 Bloomberg -- California faces another round of rolling blackouts after the operator of the states grid issued an emergency order for utilities to throttle back power to prevent a heat wave from overwhelming the system.Its the second night in a row that the California Independent System Operator has taken the drastic step as temperatures soar across the U.S. West and people blast fans and air conditioners to keep cool.The outages are the latest example of how climate change and the extreme weather it triggers take a profound toll on electrical grids. Just days ago, millions of people lost power in U.S. Midwest after a wall of lightning, hail and deadly winds tore a path of ruin from central Iowa to Chicago. Before that, Tropical Storm Isaias darkened homes from the Carolinas to Connecticut.For California, the problem is heat. An unrelenting high-pressure system is pushing temperatures upward, leaving the region facing whats expected to be its hottest two weeks in 70 years. Thats driving up power demand to extreme levels, making it hard for generating plants to keep pace.Before Friday, California hadnt imposed rolling blackouts since the energy crisis of 2001.Read More: One Million Lost Power in Storm That Spawned Chicago TornadoThe California Independent System Operator, which runs the states grid, had announced late Saturday afternoon that blackouts werent needed. Then the system unexpectedly lost generation, and it become clear the system couldnt meet demand, said Anne Gonzales, a spokeswoman for the California ISO said in an interview. She didnt know the reason the grid lost capacity and had no immediate details on how widespread the outages would be or how long they would last.The states largest utility, PGE Corp., said it expected outages to impact 200,000 to 250,000 homes and businesses in its service territory.On Friday, the states grid operator ordered blackouts after a 500-megawatt generator tripped offline unexpectedly while a 750-megawatt unit was already down. If both had been in service, grid operators wouldnt have had to call for outages, Gonzales said.The heat and the blackouts are hitting at an especially vulnerable time, with Covid-19 forcing millions to remain at home.The last time such a declaration was made during the electricity crisis of 2000 and 2001, hundreds of thousands of homes and businesses took turns going dark, power prices surged to a record and the states largest utility was forced into bankruptcy.Widespread HeatRegions around the world have been grappling with extreme heat, including parts of Europe and the eastern U.S., where temperatures last month were expected to set records for New York and Boston.Last month was tied for the worlds second-hottest July on record and the hottest ever in the northern hemisphere, according to the National Oceanic and Atmospheric Administration. But few authorities, if any, resorted to rotating outages.In addition to mass blackouts to protect the grid, a prolonged hot spell could trigger scattered outages as aging utility equipment fails in the heat. Transformers -- the metal cylinders sitting atop power poles -- can break down and even catch fire if they cant cool off at night. During a 10-day heatwave in 2006, California utilities lost more than 1,500 of the devices, with each knocking out one neighborhood in the process.Adds details of outages beginning in the sixth paragraph.For more articles like this, please visit us at bloomberg.comSubscribe now to stay ahead with the most trusted business news source.2020 Bloomberg L.P. 