As California rapidly boosts sales of electric cars and trucks over the next decade, the answer to a critical question remains uncertain: Will there be enough electricity to power them?

State officials claim that the 12.5 million electric vehicles expected on California’s roads in 2035 will not strain the grid. But their confidence that the state can avoid brownouts relies on a best-case — some say unrealistic — scenario: massive and rapid construction of offshore wind and solar farms, and drivers charging their cars in off-peak hours.