"Amazingly independent" and yet all linked. My friend who works for Progress Energy here in NC's job is to buy and sell power to and from other power companies.
I work for a 600 MW combined cycle power plant in Oregon and our company also has 4,000 MW of installed wind generation across the USA.
Only that wind installed in Calif, Oregon, Colorado, and Wyoming, effects our 600 MW CS powerplant. That which is in ND, Iowa, NY etc. are on a different grid, and are sold to central and Eastern markets. If the grid goes down as it did in what 2002? (shortly after 9-11 anyway) did not effect western or Texas grids as they are 99%autonomous.
I thought it was about 6, but it is 3 grids in the USA.