What is the secondary source of electrical power for the aircraft with an inoperative engine driven generator?

In the A320, the Auxiliary Power Unit (APU) serves as a secondary source of electrical power when an engine-driven generator is inoperative. The APU is designed to provide electrical power, pressurize the air system for starting the engines, and supply bleed air for environmental control. When an engine generator fails, the APU can automatically start and take over the electrical load, ensuring that critical systems remain operational and that the aircraft can continue to be powered for essential functions without relying solely on the main battery or external sources.

The main battery, while crucial for initial start-up and emergency power, does not have the capacity to sustain all aircraft systems for extended periods compared to the APU. The same applies to the auxiliary battery, which is primarily a backup system with limited duration and capability. An external power source can provide power but is only available when connected to the aircraft on the ground, making it irrelevant in an in-flight scenario. Therefore, the APU stands out as the most effective option for secondary power during engine generator failure.