The simple answer is “bigger than you think”. General recommendation: 5-20 gallons per day per person. In reality, it depends on how many people are going to be on the boat, where you plan to go and for how long, and what the weather’s going to be like. Most of our customers find that the ease of use, the quietness, and the convenience of our systems gives them the security to drink more water, take more showers, wash their decks down with fresh water instead of salt; all in all, they consume a lot more water than they ever dreamed they would have. The most common mistake when purchasing a watermaker is to underestimate your needs, so our best advice is to be realistic with how much water you will use, and when in doubt buy a larger capacity system. Remember, watermakers are rated by how much water they will produce in 24 hours of operation. Typically customers will only operate their watermakers for 3 - 4 hours at a time.