How committed should I be to a specific research area before and shortly after I enter a doctoral program?
This is the answer posted by Cameron Williams in a comment. I am pasting it as a CW answer - it's not "my" answer.


  I think the best course of action for most graduate students is to apply to programs that are good across the board in your general field. You may end up not liking something that you thought you'd love for one reason or another (be it tedium, incompatibility with advisors or simply finding something that you love so much more). Don't shoehorn yourself into a program that specializes only in one facet of power engineering because you may end up not liking that one thing. As an incoming graduate student, you're not expected to know much about the field you're entering so knowing whether or not a certain subfield is a good fit for you is not reasonable to assume. This comes with courses/research while you are a graduate student. 
  
  As for how much time you have, this varies a bit from university to university, but you will probably spend a lot of your first year on courses and after that you'll be expected to have an idea of what exactly you'd like to do. Of course it isn't set in stone at that point, but you should be moving in one specific direction. 
  
  And yes, you can be co-advised which can allot for multifaceted/multidisciplinary research.