Building that amount of new capacity would cost roughly $2 billion and require at least four combined cycle gas power plants, said Warren Wood, the utility’s vice president of external affairs. Wood said the utility does not yet know where it would build the new plants, but he said they will most likely be in eastern Missouri.

Constructing that many new power plants that quickly is not Ameren’s preferred plan, Wood emphasized. Delaying the rule’s implementation by several years would cut the amount of natural gas generation needed by half. “Our preferred plan is to build less combined cycle later with more renewables between now and then,” Wood said.

Ameren could meet EPA’s targets by the mid 2030s, which would only cause customer rates to increase by only “a couple percent” by 2020, Wood said.

Efficiency programs are weighted so they improve a utility’s emission rate without the need to build more generation. Gupta, at the NRDC, said efficiency carries less risk and is faster, but utilities tend to look at building more plants because of industry bias. Ameren and others would be better off putting money into efficiency rather than new power plants, he said.

“Even though people believe they’re doing all they can, all experience shows there is a lot more that can be done,” Gupta said.