The annular solar eclipse begins in the US in Oregon at 9:13 am PT and ends in Texas at 12:03 pm CT, according to NASA. An annular solar eclipse occurs when the moon passes in front of the sun, appearing slightly smaller than the sun, so it does not block the entire solar disk.

"We've been trying to get ahead of this," CAISO President and CEO Elliot Mainzer said during his CEO report to the Western Energy Imbalance Market governing body Sept. 19. "It'll be an interesting moment."

Renewables account for roughly 70% of supply during midday peak with solar making up over 80% of renewable generation at its daily peak, according to CAISO data.

The grid operator's short-term forecasting team detailed the eclipse's impacts to the region in a recent technical bulletin that shows grid-scale renewables generation is expected to decline by 9.374 GW, while at the same time, gross load will increase by 2.374 GW.