The blackout on August 14th was the biggest in U.S. history. And just like every major blackout, it raised a lot of questions about how our power distribution system works.

At a high level, the power grid is a very simple thing. It consists of a set of large power plants (hydropower plants, nuclear power plants, etc.) all connected together by wires. One grid can be as big as half of the United States. (See How Power Distribution Grids Work to learn about the different pieces of the grid.)

Photo courtesy U.S. Department of Energy
A breakdown of the major power plants in
the United States, by type

A grid works very well as a power distribution system because it allows a lot of sharing. If a power company needs to take a power plant or a transmission tower off line for maintenance, the other parts of the grid can pick up the slack.

The thing that is so amazing about the power grid is that it cannot store any power anywhere in the system. At any moment, you have millions of customers consuming megawatts of power. At that same moment you have dozens of power plants producing exactly the right amount of power to satisfy all of that demand. And you have all the transmission and distribution lines sending the power from the power plants to the consumers.

Photo courtesy U.S. Department of Energy
A map of U.S. electric control area operators (CAO). Computerized systems at each CAO monitor the power grid activity and balance power generation (supply) with power consumption (demand).

This system works great, and it can be highly reliable for years at a time. However, there can be times, particularly when there is high demand, that the interconnected nature of the grid makes the entire system vulnerable to collapse. Here's how that happens:

Let's say that the grid is running pretty close to its maximum capacity. Something causes a power plant to suddenly trip off line. The "something" might be anything from a serious lightning strike to a bearing failure and subsequent fire in a generator. When that plant disconnects from the grid, the other plants connected to it have to spin up to meet the demand. If they are all near their maximum capacity, then they cannot handle the extra load. To prevent themselves from overloading and failing, they will disconnect from the grid as well. That only makes the problem worse, and dozens of plants eventually disconnect. That leaves millions of people without power.

The same thing can happen if a big transmission line fails. In 1996 there was a major blackout in the western U.S. and Canada because the wires of a major transmission line sagged into some trees and shorted out. When that transmission line failed, all of its load shifted to neighboring transmission lines. They then overloaded and failed, and the overload cascaded through the grid.

In nearly every major blackout, the situation is the same. One piece of the system fails, then the pieces near it cannot handle the increased load caused by the failure, so they fail. The multiple failures make the problem worse and worse and a large area ends up in the dark.

One solution to the problem would be to build significant amounts of excess capacity -- extra power plants, extra transmission lines, etc. By having extra capacity, it would be able to pick up the load at the moment that something else failed. That approach would work, but it would increase our power bills. At this moment we have made the choice as a society to save the money and live with the risk of blackouts. Once we get tired of blackouts and the disruption they cause, we will make a different choice.

Big Blackouts in U.S. History
  • The Great Northeast Blackout of 1965: After a relay failure, more than 80,000 square miles of the northeastern United States and parts of Canada lost power, turning the lights out on 30 million people.

  • The New York Blackout of 1977: One hot night in July, multiple lightning strikes knocked out power to the entire city of New York, leaving 8 million people without light or air conditioning. The blackout triggered mass looting and arson across much of the city.

  • The Northwestern Blackout of 1996: Transmission lines sagged into some trees, causing an electrical short that knocked out power to more than 4 million people in Oregon, California and other western states.

  • The Blackout of 2003:Cities across the midwestern United States, northeastern United States and southern Canada lost power, apparently due to a problem with a series of transmission lines known as "The Lake Eerie Loop." Roughly 50 million people lost power.