Of course your consulting rate is a measure of power! The question about your rate is a question of how easy you are to replace. If you have skills (which are a form of capital, which is a form of power) or relationships that are hard to find then you have leverage to negotiate a higher rate. Power is at the heart of that question.
Whenever you are negotiating a rate or salary, you are forming a relationship, and the terms of that relationship depend entirely on your relative levels of power and leverage.
I guess I'll give you one thing though: whereas what two people want from each other can be complicated, power is not a scalar value like electric charge. But in many cases it acts like a scalar, especially if it takes the form of money, which is abstract and transferable.
Whenever you are negotiating a rate or salary, you are forming a relationship, and the terms of that relationship depend entirely on your relative levels of power and leverage.