The August 21 solar eclipse will obscure the sunlight needed to generate electricity at some 1,900 utility-scale solar photovoltaic (PV) power plants in the United States, according to the Energy Department's Energy Information Administration (EIA).
However, relatively little solar PV capacity lies directly in the path of totality where the sun will be completely obscured by the moon. As a result, EIA says that the North American Electric Reliability Corp. (NERC) does not expect the eclipse to create reliability issues for the bulk power system.
Solar-powered generators centered in the path of totality will be affected the most, as the moon will block all direct sunlight for up to three minutes. These generators will also be affected to a lesser extent throughout the entire eclipse event, which will last for up to three hours, measured from the onset to the ending of any blockage of direct sunlight.
Generators outside of the path of totality will be less affected, depending on how much sunlight is obscured. The path of totality spans the U.S., starting in Oregon and moving to South Carolina over around 90 minutes.
The path of totality affects 17 utility-scale solar PV generators, mostly in eastern Oregon.
Hundreds of plants totaling about 4.0 gigawatts (GW) of capacity — mostly in North Carolina and Georgia — will be at least 90 percent obscured. Another 2.2 GW and 3.9 GW of capacity are in areas that will be at least 80 percent and at least 70 percent obscured, respectively.
The values in EIA's analysis reflect utility-scale generators of at least 1 MW of capacity and do not include small-scale solar PV systems or utility-scale solar thermal generators, which in 2016 provided 0.5 percent and 0.1 percent of total U.S. electricity generation.
During the eclipse, electricity generators in the areas affected by the eclipse will have to increase output from other sources of electricity generation to supplement the decrease in solar power. NERC’s recent 2017 Summer Reliability Assessment does not anticipate any impacts on the reliability of North America’s bulk power system attributable to the eclipse, based on an analysis published in April.
Based on the amount of sunlight obscured for each of the state’s generators, the California Independent System Operator (CAISO) estimates that California will experience a reduction in solar generating capacity of almost 4.2 GW during the eclipse, which is estimated to partially darken the state from 9:02 a.m. to 11:54 a.m. local time. To ensure electricity demand is met during that time frame, CAISO plans to replace solar generation with electricity from natural gas and hydropower plants, the latter of which are generating at higher levels than previous years.
EIA says that North Carolina has the greatest amount of installed PV capacity in the band that will be at least 90 percent obscured. As of May, the state totaled 2.8 GW of utility-scale PV installations or about 13 percent of the national total.
Duke Energy, one of the largest utilities in North Carolina, estimates that solar energy output across their system will drop from about 2.5 GW to 0.2 GW at the height of the eclipse. However, solar power makes up a much smaller portion of North Carolina’s generation compared with California. In August 2016, utility-scale PV, utility-scale solar thermal and distributed PV supplied 3.1 percent of North Carolina’s electricity generation, compared with 14 percent in California.