A map showing how the Great American Eclipse will appear from different points in the United States. A map showing how the Great American Eclipse will appear from different points in the United States. (Photo: NASA)

When the Great American Eclipse passes over the United States on Aug. 21, it will bring with it a major test for the modern utility grid.

A recent report from grid operator California ISO is predicting an unprecedented drop in solar production during the eclipse, followed by an even faster recovery. While the moon's narrow 70-mile-wide shadow of totality, called the umbra, does not pass directly over California, its much wider penumbra will markedly decrease sunlight over much of North America.

For California, which contains half the nation's solar generating capacity at nearly 10,000 megawatts, this sudden dropoff in light will have a big impact on its energy mix. This past April, the state reported that for the first time ever, solar energy accounted for nearly 40 percent of all grid power produced between 11 a.m. and 2 p.m.

As the moon's shadow falls across the state, total production is expected to fall by an estimated 70 megawatts a minute, followed by a rapid increase of 90 megawatts per minute. According to Quartz, this unprecedented yo-yo effect will result in a shortfall of 6,000 MW, or enough energy to "power a large city, relative to normal supply."

During the Great American Eclipse, a loss of solar production in California will create a shortfall of nearly 6,000 MW. During the Great American Eclipse, a loss of solar production in California will create a shortfall of nearly 6,000 MW. (Photo: California ISO)

To counter this sudden increase in load, a phenomenon expected to last less than five minutes, utilities nationwide are planning now to pull additional energy from gas, hydroelectric, and other traditional sources. Lessons learned from utilities in Europe that successfully weathered a total solar eclipse in March 2015 are also being applied.

In its own report, the North American Electric Reliability Corporation (NERC), which monitors the reliability and security of the U.S. grid, said that it doesn't expect the Great American Eclipse to disrupt the nation's power supply.

“We don’t call it a reliability issue, but it’s an impact to the system operations and something operators need to do some planning to prepare for,” John Moura, NERC director of reliability assessment and system analysis, told the Financial Times.

With solar on track in the next decade to become the cheapest power source of all, lessons garnered from the August 2017 total eclipse will come in handy for a future grid even more dependent on the sun's rays. No less than six major total solar eclipses will occur over the U.S. in the 21st century, with four of them happening in a 35-year period.

Michael d'Estries ( @michaeldestries ) covers science, technology, art, and the beautiful, unusual corners of our incredible world.