Anyone planning a long cruise knows well that fresh water is the real currency on board, even more than diesel. Before ordering a boat desalination unit, however, it's worth pausing for a moment and doing some serious calculations: how many liters the crew really needs, how many hours per day the system can be operated, and how much electrical energy this requires from the batteries or generator. These are preliminary calculations, they do not replace a precise technical evaluation, but they help understand the order of magnitude before talking to those who actually install the system.
Why calculate flow rate and consumption before choosing a marine desalination unit
A marine desalination unit is often chosen by looking only at the price or size, but the right question is another: how much water is needed per day and in how many hours must the system produce it? Underestimating this step leads to two equally costly opposite mistakes:
- Oversizing the system, with unnecessary investment and bulk; energy consumption must be compared on the individual model.
- Undersizing, ending up short of water halfway through the cruise or forced to keep the desalination unit running for too many hours, impacting batteries and generator.
The starting point is always the crew's water requirement, not the technical sheet of the preferred model.
Estimate of water requirement during a cruise
Every crew has different habits: some take short showers and others don't, some wash dishes sparingly and others fill the tank to wash the cockpit. For this reason, any number should be treated as a working hypothesis, to be adapted to the real case.
An educational example (not a universal average consumption)
Let's imagine a crew of 4 people estimating a consumption of 25 liters per person per day for personal use, plus 20 liters per day for common uses such as cooking and cleaning. The calculation is simple:
- 4 people × 25 liters = 100 liters/day
- + 20 liters/day for common uses
- Estimated total: 120 liters/day
This number is not a fixed rule: it is a useful example to understand the method, not a value to be automatically applied to any boat or crew.
From daily flow rate to required hourly flow rate
Here comes into play a factor often overlooked: how many hours per day one is willing (or able) to keep the desalination unit running. The declared production over 24 continuous hours must be related to the expected hours of use and the energy available on board. Operating time depends on the system and crew habits: continuous operation should not be taken for granted.
Returning to the example of 120 liters/day:
- If the desalination unit can operate 2 hours per day, a theoretical minimum flow rate of 60 liters/hour.
- If the available time drops to 1 hour per day, the theoretical required flow rate increases to 120 liters/hour.
This means that the same water requirement can demand very different systems, depending on how many hours the owner actually intends to dedicate to water production. It is an operational margin that must always be agreed upon with the technician, along with the minimum reserve to be maintained in the tank as a safety buffer.
Electric consumption: what to consider before sizing the system
Here too, the same cautious logic applies: no generic number can replace a check on the actual consumption of the chosen model. A purely hypothetical example, not related to HP data, helps to understand the calculation logic.
Hypothetical energy example
Suppose a consumption of 500 W for 2 hours of operation:
- 500 W × 2 h = 1 kWh of energy consumed
- At 12 V, this theoretically corresponds to about 83.3 Ah, before considering conversion losses and the real constraints of the battery
This value must always be verified with the actual consumption of the model, taking into account:
- Starting current at startup, often higher than the operating regime
- Inverter efficiency, if the system does not operate directly at low voltage
- Other simultaneous loads on board (fridge, pumps, navigation electronics) that share the same power source
This is not a DIY wiring matter: the verification of consumption, available network voltage, and compatibility with the existing electrical system must be done with a competent technician.
Summary table of hypothetical examples
| Hypothetical scenario | Requirement | Available time | Minimum theoretical flow rate | Energy |
|---|---|---|---|---|
| 4 people and common uses | 120 L/day | 2 h/day | 60 L/h | 1 kWh assuming 500 W for 2 h |
| Same demand, less time | 120 L/day | 1 h/day | 120 L/h | To be calculated with the consumption of the chosen model |
The energy values are purely illustrative and do not represent consumption data of specific models.
The HP SC range and reference flow rates
To have a real point of comparison, the HP SC range from HP Watermakers covers a declared production range between 70 and 440 liters/hour, with power supplies available in AC 110/230/380/440 V depending on the model and configuration. In the official datasheet of the HP SC range appear, among others, the 140 L/h model, the 260 L/h model, and the 330 L/h model.
A concrete reference is the HP SC140/1 desalinator, designed for flow rates on the order of 140 liters/hour. However, it is important not to confuse three distinct concepts:
- Commercial flow rate indicated in the catalog
- Nominal flow rate under standard test conditions
- Actual performance in the system, influenced by water temperature, salinity, filter condition, and installation configuration
Compatibility with systems like RP Tronic and the specific configuration must always be confirmed for each product code, not generalized across the entire range.
Maintenance, filters, and water treatment on board
A desalination unit that is well sized on paper loses effectiveness if filters and membranes do not receive proper maintenance. The Official Spectra Watermakers FAQs remind how important practical access to filters is for inspections and replacement as specified in the manual, as well as checking the specific manual for your model: preservation procedures and chemicals to use are manufacturer-specific and are not interchangeable between different brands.
For this reason, those planning a long cruise should consider the desalination unit not as an isolated component, but as part of a broader system of water treatment on board, which includes filtration, possible post-treatment, and management of the quality of embarked or produced water.
For an overview of the systems and their role on board, you can read the guide to desalination units and water purifiers.
Checklist of data to provide to Technofluid for a proper evaluation
Before requesting a quote or a technical comparison, it can be useful to gather some essential information. A serious evaluation always starts from concrete data, not generic assumptions:
- Number of people on board and average cruise duration
- Measured or estimated water consumption, even if approximate
- Hours available per day for the operation of the desalination unit
- Voltage and type of electrical network available on board (battery, inverter, generator)
- Space and accessibility for the installation of the system and filters
- Tank capacity existing and desired reserve margin
- Model of desalination unit already present, if it is a replacement or an upgrade
These elements allow for a more precise technical comparison, without unverified commercial promises, and are often the same set of information required to properly evaluate also the prices of a marine desalination unit and the options available on the market.
Frequently Asked Questions
How many liters of water does a boat desalination unit produce on average?
It entirely depends on the model and the chosen configuration: the HP SC range, for example, covers a declared range between 70 and 440 liters/hour. There is no average value valid for all boats: the flow rate must be chosen based on the actual needs of the crew and the available operating hours.
How is the required hourly flow rate calculated for cruising?
Start from the estimated daily requirement (liters per person plus common uses), then divide by the actual hours the desalination unit is expected to run, obtaining the minimum required hourly flow rate. The theoretical production over 24 continuous hours does not correspond to the autonomy actually available on board.
Can the electrical consumption of the marine desalination unit strain the onboard system?
Yes, if not checked beforehand. Actual consumption, starting current, efficiency of any inverter, and other simultaneous loads must be evaluated together with a technician, because the theoretical calculation in Ah is only a starting point, not a definitive value.
How often should maintenance be done on a boat desalination unit?
The frequency depends on the model, usage, and conditions of the treated water. It is essential to follow the specific manufacturer's manual for filters, membranes, and preservation products, which are not interchangeable between different brands.
Is it advisable to rely on a technician to size the desalination unit or can it be done independently?
A preliminary estimate like the one described in this article is useful for guidance, but the final sizing — especially for the electrical part and integration with the existing system — always requires a specific technical evaluation of the real case.
Is it useful to allow a margin compared to the calculated requirement?
Yes. The theoretical result does not consider consumption peaks, variations in actual flow rate, maintenance, or periods when the desalination unit cannot operate. The margin should be defined together with the technician, also considering the tank capacity and the desired water reserve.
If you have already defined crew, available hours, and type of electrical network, contact Technofluidwe can compare this data with the available configurations and identify the most suitable solution for your cruise.
