The Supremes also ruled today in a case between the EPA and Duke Energy,
in which the utility tried to avoid the massive expense of adding
pollution controls to its coal-fired power plants. AP explains:

The utility industry has long resisted installing costly pollution
controls under the program called New Source Review. It waged vigorous
campaigns against the program starting in the 1980s and more recently
by battling it out with regulators when sued in federal courts.

The underlying issue is whether emissions increases should be measured on an annual basis, an approach favored by the EPA.

Duke
and other utilities argue that the standard for triggering Clean Air
Act requirements is an hourly rate increase in emissions. Such an
approach enables the companies to run coal-fired plants without having
to install additional pollution controls, as long as the hourly
emissions rate doesn’t rise. Annual pollution would rise, however, when
the modernized plants operate longer hours.

As a way to hedge against an unfavorable ruling, Duke (and Progress
Energy) engaged in negotiations with the North Carolina Utilities
Commission, the governor, the attorney general, and the legislature,
and together they produced the Clean Smokestacks law.
That gave the two major utilities a way to install the pollution
controls while forcing customers to pay for them through higher rates,
despite the illusory rate “freeze” that was supposedly going to spare ratepayers. Duke never believed the Smokestacks bill would improve air quality
in the state, but went along with the agreement because of the
EPA/environmentalists litigation threat. Now it looks like it was a
good decision on their part.