Transformers for solar applications are different from normal transformers in two ways. There is the possibility of a considerable amount of harmonic content due to DC/AC conversion and there is a typical daily loading cycle that is dependent on the daily sunlight cycle. The daily sunlight can be typically described as a bell-curve. Both phenomena will have an impact on the temperature rise of the different transformer parts. Two tools have been developed to predict the impact of the harmonics and to take the cyclic daily load into account.