marginal cost
the increase or the decrease in total cost within a specified period, e.g. peak, winter day, summer day, night, resulting from the addition or subtraction of one unit, i.e. kilowatt or kilowatt-hour to the quantity supplied in that period
GRM: n.
References:
SPE long-run marginal cost
SPE short-run marginal cost

ENERGY TERM COURTESY OF ENERGY.EU