The main idea of a practical method of solving probllems connected with osmotic-and-diffusive conversion of free energy of solutions with different concentration is presented in this work. We propose an application of the method to studies of the energetic efficiency of the one-mebrane osmotic motor, which is describbed by the Kedem-Katchalski formalism. Proper equations for useful power, disspated power, total power and the energetic efficiency have been derived for that osmotic motor. These equations make possible detailed analysis of energetic problems reffering to the work of the motor. They also enable us to perform its energetic optimalization just at the stage of designing.