This paper shows the results obtained from the simulated study case about the impact of harmonics on a distribution transformer useful life. These results depend on the supplied energy level (power variation along the time) under three different demand profiles with the same harmonics level. It will be presented that the transformer overload affects the transformer useful life subjected to the presence of peaks and valleys in the weekly demand profile, providing period times to the transformer for the temperature increase (peaks) or decrease (valleys). These temperature variations will affect directly the transformer loss of life and will show that not only the harmonic presence on the transformer operation will accelerate its aging, but also other conditions allow a slow aging and even a useful life beyond the rated expectation. The possible effects of more complex disturbing scenarios, like unbalanced currents and high reactive power feeding, are presented as discussion.