What is the national average cost per kilwatt-hour?
The cost of electricity as measured by kilowatt hours varies by location, making it difficult to average a national cost of any meaning. In some remote areas, where generation or wire transportation of electricity is harder, the cost goes up; in populated areas with space for closer power plants, the cost goes down. Michael Bluejay, as cited below, notes:
The cost of electricity depends on where you live, how much you use, and possibly when you use it.
Electricity rates vary widely. I found rates ranging from 12¢ to 50¢ per kWh from the same provider. The only way to know what you're actually paying is to check your bill carefully.
Wikipedia gives a national average of 11.2¢ per kilowatt hour in the United States, from the U.S. Energy Information Administration. This is substantially below many other countries, although the cost will differ widely by area. In general, the average cost of electricity in the U.S. can be anywhere from 7-25¢ per kilowatt hour, and that is considered cheap by worldwide standards.