Coffeen Power Plant was one of the five coal-fired plants constructed in Illinois. Unit 2, shown here under construction in the early 1970s, went online in 1972.

CIPS, Illinois Power Co. and Union Electric entered into a contract to link their power operations on Dec. 1, 1952. This allowed the companies to operate their systems with less generation, make maximum use of the most efficient generating plants, make emergency energy immediately available to the companies and install larger and more efficient generating units when additional capacity was required.

With new customers and increased demand for electrical service from the 1960s through the 1980s came new plants and updates to existing plants.