The output of solar energy plunged across the regional power grid during Monday's eclipse, as predicted. But there was an unexpected and even greater reduction in power demand as the temperature dipped and millions of people went outdoors to observe the event.

PJM Interconnection, the Valley Forge operator of the power grid in 13 states and the District of Columbia, said solar electricity production declined about 2,220 megawatts during the eclipse, including about 1,700 MW of "behind-the-meter" photovoltaic generation, such as from rooftop panels. That was in line with predictions.

PJM had expected the reduction in power production from rooftop panels to result in an increase in electric demand on the grid.

But utilities recorded a net decrease in demand for electricity of about 5,000 MW throughout the eclipse. PJM attributed that to reduced air-conditioning load, increased cloud cover, and "changes in human behavior related to the event" — a nation shutting down and going outside to watch.

The region experienced an average temperature decrease of about 2 degrees through the two hours of the eclipse, PJM said, which would have resulted in a reduction in air-conditioning load.

Utility operators had planned for the eclipse for months, though they did not expect it would be difficult to manage the shifting energy flows during the event. Grid-connected solar power now accounts for less than 1 percent of PJM's 185,000 MW of generation capacity.

Monday's event was a dress rehearsal for April 8, 2024, when another eclipse will occur in a northeasterly band stretching from Texas to Maine, directly affecting PJM's territory. By 2024, PJM expects solar generation's role on the grid to have grown significantly.