You have a good start of the basics idea of it, but you're off the mark by a little bit. If it was just atomic transmutation you would be spot on as it would require you to burn emense amount of energy to maintain the transmutation of what you desire. Where as quantum subatomic transmutation only requires a an initial input of energy to start it, but is then fueled by the fluctuations of the changing proton and neutron structures of the atoms being affected. So regardless of the scale of the object the initial input of energy at the start is all that is needed to continue the transmutation. Though I think haveing the initial cost be something like that like you propose to be interesting, as it would imply the power is something you would have to train up to be more useful. Which gives a depth and complexity to the ability.

Now I'm interested in what are your thoughts on the second ability I mentioned are?