In simple words -
The 747 have 4 electrical generators (1 per engine).
They produce alternating current 115V/400Hz and each are 60 KVA of power.
The APU is also equipped of 1 or (option) 2 same generators (for ground use).
The 747 Classic APU cannot be used in flight to produce electricity.
DC (28V) is produced by 4 transformers rectifiers which change the 115V current.
A small aircraft battery stores 24V DC for emergency use.
Another similar battery stores 24V DC to start the APU.
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Happy contrails