Marine watermakers should provide ample fresh water to all passengers onboard on a daily basis. As a guideline, the U.S. Coast Guard recommends that 30 gallons of water per person per day should be available onboard. So, for example, if you are planning for 4 people onboard, then you should plan on having 120 gallons of water available per day.
Next, determine how many hours per day you want to operate the watermaker to make the capacity that you need. Most people are comfortable with between two and five hours per day. Then, compute how many gallons of water you need to make per hour to make your daily demand considering the period of time you would want to operate the watermaker. Considering the example above, if a person wanted to make 120 gallons of water in a two-hour time period, then a watermaker capable of producing 60 gallons of water per hour is required.
Marine watermakers are rated based upon how many gallons of water they are capable of producing per hour. This information is also stated in terms of daily production (that is, the rated hourly production multiplied by 24). Marine water makers are typically available in ranges of production from eight to 344 gallons per hour (that is 200 to 8,300 gallons per day). Larger commercial-sized systems are typically available in outputs up to 1,750 gallons per hour (42,000 gallons per day). Unlike competitors, FCI units are engineered to deliver the advertised output under varying conditions.