What system provides onboard electrical power while an aircraft's engines or APU are not operating?

The ground power unit (GPU) is designed specifically to provide external electrical power to an aircraft when its engines or auxiliary power unit (APU) are not running. This is particularly useful during pre-flight preparations, maintenance, or when the aircraft is parked at the gate, allowing for systems such as lighting, avionics, and environmental controls to operate without relying on the aircraft's own power sources.

When connected to the GPU, the aircraft can continue receiving the necessary electrical supply needed for various equipment, which enhances operational efficiency and safety by ensuring vital systems are functional without the need to start the engines or APU. The GPU can supply a stable, high-quality electrical output, which is essential for many flight operations and checks.

While emergency power systems and battery backup systems are also important for providing electrical power during specific scenarios, they do not serve the same function as a ground power unit meant solely for external power during engine or APU inactivity. An auxiliary power unit is capable of generating onboard electrical power but it requires its engines to be running, which does not align with the question's context of needing power without engine operation.