People can define it how they want to. For me a power user is somebody who is hammering their device 24/7 doing several different things on it. From social to productive workIt actually does matter what they're doing, and how they're using the device. If you're telling me and everyone else that a thirteen year old girl that is constantly on snapchat and Facebook is a power user, I'm sure I'm not the only one that will tell you you're wrong. You can be as condescending about it as you want, but you're simply off the mark on this.
A power user is someone that's very experienced with the OS, and uses it for a variety of advanced tasks. That's not my definition - that's the definition from a variety of sources that a quick Google can provide you with.
For me, an actual power user, that includes document creation, invoicing, handling finances, managing a supply chain, heavy audio work with GarageBand and other apps, and video work with iMovie and others. I run an entire business from my iPhone, and it handles everything pertaining to it other than web design and photoshop work.
I think that and being very experienced with the OS goes hand in hand. People who are on their device a lot will likely know a lot about tech and the OS they are using.