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Solar power is booming in the U.S. So what happens when the sun disappears during the total solar eclipse on August 21, 2017? We talk to the experts.
Video courtesy of the Department of Energy
U.S. Department of Energy




MATT DOZIER: Welcome to another Direct Current Short Circuit. I’m Matt Dozier, and this episode we’re talking about something that’s been in the news a lot lately: the solar eclipse.

DOZIER: If for some reason you’re just hearing about this, on August 21st, the moon will pass between the earth and the sun, casting a shadow that will throw parts of the United States briefly into total darkness.  It’s going to start in Oregon and move across the country from west to east, all the way through South Carolina on the East Coast.

DOZIER: So, why are people so excited? Well, the last total solar eclipse we saw in the U.S. was 1979. And the last time an eclipse crossed the continental U.S. from coast to coast was 99 years ago! Back then, the country looked a lot different… Especially the power grid.

DOZIER: In fact, in 1918, the concept of a power grid hadn’t really even been around all that long. It was actually Thomas Edison’s personal assistant, a man named Samuel Insull, who pioneered the concept of a centralized electric grid in the late 1800s. In those early days, virtually all of our electricity came from one source: coal-burning power plants. But over the past century, it has grown to include lots of different sources of energy.

CHARLIE GAY: Today, we have a lot more choices for how we get electricity. We have hydropower, we have geothermal, wind, solar, nuclear. Natural gas is now playing a huge role, as well. There are many other any options than that coal-burning power plant that have become important and cost-effective in the grid.

DOZIER: That’s Charlie Gay.

GAY: My name's Charlie Gay, and I'm director of the Solar Energy Technologies Office here at the Department of Energy… Also known as SunShot.

DOZIER: Charlie said another thing that’s different about the grid today, is more and more Americans are starting to generate their own electricity. They’re doing that with things like small wind turbines, and especially, rooftop solar panels.

GAY: A decade ago there was probably about 1 gigawatt of solar installed in the U.S. Today there's about 35-36 gigawatts of solar installed across the U.S. -- photovoltaic solar -- and if I add in concentrating solar, it begins to approach about 44 gigawatts. So, roughly speaking, we've gone from 1 gigawatt to 40 gigawatts in the span of 10 years -- a very tall building that's been jumped over here.

DOZIER: Solar “photovoltaic” or “PV” systems are  those dark blue or black rectangles with silver trim you see everywhere on rooftops and lined up side by side in big open fields. These solar panels convert the sun’s energy into electricity and have  exploded in popularity among homeowners and businesses in the past decade.

DOZIER: So, back to the eclipse. Think about all these solar panels all across the US -- especially in the West -- and what happens when suddenly? The sun goes away. For a few minutes. And then just as quickly, it reappears.

GAY: So the interest is, blocking out the sun, how is that going to affect the output of solar, which of course will diminish dramatically as the sun goes by? And it will affect a pretty wide swath across the U.S. where it's not a total eclipse, but where 80 percent or more of the sun will be blocked out.

DOZIER: Of course, an eclipse isn’t the only time the sun disappears from the sky. There’s, you know, night? I asked Charlie about that.

DOZIER: (TO GAY) How is the solar eclipse different from just the sun going down, coming back up?

GAY: What's different is, at its peak will only be maybe 10-12 minutes of blocking of the sun where it's a complete eclipse. So we're not talking about the sun setting and waiting another 12 hours for the sun to rise. We're talking about a short period of time where the sun is blocked. The arrays here will be generating power before and after the eclipse happens, so it's a much different time span than will happen between day and night.


DOZIER: OK, so we know that solar power will be briefly disrupted across a big chunk of the country. But how much, exactly? And what does that mean?

VEDA: That's the biggest focus for us, to look at how does the eclipse change the PV output and what does that change for the rest of the grid operation?

DOZIER: That’s Santosh Veda, a power systems engineer at NREL, the National Renewable Energy Lab in Golden, Colorado. He’s one of several NREL researchers working on a SunShot-funded study of how this eclipse will impact the Western electric grid, where most of the solar energy generation in the U.S. is located.

DOZIER: Their first priority: figure out how just how much solar power the grid is going to lose on August 21 -- and not just how much, but how fast.

VEDA: We know that PV is going to drop off pretty quickly as the eclipse approaches its maximum in any location. And what that means is not just matching the total energy, but also matching the rate of loss of PV.

DOZIER: If you don’t have enough generation available to cover the demand for electricity at any point, that’s bad. You get blackouts. Ideally, you want to have just the right amount of electricity on the grid at all times. Makes sense, right?

