In what conditions is APU operation typically most efficient?

The operation of the Auxiliary Power Unit (APU) is most efficient under moderate temperatures with adequate fuel. In these conditions, the APU can maintain optimal combustion and performance. Moderate temperatures prevent the APU from becoming overworked or overheating, which can occur in extreme temperature conditions. Additionally, having adequate fuel ensures that the APU can run continuously at the necessary power levels without the inefficiencies associated with fuel starvation or variability in performance when operating at low fuel levels. This balanced environment allows for a more stable and effective APU output, supporting the overall operational needs of the aircraft without putting undue stress on the unit.