I highly doubt it's going to need a supply that can do at least 9V to exit shipping mode, at least I haven't yet seen a device behave like that. I more so think that's what power supplies Valve suggests to use if you want to play and charge simultaneously, that being said I believe actual power usage is going to be somewhere around those 27W at max so the battery doesn't discharge, with 45W being a buffer to charge the battery while in use, though that's still just a wild guess.