Office of Energy Efficiency & Renewable Energy

Total Eclipse of the Sun: How Will Changing Solar Power Output Impact the Grid?

August 14, 2017

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It’s been nearly a century since anyone in the U.S. has experienced anything like it: On August 21, the moon will pass between the earth and the sun, effectively blocking some or most of the sunlight that reaches the earth across a large swath of the United States. Starting above Salem, Oregon and ending above Charleston, South Carolina, this is the first eclipse in 99 years that spans the entire continent. While only lasting about two minutes in each location, the output of photovoltaic (PV) power plants across the U.S. will dramatically decrease.

Studying the Impact

The National Renewable Energy Laboratory (NREL) conducted a study of the Western Electricity Coordinating Council (WECC) territory, which covers the vast majority of the Mountain and Pacific Time zones including 14 Western states. In these areas, the eclipse will occur between 8:00 a.m. and noon. Researchers have already examined potential impacts of the eclipse on generation from power plants and rooftops. It is estimated that both sources provide the WECC with PV capacity of approximately 25 gigawatts (GW), utility scale accounting for two thirds of that. Total PV power installed across the U.S. is estimated to be over 44 GW today.

Examining the WECC as a whole, and assuming the worst case scenario—a bright and sunny day—the rolling effects of the eclipse are expected to have the biggest impact at approximately 10:30 a.m., when PV output is projected to drop 5 GW below typical generation levels. This represents the amount of energy needed to power approximately 1 million homes and, if not already anticipated, could create difficulties for portions of the grid network that use solar to meet a significant fraction of electricity demand during the day. The burden of compensating for the lost energy from solar generators will fall mostly on natural gas powered turbines, which are able to ramp up ahead of the eclipse. Hydro generation—power created from flowing water—will also help to fill the void of solar output, though conservation constraints in the West will prevent it from compensating for all of the lost generation. NREL’s modeling is expected to enable utilities to pass through this eclipse without completely disconnecting any PV, to maximize the production of solar electricity.

Preventing Future Issues

Research funded by the SunShot Initiative’s systems integration subprogram is helping to mitigate impacts of the August 21 eclipse and will continue to help utilities plan for weather events that are harder to predict. Solar forecasting technologies allow grid and solar power plant operators to predict when, where, and how much electricity will be produced—thus developing the best strategy for balancing supply and demand. SunShot funding allows NREL to conduct forecasting simulations on two large PV arrays located at a field test site near Denver. As the eclipse happens, those arrays will be monitored to verify the simulations. Denver will experience a 92% eclipse, so the impact is significant and will benefit solar producers during future eclipses.

Systems Integration

The systems integration subprogram enables the widespread deployment of secure, reliable, and cost effective solar energy on the nation’s electricity grid. Learn more

SunShot is working to develop certain energy storage solutions that are scalable, secure, reliable, and cost-effective. As more solar energy continues to be added to the grid, storage would play an important role in mitigating the intermittency of solar, which is currently not capable of meeting energy demand around the clock. These projects would enable solar generated electricity to be dispatched when and where it’s needed. Austin Energy is already beginning to integrate energy storage technology into its management tools and will soon have the capability to divert grid-connected solar to storage facilities. This work will serve as a benchmark reference for any utility to optimize its solar resources at all times.

As parts of the country prepare to experience darkness in the middle of the day, the SunShot Initiative is doing its part to help develop a more reliable and resilient electric grid, regardless of the time of day.

Learn more about SunShot’s resources on the eclipse and visit NASA’s eclipse website.