Flywheel Energy Storage Systems (FESS) present an environment-friendly solution for storing and utilizing solar energy; however, voltage and current frequent fluctuations in solar module photovoltaic (PV) systems limit the efficiency of the FESS bidirectional converter. The converter’s function is to regulate and maintain the harnessed solar energy input to the FESS through the converter’s duty cycle. In this paper, a method is proposed to select the duty cycle of the bidirectional converter for efficient energy storage of the solar module PV system. The solar module PV system is first modeled and then validated through field-testing to determine the theoretical and experimental maximum power point (MPP) voltage and current output. The solar photovoltaic module model has an average percent difference of 12% compared to the field-testing results. The FESS and the bidirectional converter are separately modeled and then integrated to determine the FESS power output, efficiency, and flywheel rotational speed. The resulting FESS efficiency is 80% which is 14% higher than the efficiencies of FESS for off-grid PV applications available in the literature. The developed duty cycle selection of the bidirectional converter will aid in the development of commercial FESS for off-grid systems.