In this paper, the focus is on energy efficiency in cooperative spectrum sensing in cognitive radio networks by considering multiple channels in a slotted time scenario. The sensing time consists of mini slots. A secondary user uses a mini-slot to sense a channel. Considering multiple channels, throughput, energy consumption, and, finally, energy efficiency are calculated. Additionally, the optimal number of mini-slots are obtained regarding throughput and energy efficiency using the particle swarm optimization algorithm. Simulation results are presented for the algorithms considered in this paper.