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Number of electricity disturbances by cause, 1992–2009


There is some uncertainty in the reliability of the U.S. electricity system in a “business-as-usual” case. Although the U.S. electricity grid has a proven track record with conventional generation mixes, power outages and grid disturbances nonetheless occur and are on the rise.

Outages and unusual grid events have doubled in the last decade, and now affect more than 5 million people a year. The majority of grid disturbances are caused by weather and system equipment failures, although human error, cyberattack, and animals are also notable causes. Estimates suggest that U.S. grid outages account for up to $160 billion in annual losses. Approximately two-thirds of this cost results from outages lasting less than five minutes.

Of particular interest are outages larger than the so-called Most Severe Single Contingency. The North American Electric Reliability Corporation (NERC) requires that utilities carry enough reserve generation capacity (with less than 30-minute startup time) to cover the loss of their single greatest contingency. In most cases, this is the grid’s largest power plant or a critical transmission connection. While such a loss should be unlikely, recent history has shown that these events do happen with some regularity.

In the past five years, every NERC region has had at least one disturbance event greater in magnitude than its greatest single contingency. Some regions, like the Southeastern Electric Reliability Council, have averaged more than ten such events per year. A future U.S. electricity system designed around large, central generators will perpetuate the need for a huge reserve capacity, while a more distributed system of interconnected microgrids will significantly reduce it, saving utilities money and improving grid reliability.


North American Electric Reliability Corporation. 2011. “Events Analysis: System Disturbance Reports.” link

U.S. Department of Energy. 2011. “Electric Disturbance Events (OE-417) Annual Summaries.” Office of Electricity Delivery & Energy Reliability. link

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