Eclipses: Sticking Solar Where the Sun Don’t Shine

Jamie Wilkie

California renewable generation, weekends in October
gigawatts throughout day

California renewable generation, weekends in October
Source: California Independent System Operator

Eclipses are neat, and this weekend’s solar eclipse (where the moon passes in front of the sun, but does not completely block it) was no exception. But while the moon does its thing, it’s also stealing sweet, free solar energy from us earthlings. 

So what is the impact of an eclipse on solar power production?

Background: The California electrical system operator publishes very detailed generation statistics on a minute-by-minute basis (can you tell where the eclipse was on the chart?).

  • At peak eclipsing (?), solar generation was two thirds lower than usual, a whopping 10 gigawatts below typical levels.

How did California keep the lights on?

Of the shortfall, roughly half was made up by charging batteries before the eclipse and discharging during. The remainder was a combination of increased natural gas generation and imports from neighboring states.

Zoom out: Eclipses last a few minutes, but they are useful case studies in what a resilient renewable grid must look like.

What if, instead of an eclipse, it was – gasp – cloudy for a week in California? How long would imports, batteries, and natural gas last?

+Bonus read: California ISO’s reports on eclipse grid management