## 4 Conclusions  
The suitability of two different electrical energy storage options--Li-ion batteries and SCs--to improve the water quantity of a PV-membrane system was investigated and compared to a battery-less performance. The tests with/without energy storage were conducted under varied weather conditions using high-temporal-resolution (one-s) SI data. The addition of one day's worth of energy storage (2.4 kWh) Li-ion batteries enabled the full-power operation of the pump for 8-11 h over the three \"solar days\", which exhibited different levels of fluctuations in solar irradiance. Consequently, the fully charged batteries allowed a 15-80% increase in the permeate production and a 3-27% increase in the permeate quality. The average permeate quality with/without Li-ion batteries all fulfilled the WHO guidelines, which highlights the good system design and appropriate choice of membrane and PV array sizing. In particular, the effects of varied energy storage capacities on the PV-membrane system on the \"partly cloudy day\" were investigated. It was found that the improvement of water production occurred at an initial SOC higher than 50% (energy capacity of 1.2 kWh), while the lower initial SOC and, therefore, the low energy storage capacity caused a system shutdown after fully discharging due to repeated attempts of charging and discharging behaviors. Finally, the system performance comparisons on the \"partly cloudy day\" between the additions of Li-ion batteries and SCs were studied. The use of SCs for short-term energy buffering resulted in a 9% increase in water production and 13% improvement in the SEC. This was compared with Li-ion batteries for providing long-term power, which resulted in a 16% increase in water production and an 8% increase in the SEC. In summary, Li-ion batteries are an interesting energy storage option for PV-membrane systems, due to their high energy intensity, large number of charging/discharging cycles and their steadily decreasing costs. When considering long-term system operations for periods up to 20 years in remote regions, the option of oversizing the PV array and allowing for a directly coupled PV-membrane system potentially offers a more reliable solution. Further investigations on this sizing approach and the associated life cycle costs need to be carried out. Moreover, the option of combining SCs and Li-ion batteries should be examined, which would enable the short-term delivery of large amounts of power (buffering) and longer-term energy storage. This approach would require further research and development on a suitable energy management system to distribute the energy flows required. S.L. : conceptualization, methodology, software, formal analysis, investigation, data curation, writing--original draft preparation and visualization; A.P.S.G.d.C. : conceptualization, methodology, software, investigation and data curation; A.I.S. : conceptualization, methodology, validation, resources, writing--review and major editing, visualization, supervision, project administration and funding acquisition and B.S.R. : conceptualization, methodology, formal analysis, validation, investigation, resources, data curation, writing--review and major editing, visualization, supervision, project administration and funding acquisition. All authors have read and agreed to the published version of the manuscript. This research received funding for equipment from the Helmholtz Association. Not applicable. The data presented in this study are available on request from the corresponding author. The Chinese Scholarship Council (CSC) is acknowledged for the provision of a PhD scholarship for SYL. The Karlsruhe School of Photonics (KSOP), Science and Technology of Nanosystems (STN) and Helmholtz are thanked for sponsoring the equipment. [PERSON] (formerly from Project Competence E) is thanked for sharing the weather data from KIT Solar Park. DuPont Chemical Company is acknowledged for donating the UF membranes and RO modules.