Steam and smoke rise on a cold day at a coal-fired power plant along the Platte River in Littleton, just outside Denver, Colorado.

(Photo: iStockphoto.com)

The Supreme Court heard arguments Wednesday over whether the Environmental Protection Agency should have considered costs when regulating coal-fired power plants.

The debate centers around mercury emissions from the plants—something Colorado has been reducing.

In 2010, Colorado became the first state to address the retirement of aging coal-fired power plants with the Clean Air Clean Jobs Act. It’s a move that was controversial with the state’s mining industry.

But it gained the support from two of the state’s largest utilities: Xcel and Black Hills Energy. The duo are currently in the process of eliminating 900 megawatts of coal-based power by switching to natural gas or renewable energy. The changes have translated into rate hikes.

This year, Xcel customers will see an average of a $1 increase per bill with $2 for Black Hills customers.

A decision from the court is expected by the end of June.