How committed should I be to a specific research area before and shortly after I enter a doctoral program? I am an electrical engineering undergraduate student and entering my final year of study.  I plan on applying to doctoral programs in the field of power engineering.  Specific advice from the field would be appreciated, but I welcome professors and students from other backgrounds to comment.

I say power engineering because I'm interested in studying problems related to the electrical grid.  However, there are a lot of problems out there: multi-agent systems, smart grid technologies, distributed grid technologies, power electronics, and many, many more.  It's such an overwhelming list that I don't know how to choose, or even how to educate myself enough to make a choice.  I'm sure other fields have this problem of "too many subjects, too little time".

I know in a general sense that I want to enter this field and do research in it, and that I would enjoy the challenge of a PhD program.   At this point, though, I don't know which specific aspect I want to study.

My questions are these: how much should I know before applying to a program?  Upon entrance to a doctoral program, how much time do I have to pick a certain area of research?  I've heard of students being co-advised by more than one professor; does this mean my study can be somewhat multifaceted?

Edit: Answers specific to doctoral study in the United States would be appreciated.
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.