The agreement was announced Wednesday by Houston-based Direct Energy. The sale will be made through Direct Energy Power Holdings Inc.
The units are located in Bastrop (BAS'-trup), Mission, and Paris and have a combined capacity of nearly 1,300 megawatts.
The agreement with New York-based Blackstone is subject to regulatory review and should close in early 2014.
Direct Energy is a subsidiary of Centrica PLC.