Will the Solar Eclipse Cause a Blackout?

Literal rather than figurative dark times are ahead next week, as a widely anticipated total eclipse of the sun occurs over parts of America on Monday. While sunlight will be reduced for much of the country, the path of “totality”—where the sun is totally blocked out—will travel from Texas up through Ohio over Niagara Falls and across parts of New England. 

The whole thing is going to be a lot of fun. But for those who are thinking like me, it raises an obvious question: what is the eclipse going to do to the electric grid? 

Without getting into the details of the physics involved, it’s a well-known fact that solar power tends not to work as well when the sun isn’t out. An eclipse will therefore substantially reduce the amount of solar-powered electricity that can be generated on Monday afternoon. During the last total eclipse in the United States, in 2017, the lack of sunlight was estimated to reduce solar generation by up to 6.5 gigawatts. Given that solar capacity has increased rapidly in recent years, we should expect an even bigger effect this time around. 

Nevertheless, it’s unlikely that the eclipse will cause serious problems for grid reliability. A total eclipse may be an unusual occurrence, but a substantial falling off of solar power is not. In fact, it happens every day, or rather, every night. Sunsets do not typically cause blackouts. Instead, as solar generation declines toward the evening, other generators come online to pick up the slack without any loss of reliability. 

It’s important to stress that this isn’t something special about solar power. The entire electric system is designed to ensure that there is enough electric capacity to meet consumer demand during its peak. Since electric demand varies considerably depending on the time of day or year, this means that most of the time there will be lots of unused electric capacity sitting around, which the grid can draw upon if other resources are temporarily unable to function.

Peak demand tends to occur at different times in different parts of the country. Colder places tend to have their peak in the winter (when electricity is needed to keep people warm) while hotter places tend to have their peaks during summer afternoons (when electricity is needed to keep people cool). The eclipse, by contrast, is happening in April, which is close to being the perfect date, at least in reliability terms. Not only that, but the eclipse itself is expected to reduce electric demand, both by making it slightly cooler and because everyone will be outside staring at the missing sun. 

A bigger issue could be the rapidity of the fall-off in solar generation. On a typical day, solar generation increases gradually in the morning to a peak in the afternoon before declining gradually again in the evening. The eclipse speeds this process up. The faster the decline (and rebound) of solar generation, the more grid operators need to be on the ball to make sure all that missing power doesn’t leave a gap between supply and demand. Fortunately, the eclipse is not exactly an unexpected occurrence, and the nation’s various grid operators have developed plans to make sure the swings in solar generation go smoothly.  

All of this means that we should feel free to enjoy the eclipse without worrying about whether the lights will be on when we go back inside.