(Bloomberg) -- The first total eclipse of its kind in 99 years will plunge broad swaths of the U.S. into darkness, sending solar supplies sliding and testing the resilience of the power grid for the first time since the rapid rise of renewable energy.

Grid operators, utilities and electricity generators are bracing for more than 12,000 megawatts of solar power to start falling offline as the moon blocks out the sun across a 70-mile-wide (113-kilometer) corridor stretching from Oregon to South Carolina.

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.

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.”

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.

Read more: What Comes of Solar Power When the Sun’s Eclipsed: QuickTake Q&A

Overnight thunderstorms have left lingering clouds from Nebraska down to Missouri, said Brendon Rubin-Oster, forecaster with the U.S. Weather Prediction Center in College Park, Maryland. There are also signs of early morning cloud cover in South Carolina, he said. The best spot to see the eclipse still seems to be the Pacific Northwest from Oregon to Idaho.

The celestial event provides an opportunity to test plants, software and markets refined in recent years in anticipation of the day when renewable energy becomes the dominant source of power. Bloomberg New Energy Finance has projected that renewables will supply more than half of the world’s electricity in 2040. U.S. utility owner Southern Co. is marking the occasion by offering special deals on energy saving and smart-home products through its Georgia Power unit.

California, home to more solar power than any other state, will tap into its network of hydropower generators and gas plants that can ramp up quickly to fill a 6,000-megawatt gap in solar energy. The state also embarked upon a public relations campaign to convince residents to conserve energy to minimize greenhouse-gas emissions while solar plants are down.

In North Carolina, part of which will see total darkness during the eclipse, Duke Energy Corp. expects about 2,000 megawatts, or 80 percent, of utility-scale solar farms to go offline. The utility will treat it like a “gradual sunset,” said Tammie McGee, a company spokeswoman, estimating that as many as 1,200 megawatts of gas generation will be called upon to pick up the slack.

For a look at how small-scale solar resources are going to displace more than $2 billion of traditional power generation, read this Bloomberg New Energy Finance report.

Electricity prices may rally on solar’s sudden slide. The eclipse will start curbing power supplies a little after 9 a.m. on the West Coast, just when the work week is starting and demand is taking off. According to energy data provider Genscape Inc., the event may extend the typical period of high power prices in California by about two hours.

Wholesale power prices for Northern California jumped 151 percent for the hour between 10 a.m. and 11 a.m. local time to $51.79 a megawatt-hour in the day-ahead market. That was the highest for that hour since August 2014. Prices before and after the peak-eclipse period are in the $30s.

Prices will probably retreat as soon as the moon starts moving past the sun and solar farms return, Genscape said. And the market impact in Texas, the Midwest and the East Coast will be limited because the region’s home to smaller concentrations of solar.

PJM Interconnection LLC, which manages the 13-state grid from the Midwest to the mid-Atlantic, sees a greater likelihood of cloud cover, particularly for parts of the Ohio Valley. Partly cloudy skies are also expected around Philadelphia and areas with a higher concentration of solar in New Jersey and North Carolina.

“As far as prices are concerned, they may jump in the afternoon due to higher loads and the need to use more expensive generation, but the impact of the eclipse should be limited,” said Jason McGovern, a spokesman for the grid operator.