California suffered its first rolling blackouts in nearly 20 years because energy planners didn’t take climate change into account and didn’t line up the right power sources to keep the lights on after sundown, according to a damning self-evaluation released Tuesday by three state agencies.

The rotating power outages didn’t last long and affected only a small fraction of the state’s 40 million people. Just under half a million homes and businesses lost power for as little as 15 minutes and as long as two and a half hours on Aug. 14, with another 321,000 utility customers going dark for anywhere from eight to 90 minutes the following evening.

But officials should have been prepared for the climate-driven extreme heat that caused electricity demand to soar and briefly left the nation’s largest state without sufficient power supplies, the state’s Energy Commission, Independent System Operator and Public Utilities Commission acknowledged in a preliminary “root cause analysis” demanded by Gov. Gavin Newsom.

The immediate cause of the power shortages was the heat storm, which saw California experience four of its five hottest August days in the last 35 years, the analysis found. Temperature records were shattered across the American West, limiting the Golden State’s ability to make up for its energy deficit by importing electrons from other states.

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