The spread that relates the revenue received from selling electricity to the cost of producing it from coal. This spread is expressed in currency units per MWh after taking into account the thermal efficiency of the input (which, in turn, refers to the relation of the electrical energy produced to the energy content of the coal used). The following formula defines the dark spread:

Dark spread = price of power – (price of input fuel/ thermal efficiency)

In order to shift from the dark spread into clean dark spread, the following formula is used:

Clean dark spread= dark spread – (emission adjustment/ cost of emission)

If the spread is positive then buying coal and selling electricity will be profitable. Otherwise, a power generator who has purchased coal forward on a fixed basis may need to sell the coal back to the market.