For the case study a 5 kWp PV system with 3 kWh FESS in a one-family house is simulated. The results show that the self sufficiency of the studied PV system can be improved from 36.8% to 51.9% by a FESS. The self-consumption is improved from 30.2% to 61.4%. Further, the flywheel is 67% of the time fully charged or at standstill. The internal losses especially during long termidling at maximum speed result in an energy efficiency of 40%, which is rather low compared to lead-acid batteries (70% – 84%) and Li-ion batteries (85% – 95%)