A method measures the electric power of a universal electric motor (4) connected to an electric power mains (6) by a Triac (7) and supplied by sinusoidal alternating voltage (V(t)). The method determines a first (ton) and a second (toff) instant at which the current (I(t)) supplied to the motor (4) passes from a zero value to a value other than zero. The current supplied to the motor (4) at a third instant between the first (ton) and second (toff) instant is measured. A fitting current time pattern (f(t)) approximating the real time pattern of the current (I(t)), and which has a zero current value at the first (ton) and second (toff) instant, and a current value equal to a sampled current at the third instant (tin), is determined. The mean electric power is calculated on the basis of the fitting current time pattern (f(t)).