An emergency power system is connected between an AC voltage source and a load. The emergency power system ensures a stable voltage on the output, even if the AC voltage received from the AC voltage source is irregular or is interrupted briefly. The emergency power system includes an AC/DC converter coupled between the load terminals and an AC regulator positioned between the AC voltage source and the load. The AC/DC converter is coupled to a chargeable battery and serves as a charging rectifier for the battery in normal operation and as an inverter in emergency operation, the battery voltage being converted to AC voltage on the output of the emergency power system. The AC regulator is built as a controllable current generator supplying on its output an AC current which is in phase with the AC voltage received from the AC voltage source. Irrespective of the energy direction through it, the AC/DC converter maintains the predetermined AC voltage (V.sub.OUT) across the output terminals of the emergency power system. The system includes a comparator which senses and compares the battery state to a predetermined reference and supplies a signal in response to this. The intensity of the current supplied by the AC regulator is controlled in response to the signal generated by the comparator so that in terms of energy the system is in power balance.