German Jamison: How the Coming Solar Eclipse Will Cast Its Shadow Across the U.S. Solar Fleet

Want to know how the coming solar eclipse will affect your local solar farm? The U.S. Energy Information Administration has the data.

The solar eclipse coming on August 21 will cast a pathway of shadow across the United States, with the moon shading at least half of the sunlight reaching the earth across all 48 continental states. A path of “totality,” or complete obscuration, will stretch from Oregon to South Carolina.

In this pathway lie about 1,900 utility-scale solar PV power plants, generating more than 20,000 megawatts of energy at peak generation, according to the EIA’s Today in Energy note published this week. While only a few lie in the path of totality, nearly all will lose at least some of their sunlight for several hours through the course of the day.

But as we’ve reported in the runup to this astronomical event, the grid operators responsible for keeping electricity supply and demand in balance are prepared. An April analysis by the North American Electricity Reliability Council (PDF) projected no threats to grid reliability through the course of the eclipse.

California, with more than 40 percent of the country’s utility-scale solar capacity, is facing the biggest challenge. But the state's grid operator and utilities say they're ready for the eclipse, which is expected to trigger a 6,000-megawatt generation shortfall (4,200 megawatts large-scale, the remainder small-scale). That drop is equivalent to losing the power demand of Los Angeles from 9 a.m. to noon. Grid operator CAISO is prepared to call on hydropower and natural gas speaker plants to manage the ramps in demand to come from the solar shortfall, while utility regulators are asking consumers to reduce their energy use during those hours.

As for the path of totality, it will cross only 17 utility-scale power plants, most of them in eastern Oregon. While the moon will completely block the sun for only about to 3 minutes, the total time during which the sun will be at least partly obscured will last up to three hours.

Hundreds of plants totaling about 4 gigawatts of capacity will be at least 90 percent obscured, most of them in North Carolina and Georgia. Another 2,200 megawatts of capacity lie in areas that will have at least 80 percent of sunlight obscured, and 3,900 megawatts of capacity are in areas with at least 70 percent of sunlight obscured. Duke Energy, one of the largest utilities in North Carolina, estimates that solar energy output across its system will drop from about 2,500 megawatts to only 200 megawatts at the height of the eclipse, EIA reported.

A recent Bloomberg calculation found that more than 9,000 megawatts of solar power may go down during the eclipse, which is equivalent to taking offline about nine nuclear reactors.

It’s important to note that EIA’s data does not include solar PV facilities of less than 1 megawatt capacity, such as small-scale rooftop solar systems -- a gap in data that has led to problems with relying on EIA data to measure the country’s solar growth. Nor does it include solar-thermal systems that use the sun’s heat to generate power.

U.S. grid operators have been able to learn from the 2015 solar eclipse that darkened skies over Germany, a country with a far greater reliance on solar power. German grid operators managed to avoid disruptions through the event, which saw solar power fall from 21 gigawatts to 6 gigawatts, and then back up, over the course of two and a half hours.