In this paper a comparison between two methods for the calculation of the hot spot temperature of a power transformer is presented. The first method is proposed by C57.91 IEEE Standard which is based on daily averages and exponential equations; on the other hand, the second method is based in differential equations and hourly measurements. To compare these methods, the ageing of a currently operating power transformer has been estimated. A comparison of the results of degradation using actual values and a hypothetical overload were performed. A brief description of each method, aging results, time calculation and analysis of the results is presented.