[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.