Mark Stultz of Xcel Energy, tells Susan McGinnis about his company's plans to convert its Cherokee power plant, which currently burns coal, to natural gas. Stultz shows Susan an aerial photo of the plant and points out the four coal-burning units, the emission control buildings, the coal pile, cooling ponds and the substation yard, where transmission towers are located. He says most scenarios under a law requiring power generators to either convert their coal-fired units to natural gas, retire them or install better emissions controls. The Cherokee plant will close down three units, and possibly the fourth. He says the plant needs to continue generating electricity while it's undergoing this conversion. The Colorado Public Utilities Commission will decide which plan will be used. The company will build other, gas-fired units, to accomplish this. The company will retire a coal-fired unit in another part of the state and leave two coal-fired units functioning with stronger emission controls. Stultz says it makes sense for many plants to switch their production to natural gas because it's abundant, domestic and burns cleaner than coal. But other utilities may be in a different situation. He says this conversion will cost his customers 1.7 to 2.3 percent more on their utility bills, but switching to renewables like wind or solar could be more expensive. He says wind and solar also have disadvantages in that they don't produce electricity 24 hours a day, while his utility has to produce electricity on that schedule. Stultz says Colorado customers support renewables, and Xcel has a more of renewables in its portfolio than any utility in the country. He says coal will always be in the mix, as will natural gas, renewables and energy efficiency and conservation, because a utility never wants to "put all its eggs in one basket." He says converting to burning biomass can be expensive because of the costs of cutting down forests and transporting the fuel. He says there are reliability issues with biomass, too.