Start with your real surplus
Not your array's rating — your surplus. A 400 W panel in Queensland yields roughly 1,600 Wh on a clear day, less in winter, much less under cloud. Out of that the fridge takes a large and non-negotiable share, then nav, lights, autopilot and charging.
What is left after all of that is your water budget. For many cruising boats with a modest array, the honest figure is somewhere between 500 and 1,500 Wh a day, and it is the number to design around.
The arithmetic
| System type | Energy per litre | Energy for 100 L | Roughly |
|---|---|---|---|
| Energy recovery, best in table | 3.7 Wh/L | 370 Wh | One 400 W panel, one clear morning |
| Energy recovery, typical | 4.5 Wh/L | 450 Wh | Comfortable on a modest array |
| Conventional, large membrane pack | 9.8 Wh/L | 980 Wh | Most of a 400 W panel's whole day |
| Conventional, small DC pump | 12.7 Wh/L | 1,270 Wh | Needs a second panel or the engine |
| Conventional, small membrane pack | 19.7 Wh/L | 1,970 Wh | Not realistic on solar alone |
Figures are your daily litres multiplied by each system's rated watt-hours per litre. A 400 W panel yields roughly 1,600 Wh on a clear Queensland day.
What fits a solar-only boat
In practice this means energy-recovery DC systems, which reuse the brine's pressure instead of dumping it overboard. Every system in our table under 6 watt-hours per litre works this way.
| System | Power | Output L/hr | Draw W | Wh per litre | Price |
|---|---|---|---|---|---|
| Zen 30 Schenker | 12/24V DC | 30 | 110 | 3.7 | $6,300 |
| Zen 100 Schenker | 12/24/48V DC or AC | 100 | 400 | 4.0 | $9,600 |
| 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 |
| Newport 400C Spectra | 12/24/48V DC | 64 | 288 | 4.5 | $14,500 |
| Zen 50 Schenker | 12/24V DC | 50 | 240 | 4.8 | $7,800 |
They cost more than a conventional unit with twice the output. That premium is the point: you are buying watt-hours, and on a solar-only boat the alternative is buying panels and battery instead, which is rarely cheaper and always heavier.
Three things that make solar water work better
- Run in the middle of the day. Make water while the panels are producing rather than out of the battery at night. It halves the effective cost because you avoid the round-trip losses of storing it first.
- Run most days, not occasionally. Short daily runs suit membranes better than long weekly ones, and they fit inside a daily surplus rather than needing a reserve.
- Size tanks as the buffer. Tankage is cheap and passive. It lets you skip a cloudy day entirely instead of oversizing the watermaker for your worst case.
When solar-only does not work
If your demand is genuinely high — a large crew, a washing machine, daily long showers — the numbers stop working regardless of efficiency, and you are looking at a generator or a much larger array. Be honest about consumption before you spend on a premium system; an efficient watermaker cannot fix an unrealistic water budget.
Equally, if your engine already runs daily for other reasons, the efficiency premium buys you something you were getting for free. That is the case where a conventional system on the same engine hours is the better buy.