This paper discusses the technical considerations when SOFCs are used for mixed DG/backup power applications, where the SOFCs work as distributed generators that output power to the grid when the grid is connected, while pick up the critical load in an acceptable time scale and continue the power supply to this critical load during a grid blackout. The principle of SOFC power generation is described, with reference to a prototype of a real 110 kWe tubular SOFC combined heat and power system, which is manufactured by Siemens Westinghouse and has operated at TurboCare S.p.A (Siemens Group) Turin, Italy. The technical requirements for a mixed DG/backup power system are presented and the system structure is detailed with special consideration of the technical aspects of the power conditioning systems. Simulation results based on a Matlab/Simulink model are reported assuming a grid blackout and a step increase of the critical load. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.