If demand is less than 10 megawatts, the wind farm can provide all of the power, and the producer get $1 per megawatt-hour. When demand rises above 10 megawatts, the gas plant is dispatched, and the wind and gas plants are both paid the $20 rate. Only when demand surpasses 20 megawatts do the operators call on the coal plant and all get $35.

So as more lower-cost resources are added to the system, coal plants contribute less frequently and thus earn less revenue. But they carry significant operating costs and are expensive to shut down and restart.

In 2008, according to data from WEC, Pleasant Prairie was available about 80 percent of the time and generating power a little more than 74 percent. A decade later, it was available 90 percent of the time but generating only about half the time.

Regulated utilities in states like Wisconsin often run the plants at a loss and pass the costs on to customers, said Mike O’Boyle, director of electricity policy for Energy Innovation, a think tank that has studied the economics of coal plants.

‘A looming problem’