[stale2002]

It is not rhetoric, it is jargon.

Marginal cost is something that people who study economics would instantly understand, to the point where they would be confused as to why others are confused.

It means something very specific, but it is very common to talk about in economics.

I will throw around the term accidently, without even realizing that it would make some people confused.

Capital costs vs marginal costs is an important concept, and definitely appropriate to talk about in the energy sector. But you of course have to explain it.

[Mendenhall]

I was not speaking to what marginal cost is, I was speaking to the reason they chose to use the term marginal cost and also chose to pin it to sunny/windy and free.

Some are assuming I did not see "marginal" in there, but the rhetoric angle is exactly why author chose marginal cost. It was simply to get the word "free" in there and then again author used "free" when speaking about shale.