AILSA CHANG, HOST:
People across the country are gearing up for the solar eclipse on Monday - and not just spectators - energy supply managers, too, especially where you are, David.
DAVID GREENE, HOST:
Yeah, that's right. Almost half the solar power in the United States is generated right here in the state of California. And as Lauren Sommer of member station KQED explains, the eclipse will mean a dip for the power grid.
LAUREN SOMMER, BYLINE: Solar power already comes with ups and downs in the form of clouds, of course. Just ask Jan Klube of the company Enphase, which monitors solar farms around the country day in and day out.
JAN KLUBE: So this was a particular cloudy day.
SOMMER: Klube pulls up a graph showing the solar output from one California home. In the morning, the sun comes up. The power starts flowing. But in the afternoon...
KLUBE: All right, you see the big drop. So there's a cloud coming and going. That's why you see the zigzag.
SOMMER: It drops by about a third. But say your solar panels are getting no sun at all because they're in the path of totality during the eclipse.
KLUBE: It will go all the way to zero.
SOMMER: Effectively, nighttime. California isn't in the full path, but the state will lose 90 percent of the sun in the north down to nearly 60 percent in the south, more than enough to cause some anxiety for the people who have to keep California's lights on.
DEANE LYON: We're doing a lot of coordination and a lot of preparation. It's probably the most work this company has done to prepare for a three-hour event in our history.
SOMMER: Deane Lyon works at the California Independent System Operator, which runs most of the state's electric grid. Solar has been booming in California. Some days, it's as much as 40 percent of the state's power supply. The eclipse will mean solar output is cut roughly in half over the course of several hours.
LYON: We're also going to see a demand increase from the loss of the commercial and residential rooftop solar.
SOMMER: All those buildings that normally use rooftop solar will need the grid instead. Add that to the loss from big solar farms, and California will need to fill a power gap equal to what 6 million homes use. Other leading solar states, like North Carolina and Nevada, will also see a hit to a smaller degree. But Lyon says not to worry.
LYON: Luckily, we had a really good water year this year, so we'll have some pretty good flexibility on the hydro.
SOMMER: And what hydropower dams can't make up for, natural gas power plants will. Supply always has to meet demand. Otherwise, you get blackouts. Grid operators say they're prepared because with renewable energy on the rise, they already deal with power dips every day and have improved their forecasting. But Lyon says the next eclipse in 2024 could be an even bigger challenge with even more solar in the mix.
LYON: It'll be a major thing for the people running the grid at that time to manage. It won't be me. I'll hopefully be retired for several years by then.
SOMMER: Energy officials are asking Californians to turn off lights and conserve on Monday just to give the grid a little extra help. For NPR News, I'm Lauren Sommer in San Francisco.
(SOUNDBITE OF MONO/POLY'S "INTERGALACTIC")
NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.