This paper aims at analysing the relationship of solar global irradiation and sunshine duration at three stations in Extremadura (Spain) at a daily and monthly basis. Studying this dependence is of great interest since it allows to estimate solar irradiation in many stations where sunshine duration is measured and then, extend the number of locations with data, in order to plot reliable solar radiation spatial distributed maps. The mentioned relationship is investigated at both daily and monthly basis, by fitting the Ångström-Prescott model by regression techniques. The correlation coefficients show notably high values for the three locations, suggesting the suitability of the model for the measured data. Moreover, the regression coefficients are in agreement with those obtained in other works for different locations in the Iberian Peninsula. In the daily analysis, it is also found that residuals show a smooth annual behaviour and, therefore, Ångström-Prescott model was fitted for each calendar month separately. The annual evolution of the regression coefficients and the atmospheric transparency index is analysed and compared for the three stations of measurements.