The annual Earth Hour event is a coordinated, mass effort to reduce electricity consumption for 1 h. Earth Hour's objective is to call attention to environmentally sustainable action through the collective impact made when individuals, businesses, governments and communities voluntarily combine electricity conservation efforts. Earth Hour events have taken place worldwide since 2007. We compiled 274 measurements of observed changes in electricity demand caused by Earth Hour events in 10 countries, spanning six years. These events reduced electricity consumption an average of 4%, with a range of +2% (New Zealand) to −28% (Canada). While the goal of Earth Hour is not to achieve measurable electricity savings, the collective events illustrate how purposeful behavior can quantitatively affect regional electricity demand. Similar actions may be a useful demand-control strategy during temporary electricity shortfalls or other crises. The policy challenge is to convert these short-term events into longer-term actions, including sustained changes in behavior and investment. Other events cause coordinated change in electrical demand, such as television programs and sporting events. These sharp drops and peaks lead to inefficient generation requirements and, potentially, grid failure. These events demonstrate the importance of short-term behavior on energy demand and possible applications to energy policies.