Entergy Louisiana LLC announced Monday it has told Shaw Group Inc. to begin building the utility’s new gas-fired generating unit in Westwego.

Ninemile Unit 6 will be capable of generating 550 megawatts of electricity -- enough to power around 550,000 homes -- and cost an estimated $721 million to build.

The Louisiana Public Service Commission approved Entergy’s application to build the new unit on March 21. Entergy filed an application with the PSC in June 2011. Entergy says the new unit could cut customers fuel costs by $26 million to $53 million a year.

The gas-fired unit is expected to begin operating during the first part of 2015. Entergy Louisiana will build and own the new power generation unit and retain 55 percent of its capacity. Entergy Gulf States Louisiana, which serves the Baton Rouge area, and Entergy New Orleans Inc. will buy 25 percent and 20 percent, respectively, of the capacity and energy of the unit via life-of-unit power purchase agreements.