Airbus A321-231 (D-AVXO / N581UW / MSN 6152) from American Airlines at Hamburg-Finkenwerder Airport (Airbus' Hamburg facility, Germany)
Notice the deployed Ram Air Turbine (RAT)?
A ram air turbine (commonly known by the acronym RAT) is a small turbine that is connected to a hydraulic pump, or electrical generator,
installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft. In normal conditions the RAT is retracted into the fuselage (or wing), and is deployed manually or automatically following complete loss of power.
In the time between power loss and RAT deployment, batteries are used.