system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How are commercial airplanes supplied with electricity?

To power screens, Air conditioning and lights etc, do they use alternators on the engines or generators inside the plane?
gpt
They have two sources of power, and there is a third (ground-power) when on the ground.  The primary source is the engines, through an internal power take-off (PTO) shaft.  When the engines aren't running, the auxiliary power unit (APU) provides starting air and electricity.  The APU is restartable in the air, to recover from loss of engines.