The objective of this work is the development of an analytical model for a hydraulic motor by using the hydraulic resistance concept. The developed model integrates the whole Hydro-Mechanic-Electric-Conversion (HMEC) line. Currently, the HMEC line represents an important technological challenge since it impacts strongly the overall efficiency of hydraulic energy conversion systems that operate under variable flow rate or pressure heads. Among these systems, we can mention wave energy converters, hydraulic systems as well as the innovative thermo-hydraulic CAPILI process based on liquid piston concept that has been recently developed in PROMES Lab. The paper describes the analytical model developed for the hydraulic motor, taking into account the main parameters of the HMEC conversion line. A small scale experimental bench of HMEC line operating under highly variable conditions has been implemented. The experimental results allowed the validation of the analytical model and the identification of the different characteristics of the HMEC line which takes into account the coupling efficiency and both volumetric and mechanical efficiencies of the hydraulic motor. Herein, the relevance of this hydraulic resistance based-model is highlighted, more specifically through the power output regulation systems and also for the optimisation of the thermo-hydraulic energy conversion systems based on liquid piston concept.