In 2010 the EPA began allowing backup generators to operate without emissions controls for 15 hours annually as part of emergency demand response programs when heavy demand could cause blackouts. Then in January 2013, EPA upped the number of hours to 100 per year, during which backup generators could operate without emissions controls.

Now, the United States Court of Appeals for the District of Columbia Circuit has vacated and remanded to the EPA the portions of the rule that contain the 100-hour exemption for emergency engines, and urged the agency to “solicit input from FERC, as necessary.”

The court says the EPA failed to consider electric market factors and efficiency implications when it created the rule, according to Argus.

Electric generating companies along with environmental groups opposed EPA’s rule.