California, struggling to balance its clean energy push with the need to boost tight power supplies and avoid rolling blackouts, will lean more on fossil fuels in coming weeks to keep the power on if scorching heatwaves stretch its grid.

The Golden State, which has among the world’s most aggressive environmental policies, faces a potential supply shortfall of up to 3,500 megawatts during peak demand hours in the coming weeks. That is about 2.6 million households worth of electricity supply.