The heart of the grid is the power transformer and it is associated to all the electrical chain for distribution, transmission and generation field. Usually the evaluation of the remaining life and condition is based on Furan analysis or dissolved gas analysis (DGA); however, both requires oil samples; in a zero downtime policy, the continuous evaluation requires an additional equipment as online DGA equipment. Due to this limitation, the alternative is the evaluation of the 'hot-spot temperature' (HST) in the transformers, with the sensors available in the oil, and winding; but an online evaluation has not been considered in the international standards as IEC 60076-7. So, this paper develops the mathematical model, algorithms and evaluates in a real case study, an optional method to obtain the HST and its impacts in the remaining life. This methodology is applied to three phase transformer of a 220 kV, 90MVA with YnD11 connection. It is demonstrates the hot-spot in a online platform and the IEC calculation with the accuracy and the restrictions.