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→Right Sizing the Capacity
The small unit would cost a lot less, too. So, the immediate temptation is to go with a smaller unit running 24 hours/day. But most watermakers are not rated for continuous duty, and even if they are, running any equipment continuously will wear it out sooner. Plus there is the factor of noise.
At the other extreme, very large watermakers are not available in DC models and require AC to keep the amperage and wire size manageable. but But on a boat AC is not as efficient as DC because DC electricity from the house bank has to be converted to AC using an inverter. The inverter also has to be larger, and more expensive, if we run the watermaker on AC.
Alternatively, a large watermaker could be run directly off the engine during its daily run to charge the batteries. The drive for the watermaker would have to be clutched, to disengage it on long engine runs.
Somewhere between these two extremes is probably best, but exactly where? A lower duty cycle increases longevity; while we would expect larger capacity gives greater efficiency, but this doesn’t hold true (see Table below). So within reason bigger is better.
For boats other than superyachts, the optimum choice is a large DC watermaker running for several hours a day, rather than an AC model running for a shorter time but requiring an over-sized inverter. The optimum, considering efficiency, backwashing, wear and tear, etc., could be running the a medium-size watermaker for two or three hours every three days.
This will probably necessitate designing the [[ElectricalCapacityDC|electrical system]] for 24 VDC; although two 12-VDC units could be put in parallel.