Green communications has received much attention recently. For cellular networks, the base stations (BSs) account for more than 50 percent of the energy consumption of the networks. Therefore, reducing the power consumption of BSs is crucial to achieve green cellular networks. In this paper, we optimize the energy utilization in cellular networks whose BSs are powered with both regular energy from the grid and the renewable energy. We minimize the on-grid energy consumption of BSs by adapting their cell sizes. The cell size optimization problem is NP-hard. We divide the problem into two subproblems: the multi-stage energy allocation problem and energy consumption minimization problem. We propose an energy allocation policy and an approximation algorithm to solve these subproblems, respectively, and subsequently solve the cell size optimization problem. Simulation results demonstrate that the proposed solution achieves significant energy savings.