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Ben Ellison is a great, dependable source.
I would add that "5V USB outlets" for boat or auto use are simple DC-DC step down converters from 12V to 5V, and that most of them are energised even if not in use; i.e., are always using a small amount of power. Therefore, it 's probably a good idea to have the possibility of switching them off easily.
Our boat 's native power is 24V and we found small cigarette lighter adapters that are 12/24V to 5V USB. Under some circumstances, this has been a great solution when we needed to charge USB electronics and did not want to power up the largish Mastervolt and Victron 24V to 12V converters that power our 12V circuits.