The first total solar eclipse in 99 years will be an unprecedented test of an American power grid that has become rapidly reliant on solar energy, according to Bloomberg. Power grids, utilities and generators are bracing for more than 12,000 megawatts of solar power to go offline starting around 9 a.m. in Oregon as the moon blocks out the sun across a 70-mile-wide (113-kilometer) corridor.

The eclipse has arrived at a time when the American power grid is becoming increasingly reliant on solar, wind and hydroelectric power.

“This is the first major test of the power grid since America started bringing large amounts of intermittent solar and wind resources onto the system. It comes just as the grid is undergoing an unprecedented transformation whereby flexible resources such as battery storage will complement growing supplies of solar and wind. Solar installations have grown ninefold since 2012 and renewable sources are forecast to supply just as much of America’s electricity demand as natural gas by 2040.”

Renewable energy sources have increased dramatically, especially over the past five to ten years, said Nicholas Steckler, an analyst at Bloomberg.

“’The U.S. power grid “hasn’t seen this sort of natural phenomenon since solar became a thing,’ Nicholas Steckler, an analyst at Bloomberg New Energy Finance, said. ‘With so many renewables coming online, especially in the last five to ten years, there is more impact from an eclipse.’”

In most cases, regional power authorities already have back-up natural gas plants and hydroelectric power lined up to help compensate for the sudden loss of solar power. They’ve also promised to keep locals apprised of the situation.

“The eclipse is expected to reach the U.S. at 9:05 a.m. local time at Lincoln Beach, Oregon, and last for about four hours. Back-up, natural-gas plants and hydroelectric dams are at the ready to fill solar’s void along with new technologies to control demand.

Regional grid operators from California to Pennsylvania plan to provide real-time updates on how their networks are handling fluctuating power flows as millions of Americans head outside to gaze at the sky.”