In this paper, the characteristics of energy discharge in a flywheel energy storage system (FESS) are analyzed according to load condition mapped in the efficiency of a generator. As time goes by, the electromotive force
(EMF) of a generator decreases in an FESS as a power source, but output power is maintained by boosting the generator voltage in a bidirectional power converter (BPC). Using the principle of operation of a boost converter, the variation of voltage gain in a BPC is investigated under the condition of load change, and current is compensated for the reduction of back EMF in a generator as its speed goes down. The loss of the FESS is separated into that of its generator, bearings, and BPC through experimental evaluation, and it has been verified that the generator plays a significant role in the efficiency of the FESS.