A process aimed at producing energy needs to produce more energy than the energy necessary to run the process itself in order to be energetically sustainable. In this paper, an energy balance of a batch anaerobic bioreactor has been defined and calculated, both for different operative conditions and for different reactor scales, in order to analyze the sustainability of hydrogen production through dark anaerobic fermentation. Energy production in the form of hydrogen and methane, energy to warm up the fermentation broth, energy loss during fermentation and energy for mixing and pumping have been considered in the energy balance. Experimental data and literature data for mesophilic microorganism consortia have been used to calculate the energy balance. The energy production of a mesophilic microorganism consortium in a batch reactor has been studied in the 16-50 degrees C temperature range. The hydrogen batch dark fermentation resulted to only have a positive net production of energy over a minimal reactor dimension in summer conditions with an energy recovery strategy. The best working temperature resulted to be 20 degrees C with 20% of available energy. Hydrogen batch dark fermentation may be coupled with other processes to obtain a positive net energy by recovering energy from the end products of hydrogen dark fermentation. As an example, methane fermentation has been considered to energetically valorize the end products of hydrogen fermentation. The combined process resulted in a positive net energy over the whole range of tested reactor dimension with 45-90% of available energy. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.