A total eclipse on Monday will mean the loss of thousands of megawatt-hours of solar generation across portions of the United States — likely impacting more generation than in any previous solar eclipse, experts say.

Grid operators across the nation have been preparing for months and say they can navigate the event with no impact to reliability. But they also say their efforts are complicated by uncertain weather outlooks, and eclipse-related behavioral changes that make forecasting energy demand difficult.

While the “path of totality” will pass over just a portion of the solar facilities in the U.S., and cloud cover will impact the extent of lost production, “the effects of this eclipse will be significant on solar generation across the country,” said Solcast lead data scientist Hugh Cutcher. The company offers solar irradiance data and forecasting technology.

Solcast estimates that across all U.S. grids, maximum generation losses over the span of the eclipse could be up to 39.9 GWh, with about 16.2 GWh of the loss from rooftop solar.