As some people have stated, there might be no hard definition of a power user, so I will just tell you how I loosely conceive of it, and what I mean when I call myself one.
A power user, to me, is not mainly about how much he uses a device, but about how much he knows about it, what it can do, how to use it, how to troubleshoot it, etc.
He would know more than the average user, but he might not be a technical professional or a developer. A technical professional or a developer, of course, would also be a power user.
What is someone like who is NOT a power user?
I have met some teenage girls who used their iPhones more hours than I do, but they complained that the iPhone keeps "sending them notifications" -- they said this as if that's one of the design flaws of Apple or the iPhone. And when it was mentioned that they needed such and such an app to this this or that, they said they didn't want to get more notifications.
When I brought up the fact that it was the apps that were responsible for the notifications and not Apple as such, and that they could customize their notifications with each app -- they could turn notifications on or off, choose whether they make sounds, display on the lock screen, etc. -- they had no idea. They had never heard of such a thing.
And they had been putting up with this issue for years. This meant that, for the years they had been using their phones, they had never gone through the settings app to discover this themselves, nor had they actively searched for answers about their phones.
I have come across people like these, surprisingly often with this exact issue with notifications (that's why I like to use it as an example, but sometimes with other issues that you and I have long taken for granted.
Now, these are NOT power users.