The detailed balance model for quantum dot solar cells with quantum efficiencies above unity is extended to the case of non-ideal extraction of carriers from dots and finite mobilities of free carriers. For more realistic estimations of maximum efficiencies, we include experimental values for Auger lifetimes and absorption coefficients. Thus. for a given material, the model is capable of determining the critical values for the mobilities and extraction lifetimes, which are necessary to make the efficiency benefit from multiple exciton generation. (C) 2008 Elsevier B.V. All rights reserved.