How Blackouts Work

The sun sets over the Manhattan skyline Aug. 14, 2003, during a major power outage.
The sun sets over the Manhattan skyline Aug. 14, 2003, during a major power outage.
Robert Giroux/Getty Images

You may remember the blackout that occurred on Aug. 14, 2003. It was the biggest in U.S. history. And just like every major blackout, it raised a lot of questions about how the 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.)


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.

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. In this article, we'll find out what happens to cause a blackout and see what steps you can take to prepare yourself for an extended power outage.

First, why do widespread blackouts occur if there are so many potential back-ups in the system?

Blackouts: Grid System Failure

Hundreds of New Yorkers make their way home during a power cut after a power station was struck by lightning on July 13, 1977.
Hundreds of New Yorkers make their way home during a power cut after a power station was struck by lightning on July 13, 1977.
Bryan Alpert/Hulton Archive/Getty Images

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 geomagnetic storm 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 huge blackout in the western United States and Canada because the wires of a major transmission line sagged into some trees and shorted out. When that transmission line failed, its entire 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, and 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 the moment 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 losing power. Until we make a different choice, then, it makes sense to be prepared for those times when electricity is not an option.

Blackout Preparation

The risk of a blackout is always present, and while certain electrically powered appliances are mere conveniences -- washing machines and air conditioning, for instance -- others are much harder to live without, even for a few days.

The best approach, then, is to be prepared. By following certain tips, you can make yourself a bit more immune to some of the problems associated with extended blackouts.


  • Always have at least one large cooler and a supply of ice on hand. If you lose power for more than two hours, you'll need them to keep your food cool so you don't lose an entire refrigerator full of sustenance.
  • Stock up on water. If the blackout turns into a long-term disaster-type scenario, you may not have easy access to clean water. Store one gallon (3.7 liters) of potable water per person per day, for anywhere from three days (if you think you would evacuate) to two weeks (if you plan to stay at home through the blackout).
  • Create a general emergency-preparedness kit. In addition to water, this includes a first-aid kit, personal hygiene items, flashlights, copies of important documents, emergency contact numbers, batteries, a non-cordless phone and an extra supply of any necessary medications.
  • Stash some cash. In case of a major blackout, you probably won't be able to use a credit card to buy the things you need.

Hopefully, any blackout you experience will only last for an hour or two. But if it turns into a long one, following these tips should help you weather the storm.

For more information on blackouts, power grids, emergency preparedness and related topics, look over the links on the next page.

Lots More Information

Related HowStuffWorks Articles

More Great Links

  • "Blackouts." Ready America.
  • Brain, Marshall. "How Power Grids Work." HowStuffWorks.
  • "Power Outage Checklist." American Red Cross.