The invention relates to an electrical emergency source device fitted on an aircraft, which comprises an energy source capable of outputting the power necessary for vital electrical loads (10) to operate correctly, wherein there are two types of these loads: first loads, that absorb a constant power with time, second loads, that absorb variable power with time. Wherein this device comprises: a power source (12) sized to output the average power absorbed by all loads, a power source (15) capable of supplying power peaks absorbed by the second loads, and wherein, since the power of several loads is reversible, the power of the said device is also reversible.