For months, the nation’s grid overseers have been preparing for any disruptions in solar power that the eclipse might cause, by running models and training operators in simulators for worst cases. Because solar still provides less than 1 percent of electricity nationwide, regulators are confident that the lights will stay on, other energy sources will compensate and the costs will be minimal.

But many operators also see the event as a rare trial run for a future in which solar power will become far more prevalent — and they will have to accommodate a fast-growing source of energy that, unlike older coal or nuclear plants, can wax and wane considerably during the day, and drop off at night.

“An eclipse is obviously not something we see every day, but this is going to be a good exercise for us,” said Randy Wheeless, a spokesman for Duke Energy, which expects solar capacity in North Carolina to dip from 2,500 megawatts to just 200 on Monday afternoon, affecting roughly 3 percent of electricity generation in the state. “There’s no doubt more solar power is going to come onto the grid in the future, and that does increase the challenge of balancing the grid even on days when there’s not an eclipse.”

Nowhere is this challenge more stark than in California, home to nearly half the nation’s photovoltaic panels. The California I.S.O. already has to handle a steep ramp-up of solar power every morning and a similarly steep ramp-down every evening. Because it can be difficult and costly to quickly switch gas turbines on and off to compensate, the state is getting creative in juggling its fast-growing solar load.

Some of its tactics will be put to the test on Monday. For example, California has agreements with Arizona, Nevada and Oregon to transfer small amounts of electricity between states to balance out fluctuations. The state may need to call on this “energy imbalance market” during the eclipse, Mr. Schmitt said.