C. Efficiency and Self-Discharge
FESSs and Li-ion batteries have similar round-trip power efficiencies if the self-discharge over the storage time is not taken into account. The evaluation of FESS for residential PV systems conducted in this paper shows an energy efficiency of approx. 40%. Compared to LA batteries (70% – 80%) and Li-ion batteries (85% – 95%) this is rather low. The difference in performance is mainly caused by the unequal selfdischarge rate of the storages. Chemical batteries have a selfdischarge of 0.1% – 0.3% per day, while FESSs show a rate of 20% – 100% per day. In residential PV installations the storage device works in daily cycles with a characteristic storage time of several hours causing a significant selfdischarge which drives down the energy efficiency.