How about “solar eclipse?”

That’s how important solar generation has become in the state: Utilities and CAISO are preparing to handle fluctuations in generation and demand during a partial eclipse this summer.

For 82 minutes on the morning of August 21, the moon will move across the path of the sun. Sunny California will be a lot less so as the moon blocks 76% of the sun in Northern California, and 62% in the Southland. Besides the rare spectacle, it also means a significant reduction in the power available from solar plants for about two hours of the business morning.

In fact, CAISO forecasts a 64 percent reduction in power from commercial solar facilities at the deepest portion of the eclipse. That’s highly significant in a state that will have 10,000 MWs of installed commercial solar capacity by August.

Of course, residential solar installations will be affected, too – and they account for 65% of the 5,800 MW of rooftop solar power. That’s a lot of power that will be unavailable – the projection is that other grid-based supplies will have to make up 1,365 MW during the event. (The forecast also assumes a drop in wind power at the same time, based on the experience in Europe during an eclipse in 2015.)

Eclipses don’t happen every day and they are predictable to the minute and to a precise “shadow,” unlike heavy fog or other weather-related reductions in solar energy. Utilities have plenty of time to plan for this event, and to observe the effects on the grid from such a dramatic loss of renewable power.

There is one group of people who don’t have to worry, though: Homeowners who have storage connected to their solar panels. Not only will they have plenty of reserve power to see them through the “dark hours” of August 21, their increased demand won’t draw on the grid.

The CAISO plan already calls for studying the effect of rooftop solar on the load forecast during the eclipse. It also provides an ideal opportunity for CAISO and California utilities to observe the way in which storage contributes to the resiliency of the grid when mean solar production is reduced.

Of course, installing local storage to prepare for an eclipse isn’t the most efficient plan — the next solar eclipse visible in California occurs in October 2023. But it does get foggy, and summer loads do stress the grid, and as California becomes more dependent on solar, it will become more dependent on storage as well.