When the total solar eclipse hits on Aug. 21, it’s going to be quite the party. The problem? Solar panels need the sun.

With the sun blocked, solar energy sources will be affected. More than 100 million solar panels are expected to be darkened, which will drop output by 20 percent — or the equivalent of all the energy the city of San Francisco uses in a week.

SEE ALSO: 20 questions you’re too embarrassed to ask about the solar eclipse

Previous eclipses like the one that blotted out Europe in 2015 can give us a heads up on how much solar power will plummet. In Germany, output dropped from 14 gigawatts (GW) to 7 gigawatts, according to …

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