APU Start, and ETOPS
Quote from cod liver oil:
“Power source for B787 APU starting may be airplane battery, a ground power source, or an engine-driven generator.
Wouldn't engine-driven generator be used as primary power source for APU in-flight lightup?”
Probably not. I cannot quote the ETOPS/EROPS requirements, but suspect one of the scenarios they address is likely to be that of complete electrical-generation failure. That MAY mean that the ability to start the APU is mandatory for some or all ETOPS legs. I doubt it would be acceptable to rely on the RAT and its limited generation capability to provide essential services for several hours (although I stand to be corrected).
However, as an interim measure, it might be acceptable to start the APU as a pre-emptive measure prior to commencing the part of the sector where the ETOPS rules apply. If the APU failed to start (due to cold-soak, or whatever), you would have to divert somewhere. So it might make sense to leave it running on departure (off-load), and not shut it down until the ETOPS leg has been completed. The fuel-flow on an idle APU at high altitude is relatively modest.
FOOTNOTE
Wouldn't it be nice to have full schematics of the B787 electrical systems in the public domain, nearly 18 months after this civil aircraft entered service? In April 1988, when we started airline ops on the cutting-edge A320, no one put any pressure on line pilots to treat the contents of the FCOM as sensitive commercial information. Why is Boeing so shy of divulging its engineering achievements?
In the present circumstances, public rumour and speculation becomes all the more rampant when hard facts are so thin on the ground.