On paper, a watermaker might promise 20 LPH, 30 LPH, or even 60 LPH. But anyone who’s actually run a desalination system offshore knows those numbers don’t always match reality. Conditions change. Power availability changes. Seawater changes. And your watermaker’s actual output can swing by 10–30% depending on the day. This post breaks down why rated output is just a baseline – and what really determines how much water you can make at sea.
The Truth About Output Ratings
Manufacturers publish output ratings based on ideal conditions: stable voltage, optimal pump efficiency, clean membrane, and moderate seawater temperature. Think of them as “laboratory figures.” They’re still useful, but only if you understand how real-world variables influence them. In practice, your watermaker will generally produce a bit less than the rated maximum once you’re in open water with fluctuating power supply and warmer, saltier seawater.
Power: The Silent Output Killer
If there’s one variable that sabotages watermaker performance the most, it’s voltage drop. A 12V system that promises 180 watts needs a healthy battery bank and clean wiring to actually deliver that. A sagging battery won’t hit full pressure, and without full pressure the membrane can’t do its job efficiently. The result: lower flow rate, lower output.
This is why ultra-efficient, low-draw units have become so popular with cruisers and off-grid travellers. Less power demand = more stable performance, even when you're relying on solar or a modest house bank.
Salinity and Temperature: Two Factors You Can’t Control
Seawater isn’t constant. It varies by region, depth, and season.
• Warm water produces less — higher temperature means lower density, which reduces net driving pressure inside the membrane.
• Saltier water produces less — areas like the Red Sea, Coral Sea, or shallow lagoons can push total dissolved solids well above normal seawater. The saltier the feed water, the more pressure required to maintain the same output.
• Cold water produces more — the membrane loves dense, cool water. It’s why units often exceed their rating in southern latitudes.
Understanding these shifts helps you plan daily water production more realistically, especially on long passages.
Membrane Condition: The Long Game
A membrane doesn’t fail instantly; it slowly loses efficiency. Fouling, mineral scaling, or improper storage (like skipping freshwater flushes) gradually reduce output. When output drops 15–20% from your normal baseline, it’s usually a sign it’s time to clean or replace your membrane.
This is also why high-quality membranes matter. A good one will hold consistent output for years if treated properly.
Pump Efficiency and System Design
High-pressure pumps are the backbone of output. A worn pump, tired seals, or misadjusted pressure valve will quietly trim litres off your hourly production without you noticing at first. Efficient systems with low internal friction and smart plumbing layouts tend to outperform their published LPH numbers.
Portable units with direct-feed layouts (fewer bends, fewer fittings) often perform better in the real world than over-engineered fixed systems.
So How Much Water Can You Really Make?
Here’s the practical rule sailors follow:
Rated LPH × 0.8 = realistic cruising output
A 30 LPH unit? Expect 24–26 LPH. Some days you’ll get the full rating. Some days you’ll get a bit under. What matters is consistency – the ability to hit pressure, stay efficient, and keep making water without fuss.
And that’s where well-engineered, low-power desalination systems shine. They don’t rely on big generators or perfect conditions; they just quietly make water whenever you need it.
Want reliability without the guesswork? Explore LEDI’s ultra-efficient, solar-ready watermakers engineered for real-world offshore conditions, not just lab numbers.
At LEDI we provide output numbers that are actually real.
https://ledi.com.au/products/scout-sw20
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