What is the power output from the US national grid?

Wikipedia has figures for electrical generation ie input to the grid, rather than output from the grid, but it's the same apart from transmission losses. For 2006, the latest given, it is a total of 4.063 billion megawatthours. Since there are 8760 hours in a year, this gives the average power output as 578,000 MWe. This compares to a total nameplate rating for all the plants in the country of 1,075,171 MWe.

See Wikipedia 'Energy use in the United States'