It depends on how much water you think you will use each day and where you are going to locate the RO. Our boat has a 12 volt Katadyn 80 about 5g/hr. The fresh water heads use a lot of water. We have to run the water maker about 4-6 hours a day to keep up with demand including showering. This adds to the amount of amps we need to generate to replenish the batteries. However; we live in remote areas where fresh water is not readily available so RO is essential. If I was going to do it again, I think I would go with a higher output system that ran off 120 volts. We have friends with 120 volt systems that make 40g/hr. That being said we are able to maintain our batteries with 780 watts of solar plus wind and produce enough water for our needs