What factors determine whether to use Ground Power Unit (GPU) or aircraft power (APU/aircraft generators) on the ground?

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The key idea is choosing the power source based on what is available at the gate, what the aircraft needs electrically, and the benefits of not using the APU. If a suitable Ground Power Unit is available at the aircraft’s position and can meet the aircraft’s electrical load (voltage, frequency, and required current), it’s preferred because it saves fuel, reduces noise and emissions, and helps preserve the APU for times when ground power isn’t available. The aircraft’s power requirements matter because some operations or phases demand more power than a single GPU can provide, or require certain loads (like air conditioning or high startup power) that may necessitate using the APU or multiple power sources. Redundancy also plays a role: having an alternate power source or using the APU as backup ensures essential systems stay powered if the external power fails. Noise considerations exist, but they are secondary to availability, capability, and the fuel/backup benefits.