Note that H2/O2 Generators cannot be powered by Fuel Pack (Hydrogen); they can only use Fuel Pack (Promethium) (large or small). At present, the generator's power usage is about 70kW (not the 10kW stated in-game). One Fuel Pack (Promethium) therefore powers it for about 42 minutes (real time), and generates 10-11 Oxygen Bottles and Hydrogen Bottles.
H2/O2 generators are one of the most efficient uses of your Fuel Pack (Promethium). Empyrion survivors know: Power this advanced generator with the Promethium packs, and get more than enough Hydrogen packs to power everything else.