Which electrical buses are powered when only the APU is online while on the ground?

When the Auxiliary Power Unit (APU) is online while the aircraft is on the ground, it typically powers essential systems that are necessary for the operation of ground services and avionics. In most aircraft configurations, this includes not just the essential buses (which provide power to critical systems required for safety and operation) but also the main and auxiliary buses.

The main and auxiliary buses are responsible for powering a broader range of systems, including cabin lighting, inflight entertainment, and other non-essential systems that enhance passenger comfort and convenience while the aircraft is stationary. Thus, with the APU functioning, all six buses, including essential, main, and auxiliary, are indeed operational. This provides the necessary power for both vital and ancillary systems without needing to start the engines, enhancing efficiency and convenience during ground operations.