What does Wh/L mean on a watermaker?

Watt-hours per litre: the system's rated electrical draw divided by its rated output. It tells you what your battery bank pays for each litre of fresh water. Manufacturers almost never publish it, which is why we calculate it for every system.

What is a Clark pump?

Spectra's pressure intensifier. The brine leaving the membrane is still at nearly full pressure, and a Clark pump uses it to drive a piston that pressurises the incoming feed, removing the need for a large high-pressure pump. Schenker's ERS does the same job differently.

What is a good TDS for watermaker product water?

Lower is better, and what matters more than any single figure is the trend over time on your own system. A rising reading at normal operating pressure suggests a membrane or seal problem; a high reading in the first minutes of a run is normal.

What is brine on a watermaker?

The concentrated seawater left after fresh water has passed through the membrane. On a conventional system it goes overboard still at close to full pressure, which wastes that energy. Energy-recovery systems route it back to help pressurise the incoming feed instead.

Reference

Watermaker glossary: the terms on every spec sheet, explained

Wh/L, Clark pump, ERS, TDS, brine, pickling. Plain definitions for the words dealers use without explaining them.

4 min read · checked 5 September 2026

Marine desalination has a vocabulary that is assumed rather than taught. Here is the short version, in the order the terms tend to come up.

The process

Term	What it means
Reverse osmosis (RO)	Forcing seawater through a semi-permeable membrane at high pressure so fresh water passes and salt does not. The core of every marine watermaker.
Membrane	The cylindrical element the separation happens in. Described by diameter and length in inches — 2.5 × 40 in, for example.
Feed	The seawater going in, and the low-pressure side of the system that delivers it.
Product water / permeate	The fresh water that comes out. Should be checked before it is sent to the tank.
Brine / concentrate / reject	The saltier water left over. What happens to its pressure is the single biggest efficiency question in the whole system.
Recovery	The proportion of feed water that becomes product water. Marine systems run at low recovery, which is why so much brine goes overboard.

The hardware

Term	What it means
Feed pump / lift pump	The low-pressure pump that draws seawater in and pushes it through the prefilters.
High-pressure pump	On a conventional system, the pump that raises feed pressure to roughly 55–60 bar. It is where almost all the power goes.
Prefilter	Sediment filtration protecting the pump and membrane. A consumable, and the first thing to check when output drops.
Energy recovery / pressure intensifier	A mechanism that uses the outgoing brine's pressure to help pressurise incoming feed, removing the need for a large high-pressure pump.
Clark pump	Spectra's implementation of a pressure intensifier: returning brine drives a piston that pressurises the feed.
ERS	Schenker's Energy Recovery System — the same principle in a different mechanical package.
Autoflush	An automatic fresh-water flush that runs on a timer or at shutdown, so the membrane is not left sitting in seawater.

The numbers

Term	What it means
L/hr	Rated output in litres per hour, normally quoted at around 25 °C seawater. Output falls in colder water.
Wh/L (watt-hours per litre)	Rated draw divided by rated output: what your battery bank pays for each litre. Almost never printed by manufacturers, and the spec this site exists to publish.
TDS	Total dissolved solids, in parts per million — the practical measure of how salty the product water is. A meter is the most useful diagnostic you can add.
Bar / psi	Pressure at the membrane. Marine RO needs roughly 55–60 bar (about 800–870 psi).
Salt rejection / salt passage	How much salt the membrane blocks, and how much slips through. Rejection falls if pressure is too low.

Operation and care

Term	What it means
Flushing	Displacing seawater with fresh, chlorine-free water so nothing biological grows while the system sits.
Pickling / preservation	Filling the system with a preservation chemical before a lay-up of weeks or more. Use only what the manufacturer specifies.
Fouling	Biological growth or scale reducing membrane performance. Often cleanable, and not the same as a failed membrane.
Commissioning	Bringing a newly installed system into service: pressure test, TDS check, and being shown the flush and pickle procedure on your own boat.

See these terms against real systems

Every figure in this glossary appears as a column you can sort in the comparison table.

Open the table

Common questions

What does Wh/L mean on a watermaker?: Watt-hours per litre: the system's rated electrical draw divided by its rated output. It tells you what your battery bank pays for each litre of fresh water. Manufacturers almost never publish it, which is why we calculate it for every system.
What is a Clark pump?: Spectra's pressure intensifier. The brine leaving the membrane is still at nearly full pressure, and a Clark pump uses it to drive a piston that pressurises the incoming feed, removing the need for a large high-pressure pump. Schenker's ERS does the same job differently.
What is a good TDS for watermaker product water?: Lower is better, and what matters more than any single figure is the trend over time on your own system. A rising reading at normal operating pressure suggests a membrane or seal problem; a high reading in the first minutes of a run is normal.
What is brine on a watermaker?: The concentrated seawater left after fresh water has passed through the membrane. On a conventional system it goes overboard still at close to full pressure, which wastes that energy. Energy-recovery systems route it back to help pressurise the incoming feed instead.