The spread that results from power minus coal and carbon costs. That is this spread captures the profit realized by a power generator after subtracting the cost of coal fuel and carbon allowances. In this sense, the clean dark spread is similar to the clean spark spread, but is related to coal-fired (as opposed to gas-fired) power plants. This spread reflects the profitability of a coal-fired generating facility. In view of the fact that the coal burning process results in emission of more than two times the carbon dioxide, the cost of purchasing emission allowances for such facilities is distinctly high.

The dark green spread is also known as the clean dark spread.