On Feb. 26, 1979, at 11 a.m. Eastern time, ABC News broke from regular programming for a special astronomical event: the last time in the 20th century that any place in the United States would experience a total solar eclipse. Fighting the infamous cloud cover for a shot of the vanishing sun over Portland, Oregon, ABC showed the world images of a city plunged into a darkness like “midnight, practically.” When Oregonians would have typically been enjoying breakfast, they were, instead, turning on the lights.

Back then, very little connected the natural sources of light and the electric ones. Solar-generated electricity existed, but it was a niche product. The primary buyer of solar modules in the U.S. in the 1970s was the oil industry, which used them to power offshore drilling platforms. When the solar eclipse darkens the skies over the U.S. on Aug. 21 this year — the first total solar eclipse visible here since ’79 — it will envelop a national electric grid that is deeply different from what existed 38 years ago. Today, electricity made by the sun is no longer for extreme environments or sci-fi novels. It’s an everyday reality. And when the sun vanishes for 2 minutes and 43 seconds, the electricity it produces will vanish with it, potentially destabilizing any part of the grid that relies on much solar power.

Experts don’t expect the country to have eclipse-related blackouts. But for the first time, they’re paying attention to and planning for the impact of an eclipse on the electric grid. And, they say, when a total eclipse crosses the country again seven years from now, it’s likely those plans will be even more important — and the risks could be higher.

Solar still makes up a tiny fraction of the overall electricity produced in this country — just under 1 percent in 2016, according to the U.S. Energy Information Administration. But that tiny percentage represents huge growth. Between 2010 and 2016, the installed solar capacity — think of it as potential power, the electric generation that is constructed and operational and ready to use — grew by 72 percent every year on average. Meanwhile, the amount of solar we actually use varies a lot by time of year and location. In March 2017, solar provided a record 2 percent of all the electricity used nationwide. And for three hours on March 11, the electricity used by Californians was about 50 percent solar.

That’s a big deal because our electric grid wasn’t designed with solar power in mind. Frankly, our electric grid — the network of wires connecting power plants to your home outlets and back again — wasn’t really designed at all. Instead, it evolved as individual houses powered by basement generators were linked into neighborhoods powered by larger equipment, which were linked into municipal networks, which were eventually lashed together to crisscross the entire country. The process was ad hoc, and there’s still no single regulating authority in charge.

It’s also remarkably fragile. Keeping the lights on is a minute-to-minute balancing act in which grid operators have to maintain an almost perfect match between the supply of electricity coming onto the wires and the demand for electricity being pulled off. A mismatch of even a fraction of a percent can mean blackouts. Historically, that less-than-ideal construction has worked out generally OK, because the sources of energy we were using could be, to varying degrees, turned off and on at will. Too much demand and not enough supply? Increase production at your natural gas power plant. Too much supply and not enough demand? Sell the excess to a different part of the grid and slow down production at a coal plant or two. It takes people working 24 hours a day, seven days a week, but we’ve been able to make a teetering colt of a system function like a full-grown draft horse.

Solar is less controllable. If the sun isn’t there, it’s not there. You can’t force it to shine at night. And solar resources are variable throughout the day — the eclipse is an extreme example of that. That doesn’t make solar a “bad” source of energy, said John Moura, director of reliability assessment and system analysis for the North American Electric Reliability Corporation (NERC), a nonprofit regulatory body that helps coordinate between the many entities that control the grid. But solar is different, and people are still figuring out how to mesh the source with aging infrastructure that was optimized for something else.

All of this explains why a total eclipse has suddenly become a matter of interest for people like Moura. Where the sun disappears on Aug. 21, the supply of electricity from solar generation will go with it. When the sun comes back, there will quickly be a lot of supply added. It’s crucial to make sure those changes don’t upset the balance between supply and demand. Ten years ago, Moura told me, nobody would have thought of this event as even a possible risk. But now, there’s enough solar that grid controllers, utility companies and organizations like NERC are paying attention. Moura was part of a team behind a white paper that NERC published in April outlining the results of a series of computer simulations showing what could happen during the eclipse and what grid operators needed to be aware of.

At first, he told me, his team didn’t think there was going to be much effect on solar electricity at all. The total eclipse will first be visible in the U.S. in Oregon and then will travel on a diagonal path toward South Carolina — at first glance, it wasn’t hitting the states with major solar resources. “But after working with NASA and getting the lines all mapped out, even Southern California is going to receive up to a 60 percent obscuration of the sun,” he said. “That’s directly correlated with solar [electricity generation] output.” At the end of 2016, there were 14 gigawatts of solar capacity in California. (For context, the Hoover Dam has a capacity of 2 gigawatts and powered 700,000 homes at its productivity peak in the 1980s.) Even a small dip in productivity can add up at that scale.

But California isn’t even the place Moura will be watching most closely. That would be North Carolina. Yes. Seriously. Not only is the Tarheel State second in the nation in solar capacity with more than 3,200 megawatts (the largest nuclear power plant in the U.S., Palo Verde in Arizona, has a capacity of 3,937 megawatts), but 3.24 percent of the electricity generated in the state in 2016 was from solar — far higher than the national average. Most of North Carolina isn’t directly in the path of the total eclipse, but, like California, it will see a reduction in sunlight. And, unlike California, North Carolina will get that reduction somewhere around 2:45 in the afternoon — a time of day (and time of year) when electricity demand is usually near its peak, solar supply also peaks and energy companies tend to rely on solar resources most heavily.

The eclipse will have a major impact on solar electricity production in North Carolina, said Randy Wheeless, a spokesman for Duke Energy, the company that controls most of the grid in the state. Duke has a team of engineers essentially running war games to prepare — modeling the possible outcomes in computer simulations, preparing purchases of fuel for traditional generation, and arranging access to electricity generated elsewhere. In 2015, when the solar-heavy German grid went through an eclipse, they took a similar approach, said Ulrike Hörchens, a press officer for Tennet, a European electric transmission company. Standby generation — power plants ready to start pumping out electricity at a moment’s notice — are a big deal in these situations.

North Carolina will be ready, Wheeless said, but preparation for the eclipse has also highlighted the importance of infrastructure upgrades that require big investments. In Europe, the 2015 eclipse drove home the importance of electricity storage — something in short supply in the U.S. — that could be tapped to balance supply and demand. Back in his territory, Wheeless said the biggest issue is turning out to be that construction of new transmission lines hasn’t kept pace with the construction of solar power plants. The majority of these facilities are in lightly populated rural regions, creating geographic pockets where a lot of electricity now flows over an old network of wires and circuits that was built to distribute small amounts of electricity to local farmers. Those lines are already stressed. The wink of the sun — and the quick decrease and increase in solar generation it represents — will stress them further. He’s certain there won’t be blackouts, but the eclipse is definitely a test of how robust a jury-rigged system can be.

By the time the next total eclipse comes around in 2024, there’s likely to be a lot more solar on the grid, Moura said. What we learn from this eclipse will help us better understand how the grid will have to change to make that new influx of solar electricity work — both on an everyday basis and during extreme events. So while millions of Americans will be staring at the sky on Aug. 21, grid operators will be checking their numbers, watching supply and demand, and making sure our electric system gets a passing score.