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Solar power will take a hit during the Aug. 21 eclipse

A solar eclipse that will sweep across the country on Aug. 21 is expected to cause a noticeable dent in solar-energy collection, and utilities are concocting workarounds to meet power demands during the event.

Utility workers already have a game plan in California, where 9 percent of electricity came from large solar plants in 2016. During the eclipse, when the sun disappears behind the moon, they plan to ramp up output from other sources, including from hydroelectricity and natural gas, and then quickly reintroduce solar power as the sun reappears.

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California's residents shouldn't notice a difference in their power supply during the eclipse, said Steven Greenlee, a spokesman for California Independent System Operator (ISO), a nonprofit that manages California's power grid.

Even though the totality of the eclipse isn't passing directly over California — it's traveling in a curved path from Oregon to South Carolina — it will affect the Golden State, which has nearly half of the nation's solar-electricity-generating capacity, according to the U.S. Energy Information Administration.

"We will see between 50 and 75 percent of solar production from our solar plants reduced," Greenlee said.

The state is expected to lose almost 4,200 megawatts during the eclipse, which will partially darken the state from about 7:45 a.m. to 12:45 p.m. local time, on Aug. 21, according to the ISO. To put that in perspective, one megawatt powers about 1,000 typical homes, Greenlee said. In addition, small systems that rely on power from rooftop solar panels will go offline, "which means that those mostly residential units will then look to the grid for their power support," Greenlee said.

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When these units are added to the expected loss of 4,200 megawatts, the total estimated demand on the power grid will be about 6,000 megawatts, or the equivalent of the electricity used by about 6 million homes, according to the ISO.

Six thousand "is the amount of megawatts that we expect that we'll have to generate to make up for the effect of the solar eclipse," Greenlee said. "For us here at the ISO, we're going to need to make sure that we have our reserves properly procured," which involves having 100 percent of the expected demand plus an additional 6 percent in reserve, he said.

The ISO is talking with providers of other types of energy, particularly natural gas, which provides about 53 percent of California's energy resources, Greenlee said. Those firms will need to acquire more natural gas for the morning of the eclipse.

In addition, the ISO will have to properly time the fading out and the return of the sun's light. As the eclipse begins passing over California, solar-energy collection will decrease at 70 megawatts per minute. As the eclipse ends, it will return at 90 megawatts per minute, which is "very fast," Greenlee said.

"The challenge on our end is going to be able to time bringing up the resources, the natural gas or the hydro generation that we're going to need as the solar is ramping down," he said. "And then we're going to have to ramp the gas and hydro generation down as the solar is ramping up."

Live Science