While many companies have pledged to reduce greenhouse-gas emissions with “net zero” plans that involve buying carbon credits, America’s largest power company is attempting something quite different: “real zero.”

NextEra Energy Inc. NEE, -4.61%,   the owner of Florida Power & Light and one of the world’s biggest renewable energy developers, is set to announce Tuesday that it plans to make most of its operations completely carbon-free by 2045, by building huge solar farms and converting its power plants to run on hydrogen fuel.