Colorado regulators will kick off a series of hearings Wednesday that could determine how the state’s largest utility generates its energy in the coming decades — and how much it will contribute to global warming. Xcel Energy, which supplies electricity to 1.5 million people across Colorado, announced plans to phase out its remaining coal plants earlier this year and instead rely more on natural gas, renewable energy, and emerging technology such as hydrogen power. The utility’s plans came as the state, pushed by activists and environmental groups, set a goal to slash its greenhouse gas emissions by half of its...