How much power does the APU generate in a Boeing 737-300? I have just watched Mayday episode 11, season 11, "Nowhere to Land" about TACA Flight 110 incident and I was more than surprised to learn that:


APU must be turned on,
APU needs about 5-10 minutes to start powering anything,
APU can be used to power lights in the passenger cabin (???).


This is the exact flow of events (plus voice description) as it is shown in above cited episode:


a pilot is shown turning APU's switch,
narrator explains that a certain amount of time must pass before APU starts working,
lights in passenger section are turned on after this period of time passes.


Contrary to the above, I was always told (based on Air Transat Flight 236 incident, shown in season 1, episode 6, "Flying on Empty" of Mayday, and based on other sources):


APU is deployed automatically (gravitational solution) whenever power from engines is lost,
APU starts powering equipment nearly immediately,
APU is able to power basic instruments only, even pilots cabin remains dark, not to mentioning passenger sections.


What is the reality?
You are getting the APU mixed up with the RAT:


The Auxiliary Power Unit (APU) is an engine powered generator, either a gas turbine or a piston engine. APUs deliver enough power to start engines, power cabin lights,  cockpit instruments and radios, and in some cases power hydraulics. Often the APU can only be run at lower altitudes where the air is thicker
A Ram Air Turbine (RAT) is a small, automatically deployed propellor run generator which is lowered into the airflow if there is a loss of power. The RAT generates enough power for the most critical systems only, and is designed to give pilots enough electricity to glide until the APU can be started


Often the APU runs from the same fuel supply as the engines, so if the airplane runs out of fuel the APU is unavailable, so the RAT is all they'd have.