How much power does the APU generate in a Boeing 737-300?
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.