Step one: your real daily litres
Multiply crew by litres per person per day. The second number is where honesty pays off, because it varies by a factor of three or more depending on how you live aboard.
| How you cruise | Litres per person per day | Four-person total |
|---|---|---|
| Frugal passage-making: drinking, cooking, saltwater deck wash | 10–15 L | 40–60 L |
| Normal cruising: short showers, dishes, occasional rinse | 20–30 L | 80–120 L |
| Liveaboard comfort: daily showers, washing machine, deck rinsing | 35–60 L | 140–240 L |
If you have been cruising with tanks, you already know your number: divide tank capacity by how many days it lasts.
Step two: turn litres into run time
Divide your daily litres by a candidate system's rated output. A hundred litres from a 30 L/hr system is about three and a half hours; from a 140 L/hr system it is under an hour.
Under two hours a day is comfortable and easy to fit around an engine run or a sunny afternoon. Over six hours is a system you will resent, and one that ties you to the boat. Somewhere in between is the target for most cruisers.
Step three: check it against your power
This is the step that gets skipped, and it is the one that decides whether the system gets used. Multiply your daily litres by the system's watt-hours per litre. That is the energy it removes from your day.
Then compare it with what you can genuinely spare after the fridge, nav, lights and autopilot have taken their share. A 400 W panel in Queensland gives roughly 1,600 Wh on a clear day; a cloudy week gives a fraction of that. If the watermaker needs more than your surplus, it will only run when the engine runs, which may be exactly what you intended — or a nasty surprise.
| System | Power | Output L/hr | Draw W | Wh per litre | Price |
|---|---|---|---|---|---|
| Zen 30 Schenker | 12/24V DC | 30 | 110 | 3.7 | $6,300 |
| Ventura 150 Deluxe Spectra | 12/24V DC | 24 | 108 | 4.5 | $6,000 |
| Ventura 150 Connect Spectra | 12/24V DC | 24 | 108 | 4.5 | $7,400 |
| Scout SW20 LEDIreported | 12V DC | 20 | 200 | 10.0 | on request |
| Portable 12VDC 34L Rainman | 12V DC | 26–37 | 400 | 12.7 | $4,898 |
| Modular 12VDC 34L Rainman | 12V DC | 26–37 | 400 | 12.7 | $5,053 |
| Ventura 200 Tropical Spectra · spec unconfirmed | 12/24V DC | 31 | — | not published | $8,200 |
Under 50 L/hr covers most couples and small crews. The energy-recovery systems here make 100 litres for a few hundred watt-hours.
Step four: the practical constraints
- Space. Modular systems split the pump, filters and membranes so they can be tucked around a machinery space. Framed units are quicker to install and service if you have the room in one place.
- Power source. A DC system runs off the bank; an AC system needs a generator, shore power or a large inverter; petrol needs fuel and ventilation.
- Access. Prefilters get changed often. If they end up behind permanent joinery, they will not get changed often enough.
- Use pattern. Watermakers reward regular running. A system too big for your needs sits idle and needs pickling more often than one sized to run most days.
A worked example
Two adults and two children, cruising the Queensland coast, showering most days: call it 25 litres a head, so 100 litres a day. They have 600 Ah at 12 V and about 800 W of solar, of which maybe 1,200 Wh a day is genuinely spare after the fridge.
A conventional system at 12.7 Wh/L would need about 1,270 Wh a day — right at the edge of their surplus, and over it on a cloudy day. An energy-recovery system at 3.7 Wh/L needs about 370 Wh, which leaves room to spare and means the tanks fill whether or not the engine runs. On those numbers the efficient system is not a luxury; it is the one that works.