This is an important question. As of now, it is not possible to use alternative energy exclusively in America for three reasons.
First, we use an incredible amount of energy every day and it only increases. Think about all of our electronic gadgets, air conditioning, traveling and the like. In 2009 we used almost 19,000,000 barrels of oil everyday. Today it is even more.
Second, alternative energy is not as developed as we like. For example, a minority of people use alternative energy now. Things are changing, but it is still slow. For example, there are not many hybrid cars on the road, solar energy is slow to be used, and the same can be said for wind and hydraulic power. More research has to be done and the infrastructure has to be laid out.
Third, nuclear energy is powerful, but with the dangers associated with this, people are cautious.
If you are talking about the future, it is possible with the advancement of technology. We will have to wait and see.