Abstract Increasing extreme climate events driven by climate change raise the question of their impacts on the power production system, and implications for renewable versus fossil power supply. Here, using climate reanalysis data and daily electricity generation (2018–2023), we systematically quantify the impact of extreme climate events, specifically extreme cold events and extreme hot events on United States state-level carbon emissions and on the carbon intensity of electricity. We find that extreme climate events increase the carbon intensity of the energy production, increasing the reliance on fossil-based sources of energy and reducing the capacity of renewables. The states with more renewable electricity generation were also more affected by extreme temperatures. Our results reveal the extent to which the reliability and resilience of the current United States electricity system depends on fossil energy during extreme climate events, and suggest a need for adaptation measures as the country will transition to higher shares of renewable energy while extreme events will become more frequent.