Abstract: Described is a fuel cell system wherein at least one fuel cell (8) is supplied with a compressed oxidation gas at least intermittently and/or at least partially, as well as a method for starting up a fuel cell system, said method comprising the following steps: a valve (6) connecting the at least one fuel cell (8) to a compressed gas source (1) supplying a compressed oxidation gas is opened; fuel is supplied to the at least one fuel cell (8); it is checked that the fuel cell system has reached an operating point at which sufficient power is generated for autonomous operation of the fuel cell system and for the consumers to be supplied; and a discrete or gradual switching is carried out from the supply of a compressed oxidation gas stored in the compressed gas storage source (1) to the ambient air supplied by a fan (5). The use of the fuel cell system in an emergency power supply is described.