Regulated utilities cost ratepayers over $3.5 billion from 2015 to 2017 through uneconomic coal practices, according to a report released Tuesday from the Sierra Club.

Vertically-integrated utilities consistently operated coal units based on their own scheduling rather than relying on market signals to determine when running that plant would be most economic, the report found. The practice, known as self-scheduling, became common when there were fewer cost-effective alternative resources, but now hinders the ability of other resources, wind and solar, to compete in power markets, research has previously found.

Without self-scheduling, coal-powered generation would have dropped 10% and the median market price would have risen 30% or $7.7/kWh in the Midcontinent Independent System Operator (MISO) from 2015 to 2017, according to modeling scenarios run for the report by Synapse Energy Economics.