In this work, an efficient formulation for the aggregation of multiple identical generating units for the mixed integer linear programming unit commitment problem is presented. To this end, as already done in literature identical units are clustered with a single integer variable defining the number of currently committed units. To correctly model the feasible aggregated ramp speed, a shifting generation level method with low computational effort is introduced, which is the main contribution of this work. In contrast to the reference binary formulation, the number of integer and continuous variables and constraints for the integer formulation does not scale with the number of aggregated units. Thus, depending on the UC problem, the computational effort can be drastically decreased, which is shown in test cases. (C) 2019 Elsevier Ltd. All rights reserved.