Westar Energy is an investor owned electric utility headquartered in Topeka, Kansas. Westar is the largest electric company in Kansas, serving 690,000 residential, commercial and industrial customers in the eastern third of the state. Westar Energy generates more than 27,000,000 megawatt-hours of electricity per year at its 12 power plants. Westar Energy service territory covers 10,130 square miles (26,200 km) in east and east-central Kansas. Westar Energy owns more than 6,300 miles (10,100 km) of transmission lines and about 28,100 miles of distribution lines.

What is now Westar is the product of a 1992 merger between the two major electric companies in eastern Kansas, Kansas Gas and Electric (KG&E) of Wichita and Kansas Power and Light (KPL) of Topeka.

KG&E was founded in 1909 when the American Power and Light Company took over electric companies in Wichita, Pittsburg and Frontenac. Within a decade, it served over 48,000 people in 50 cities and towns. It also provided natural gas to several of the larger cities in its service territory.

KPL was founded in 1924, and quickly expanded across northeastern Kansas. In 1983, it merged with The Gas Service Company, a natural gas utility serving customers in Kansas, Missouri, Nebraska and Oklahoma.

In 1992, KPL merged with KG&E to become Western Resources, with KPL and KG&E as operating companies. The merger created one of the largest utilities in the Midwest, serving 560,000 electric customers and 1.06 million natural gas customers in three states. In 1996, Western Resources sold its natural gas business to ONEOK as Kansas Gas Service; this company is now part of ONE Gas. In return, Western Resources acquired a 45 percent stake in ONEOK; it sold this stake in 2003.

In 2002, Western Resources officially changed its name to Westar Energy, and all of its subsidiaries began doing business under that name.

In May 2016, Great Plains Energy, parent company of regional utility neighbor Kansas City Power and Light Company, purchased Westar for $12.2 billion including $3.6 billion in debt. Under the terms of the purchase, Westar shareholders will receive $51 in cash and $9 in stock options per share. Great Plains also agreed to retain Westar's current headquarters location in downtown Topeka as a condition of the deal.

In 1999, Westar Energy restated its consolidated financial statements for 1999, 1998 and 1997 and for each of the periods of 2000, related to the Westinghouse Security Systems (WSS)acquisition.

On January 14, 2003, Westar Energy Inc was charged for transactions involved power sales from one Cleco Corporation affiliate to Westar and then back to another or the same Cleco affiliate, and paid $30,000,000 USD for settlement.

On Nov 1, 2002, Westar Energy announced to restate results for its first and second quarter, to account for additional impairment at its Protection One Inc.(POI) unit.

On March 25, 2004, Westar Energy said to restate its 2003 annual financial results, after realizing that it might have understated its cash flow from operations for the year.