My favourite definition for consciousness is the following:
If it is something to be like a particular thing, then that thing is conscious.
If it is something to be like a bat, then a bat is conscious. If it isn’t something to be like a bat - the “lights are off” so to speak - then it’s not conscious.
Edit: clarity
After you are yourself again you'll need to interpret the acquired memories. You remember echolocview of the room (bats have memory, it's testable), hunger, disorientation and some other sensations. Does it mean that bat is conscious? What will you remember differently if bat wasn't conscious? You'll remember nothing? But memories are there and you should have access to them. The lack of self-awareness? But you don't know how you would remember the presence of self-awareness in a bat.
Nothing is really static in life. So nothing ever really is always just a thing, it was one thing, now it's a slightly different thing.
It is like something to be a human. But the conscious experience of a bat (if it exists) may be completely alien to us in every way. That’s not relevant to what it is to be like a bat. The only important thing is if it is like something to be a bat.
It is based on our illusion of understanding what it is like to be a human, but upon closer inspection it bears no real content.
At least as you've stated it, that's a circular definition.
I guess I'm focusing more on the "lights are off" part, although that's just an aside. "Conscious" and "lights are on" seem like alternate ways of saying exactly the same thing.
Nagel's "is it like something to be" terminology has always struck me as wilfully contorted and confusing, but amounting to the same thing again -- you're conscious if you're conscious. It doesn't help us explain consciousness and it doesn't even help figure out what things are conscious and which aren't. Is a plant conscious, or a rock? Writers can certainly imagine what it might be like to be those things.