A system for generating back-up electrical power. The system includes a vessel adapted to contain a volume of compressed gas and a valve to release gas from the vessel at a predetermined pressure. A scroll expander is adapted to receive and pass the released gas. A rotary member is rotated by the flow of the released gas passed by the expander. An electrical generator is drivingly connected to the rotatable member of the expander to generate a supply of electrical power. A power conditioning unit has at least one capacitor to store electrical energy to provide a back-up electrical supply for a brief period until the scroll expander and generator can produce the required supply of electrical power.