On Monday, August 21, 2017, a total solar eclipse will sweep across the continental United States from Oregon to South Carolina. A solar eclipse occurs when the moon moves between the Earth and the sun, thereby obscuring the sun from Earth’s view and creating a shadow on Earth’s surface below. While a partial solar eclipse occurs every 18 months, this will be the first total eclipse visible since 1979. Because the moon and the Earth are constantly in motion, the eclipse will only remain visible at a particular location for a short period.
This rare eclipse event presents one interesting dilemma for utilities that rely on solar energy. When the sun is obscured, energy generated from photovoltaic solar panels drops rapidly, thereby resulting in a significant decrease of the amount of power contributed to the power grid by those panels. Utilities have to compensate for the drop by getting power from other sources.
Of course, the drop in solar energy occurs every night when the sun goes down. Sometimes cloud cover can even diminish solar PV output in a particular area during high demand. The difference on Monday is that the eclipse will occur across an entire state at once, during the day, when solar systems are typically near peak output and when load demand is especially high from air conditioning use.
Some parts of the sunny Southwest—states that generate a large amount of solar energy—will experience a momentary full eclipse on Monday. However, the utilities in these areas expect to have sufficient alternative energy sources to meet the demand on the grid during the five minute window when the eclipse hits.
The bigger concern lies in California, where the eclipse is set to cast a longer shadow on Monday morning. Since solar accounts for a large part of the state’s electricity, California utilities and utility operators plan to ramp up generation at their natural gas and hydroelectric plants to cover the gap. When the eclipse passes a few minutes later, and all of the solar panels come roaring back to life, operators will have to move quickly to scale back gas and hydropower to accommodate the sharp and quick resurgence of solar power.
The North American Electric Reliability Corporation (NERC) does not expect the eclipse to create reliability issues for the bulk power system. Still, the eclipse will test the reliability of solar generation on a national scale, and challenge the preparedness of operators in California and other highly affected states like North Carolina in the Southeast.