Electric companies’ power grids are all interconnected, so if one company is running a little short of the demand, a neighboring utility can sell some of its excess to make up the difference.
How is that accomplished?
To make the diagram simple, in the grid you have transmission lines (the big ones), and distribution lines (the small ones). What is the actual process in diverting “electric power” to the neighbors’ transmission lines, and what stops that from happening normally?
The inter-ties between utilities are not as closely regulated as you think. Essentially, the electricity flows where it will through the inter-tie points. This can be regulated somewhat by switches, but typically it is not done aggressively. In rare or emergency cases, the inter-ties can be shut off completely.
This is the reason when one company’s service region has a major blackout, several others of the neighbors will also have disruptions of service or blackouts too. There is all of a sudden a phenomenal drain through the inter-ties when this happens.
Inter-tie transaction billing and accounting is an art in itself. There are very accurate meters on each utility’s side of the inter-tie, which measure exactly how much electricity flows when. And at the end of the month, the utilities settle between themselves who owes who how much. The amount owed per kW-hour can vary from hour to hour, depending on the overall system demand, and the rate of flow through from one system to another.