With the added generating capabilities of Keokuk Plant, UE began offering electricity to the then-rural areas outside St. Louis. In 1917, UE moved into the Missouri counties of Franklin, Jefferson, St. Charles, St. Francois and St. Louis.

Keokuk Dam, under construction in 1912, allowed Union Electric to generate electricity in an innovative way -- through hydroelectricity.

Union Electric began the process of rural electrification with added generation from Keokuk Plant.

In the 1920s, Union Electric began to add generation sources, by acquiring or building fossil fuel plants. Cahokia Plant, which was built in 1928 and is now closed, and the first Venice Plant unit, built in 1942, boosted UE's peak energy output to 350 megawatts.

While the company was bringing electricity to more and more Missourians, another division of the Union Electric Light and Power Company was also expanding and providing a different service.

That division, St. Louis County Gas Company, was engaged in the manufacture and distribution of gas. The company had been manufacturing gas from coke and oil at its 43-acre Shrewsbury plant since 1911 and selling to customers in the county. At the time, it was the second largest gas system in Missouri.

In 1945-46, the Securities and Exchange Commission ordered a horse trade: Union Electric transferred the County Gas Company to the Laclede Gas Company in exchange for Laclede's Mound Street Plant and electric customers. This swap of customers and facilities left Laclede the sole supplier of gas in the city and county and Union Electric the sole supplier of electricity in the St. Louis area.