For the design of solar systems of flat plate is required to know the average value and extreme values of average global radiation. For the particular Colombian case this information is usually consulted in the NASA database or in the official Atlas of solar radiation. This paper presents a review of the models that relate global radiation to solar brightness and is applied to a particular case study that allows determining which of them presents the greatest effectiveness. Ten models were analyzed in total and greater emphasis was placed on polynomial approximation models of first and second order because it is the simplest and reaches the best approximation. It was observed that due to the dispersion of the data, due to the geographic conditions of the region, the model that best results presents is the order two polynomial approximation.