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Dense deployment of small cell base stations (SBSs) powered by renewable energy is spotlighted as a solution of the increasing demand of communication services. How to utilize renewable energy efficiently under the temporal change of traffic loads is a brand new challenge. In this paper, we design real-time controllers for cell zooming, that is, to determine transmission power and activation/deactivation of energy harvesting SBSs. The proposed method maximizes the total number of accommodated users, considering the remaining energies of batteries. First, we solve a problem of finding a number of active SBSs that maximizes energy efficiency with full accommodation of active users. Then, we apply model predictive control to the real-time computation of an optimal transmission power. Moreover, we present an explicit formula of an approximate optimal transmission power, by simplifying the proposed method …