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The power for all systems come from GEN1 and GEN2 which are powered by APU. The APU can be started if there is power on the start bus, the start bus is powered either by EPU (on ground or when docked) BATT (which is charged during flight by GEN1) or Emergency power. On the ground, if you select EPU/BOTH and the EPU button is off you can set the GEN and APU button to OFF and all power will come from EPU (External Power Unit)