A method for controlling the voltage output of a power distribution system for powering a plurality of electrical loads of a transport conditioning unit, the transport conditioning unit comprising an internal combustion engine, an electric AC generator driven by the internal combustion engine, a compressor, a condenser, and a condenser fan associated with the condenser for providing air flow thereover, and a plurality of electrical loads (12', 15, 15', 14) comprising an electric compressor motor for driving the compressor and a first electric fan motor for driving the condenser fan, and a power distribution system comprising an AC-DC voltage converter electrically coupled as input to the AC generator and as a output to a DC power distribution bus configured to carry DC voltage, the DC power distribution bus being electrically coupled to at least one of the plurality of electrical loads, the method comprising: a) measuring an instantaneous DC voltage value, V DC (t), and an instantaneous direct current value, I DC (t), at the output of the AC-DC voltage converter; b) computing an instantaneous DC power value, P DC (t), based on the measured values I DC (t) and V DC (t); c) determining an engine horsepower value, P m t, of the internal combustion engine, the horsepower value, P m t, being associated with the instantaneous DC power value; d) computing a nominal running speed value of the internal combustion engine corresponding to the engine horsepower value, P m t, based on an equation or on a lookup table describing a numerical relationship between engine horsepower values and running speed values associated with the internal combustion engine; and e) running the engine at the nominal running speed value. a power distribution system comprising an AC-DC voltage converter electrically coupled as input to the AC generator and as a output to a DC power distribution bus configured to carry DC voltage, the DC power distribution bus being electrically coupled to at least one of the plurality of electrical loads, the method comprising: a) measuring an instantaneous DC voltage value, V DC (t), and an instantaneous direct current value, I DC (t), at the output of the AC-DC voltage converter; b) computing an instantaneous DC power value, P DC (t), based on the measured values I DC (t) and V DC (t); c) determining an engine horsepower value, P m t, of the internal combustion engine, the horsepower value, P m t, being associated with the instantaneous DC power value; d) computing a nominal running speed value of the internal combustion engine corresponding to the engine horsepower value, P m t, based on an equation or on a lookup table describing a numerical relationship between engine horsepower values and running speed values associated with the internal combustion engine; and e) running the engine at the nominal running speed value. a) measuring an instantaneous DC voltage value, V DC (t), and an instantaneous direct current value, I DC (t), at the output of the AC-DC voltage converter; b) computing an instantaneous DC power value, P DC (t), based on the measured values I DC (t) and V DC (t); c) determining an engine horsepower value, P m t, of the internal combustion engine, the horsepower value, P m t, being associated with the instantaneous DC power value; d) computing a nominal running speed value of the internal combustion engine corresponding to the engine horsepower value, P m t, based on an equation or on a lookup table describing a numerical relationship between engine horsepower values and running speed values associated with the internal combustion engine; and e) running the engine at the nominal running speed value.