In future, a high proportion of electrical energy will be generated from renewable sources. One option for using electricity generated from renewable sources, for example from wind power or photovoltaics, is to produce hydrogen by electrolysis. Different system configurations and operating conditions were investigated in order to study the efficiencies of producing hydrogen by solid oxide electrolysis. Important factors for system efficiencies are the voltage at which the stack is operated, the source of heat for the water evaporation, the pressure level of the hydrogen storage, and the system configuration. Based on these factors, four efficiencies were defined taking into account the different conditions and configurations. The calculated efficiencies vary from 104% in the best case down to 62% in the worst case, all based on the lower heating value of the hydrogen produced. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.