This paper presents a research of the behavior of the variable costs of fuel supply (CSC) in the generation of electricity by the use of gas and coal. This research was made in Colombia for the period 2012-2017. The information gathered allows coal plants to estimate their future performance under different price scenarios. It is also helps power planning organizations to evaluate the thermal generation plants in the country. Initially in the study was only considered fuels current situation and the efficiency of thermal plants in the country. We determined the procedure to calculate the CSC and last we established a financial analysis in order to compare them under the same time scenario.