A method and system limiting specific consumption of an aircraft by matching sizing of a power supply to actual power needs of a cabin pressure control system. The method optimizes overall efficiency of energy supplied onboard an aircraft including, in an environment near the cabin, at least one main power-generating engine, sized to serve as a single pneumatic energy-generating source for the cabin and as an at most partial propulsive, hydraulic, and/or electric energy-generating source for the rest of the aircraft. The method minimizes power differential between a nominal point of the power sources when the sources are operating, and a sizing point of non-propulsive energy contributions of the sources when the main engine has failed, by equally dividing power contributions of the main engines and the main power generator under nominal operating conditions and in an event of failure of a main engine.