing model, Power efficiency, QoS requirements