VEDA: So we need to make sure that we have generators that are capable of picking up at the speed at which we lose PV. That's part of our analysis as well. That's information we'd like to provide to our partner, Peak Reliability.

DOZIER: Peak Reliability is what’s known as the “reliability coordinator” for utility companies across 14 western states and even parts of Canada and Mexico. Peak works with more than 130 utilities to ensure that the grid is stable and flexible so people’s lights stay on, even during potential disruptions… like, say a solar eclipse.

DOZIER: So, Santosh and the NREL team took data from Peak and used their expertise to predict what will happen during the eclipse, which is… well, not much. The roughly 5 gigawatts of lost solar output will be picked up by natural gas and hydropower, so no one will be the wiser.

DOZIER: But researchers aren’t just looking at the eclipse as a potential disruption. It’s also an opportunity to gain valuable insight into how the grid behaves in places where solar power is becoming increasingly common. They’ll get the chance to answer questions like, if the eclipse cuts the sunlight shining on a solar panel by 60 percent, how much power will it actually produce?

VEDA: So, I think right now we assume that the obscuration percentage represents the percentage of PV drop, but that is something we have not had a chance to test in the field. We have large utility-scale PV panels installed at NREL’s facility and we will observe the PV output to know the impact as the eclipse progresses.


DOZIER: Solar energy’s rise in America has been explosive. As it continues to grow, the things NREL and SunShot learn during the eclipse will help utilities better understand how it interacts with the grid at large. And that’ll further reduce costs for consumers. Everybody wins.

VEDA: Oh, absolutely. Absolutely. And i think that's where the funding from DOE has been very helpful, as well. What we're trying to do is we are developing the tools to do this study for this particular eclipse. But what we're also focusing on would be to expand this framework to be applicable for any type of wide area event, like a storm or maybe another future eclipse. And that way we would have the lessons learned from this and be able to apply it in the future.

DOZIER: After all, it’s not every day you get to see the sun turn off and back on again in the span of just a few minutes.

GAY: It's certainly a phenomenal opportunity. If you think back to... we all know Stonehenge, we all are aware of the sun affecting our lives and sort of encoded in our DNA here probably is a lot related to the importance of sun, because that's where ALL of our energy has come from.

DOZIER: And for the folks at NREL, there’s more than one reason to be excited about the big day.

DOZIER: (TO VEDA) Do you have plans to watch the eclipse? I mean, are you going to get to watch it or are you going to be staring at a computer?

VEDA: At NREL we actually have a bit of an eclipse party going on, where we're going to go up to the solar panels. There are a lot of trails close by and we plan to go up to one of these trails and watch the eclipse. And since in Colorado it's going to be 92% obscuration (not 100%), but this is good enough for us! (LAUGHS).

DOZIER: (TO VEDA) It’s going to be pretty dark.

VEDA: It's going to be pretty dark, yes!

DOZIER: (TO VEDA) All right, good luck with everything.

VEDA: Thank you. Bye.


DOZIER: That’s our show! If you have questions about this episode or any other episode you can email us at or tweet @ENERGY. If you’re enjoying Direct Current, help us spread the word! Tell your friends about the show, and leave us a rating or review on iTunes. We appreciate your feedback.

DOZIER: A big thanks to Charlie Gay, Santosh Veda and the rest of the folks at SunShot and NREL for their help on this story.

DOZIER: Thank you to the Energy Public Affairs team, and Taylor Gray at Transition Music.

DOZIER: Direct Current is produced by Simon Edelman, Allison Lantero and me, Matt Dozier. Art and design by Cort Kreer. With support from Paul Lester, Ernie Ambrose and Atiq Warraich.

LANTERO: We’re a production of the U.S. Department of Energy and published from our nation’s capitol in Washington, D.C. Until next time thanks for listening!

Check out more great resources on how the eclipse will affect solar energy, the electric grid, and the National Labs located along the "path of totality"...

Throwing Shade: 3 Things to Know About the Solar Eclipse

Office of Energy Efficiency & Renewable Energy (EERE)

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


SunShot Spotlight: Solar Eclipse 2017

National Renewable Energy Laboratory (NREL)

NREL Analysis Shows Clear-Sky Probabilities for the August Solar Eclipse

NREL Researchers to Estimate and Observe Grid Impacts of the August Solar Eclipse

Energy Information Administration (EIA)

Solar eclipse on August 21 will affect photovoltaic generators across the country