the residual curve obtained can be directly compared with those resulting from the integration of solar and wind technologies it is thus possible to derive for each scenario the optimal generation mix which satisfies that residual load at the leastcost it is then possible to show and quantify those additional costs borne by the whole electrical system when integrating intermittent renewables are present in the electrical system dispatchable sources must provide more electricity in the periods of high residual demand when the electricity is more costly and less electricity in period of low residual demand when the electricity is cheaper those are represented by the areas in blue and in pink respectively in figure dthe dotted black line in the figure shows the minimal cost for producing electricity as a function of the utilisation time