Duke Energy's merger with Progress Energy, completed in July 2012, created the largest electric utility in the United States. 

In fact, two of the three largest U.S. utilities are the result of recent mergers. Exelon, third on the list of the 10 biggest U.S. utilities, merged with Constellation Energy in March 2012.

Duke will now service 7 million retail customers across six states. The company also owns 67,000 MW of generating capacity across its footprint.

Take a look at this chart from the U.S. Energy Information Administration to see the 10 largest utilities in the U.S. and how big their customer bases are:

1. Duke Energy

2. Pacific Gas & Electric

3. Exelon

4. Edison International

5. NextEra Energy

6. Southern Company

7. American Electric Power

8. First Energy

9. Xcel Energy

10. National Grid