The last coal-fired power plant in Hawaii closes in 2022, emitting 1.5 million tons of greenhouse gases a year. Energy storage farms could replace it, with potential uses such as battery storage, solar and wind power. The state's largest stand-alone energy system, with 565 megawatt-hours of storage capacity, was developed by San Francisco-based Plus Power. Hawaii has mandated a transition to 100% renewable energy by 2045.