This report presents a hybrid parabolic trough plant performance analysis, in order to estimate the backup fraction needed for a 100MW hybrid solar-fossil power plant at different locations, and obtain better locations where consumption of fossil fuels is reduced. A thermodynamical model is formulated in order to obtain the backup requirement. As only monthly means of solar radiation data are available, two different alternatives are used as input data for the model: the first uses the monthly average considering that all months are composed of identical days, and the second models an artificial month of different days in hourly basis for the same monthly mean. It is found that the second alternative is more accurate than using monthly means, at an increased computational cost. A comparison between the two models is performed, and a maximum 12% difference is found. The simpler monthly mean model predicts a higher back-up fraction than the artificial hourly data. This highlights the importance of obtaining accurate data for solar power plant dimensioning and decision making processes. Copyright © (2009) by the International Solar Energy Society.