A practical method to calculate daily amp-hours, size batteries and solar, and build a Privilège catamaran around the owner’s real life at sea.
Calculating power consumption on a sailing boat starts with a simple rule: electrical autonomy must be measured in energy, not merely in amps. Every refrigerator, autopilot, watermaker, communication system and air-conditioning unit has a power rating and an operating time. Multiply the two, correct for duty cycle and conversion losses, and the yacht’s daily energy requirement emerges.
A refrigerator may draw 3 to 9 amps while its compressor is running, yet consume far less over 24 hours because it cycles on and off. Air conditioning is different. A single marine unit can consume more energy overnight than the refrigerator, navigation electronics and lighting combined.
For Privilège Marine, the opportunity lies in designing the entire energy architecture around the owner. The Signature 510, 580, 600 and 650 do not need identical batteries, solar arrays or generators. A couple crossing oceans, a family working remotely and an owner expecting silent tropical air conditioning require fundamentally different yachts.
The Amp-Hour Figure Is Useful but Often Misleading
Owners frequently ask how to calculate power consumption on a sailing boat. The calculation itself is straightforward. The difficulty lies in using the correct units.
An amp is a measure of electrical current. It does not describe total energy consumption unless voltage and time are also known. Ten amps at 12 volts represents 120 watts. Ten amps at 24 volts represents 240 watts. The current is identical, but the power has doubled.
The basic formula is:
power equals voltage multiplied by current
A 24-volt pump drawing 5 amps therefore consumes 120 watts.
Energy adds time to the calculation:
daily energy equals power multiplied by time
If that pump operates for two hours, it consumes 240 watt-hours, or 0.24 kilowatt-hours.
This is why Use watt-hours, not amps alone is the safest rule for a catamaran energy audit. Watt-hours allow an owner to compare a 24-volt refrigerator, a 230-volt induction hob and a 48-volt battery system without confusing current with energy.
Daily amp hours cruising remain useful, but only after the system voltage has been defined. A yacht consuming 12 kilowatt-hours a day requires about 500 amp-hours at 24 volts. The same energy represents 250 amp-hours at 48 volts.
The yacht has not become twice as efficient. Its electrical architecture simply operates at a higher voltage.
The Energy Audit Must Separate Rating From Real Consumption
Every appliance should be entered into an energy audit with four values:
- Its rated power
- Its operating time
- Its duty cycle
- Its electrical supply, whether direct current or alternating current
A 100-watt appliance running continuously consumes 2.4 kilowatt-hours per day. A 100-watt compressor operating 40 per cent of the time consumes 0.96 kilowatt-hours.
The distinction is essential. Refrigerators, freezers, water pumps and air-conditioning compressors cycle. Navigation lights and communication equipment may run continuously. Induction hobs and hairdryers draw substantial power, but only for short periods.
A proper boat power consumption calculator should therefore use the following equation:
Daily consumption equals rated watts multiplied by operating hours multiplied by duty cycle.
Alternating-current appliances introduce another correction. Energy taken from the batteries passes through an inverter. If an appliance requires 1,000 watts and the inverter operates at 90 per cent efficiency, the batteries must supply roughly 1,111 watts.
Those inverter losses become significant on yachts with electric cooking, domestic appliances and extensive entertainment systems.
Standby consumption also matters. An inverter, network switch, television, charger or satellite terminal may appear insignificant on its own. Ten devices averaging only 10 watts each create a permanent 100-watt load. Over 24 hours, that is another 2.4 kilowatt-hours.
Small loads become large when they never stop.
The Refrigerator Draws Amps Only When the Compressor Runs
The question “how many amps does a fridge use on a boat?” has no single answer.
Modern marine refrigerators commonly use variable-speed direct-current compressors. Secop’s BD35F and BD50F families, widely used in mobile refrigeration, show maximum-speed power inputs ranging broadly from about 35 to 104 watts, depending on compressor model, refrigerant and operating temperature.
At 12 volts, that represents approximately 3 to 9 amps while the compressor is running. At 24 volts, it represents roughly 1.5 to 4.5 amps.
The daily result depends on insulation, ventilation, ambient temperature, door openings, thermostat setting and the quantity of warm food placed inside.
Consider a 70-watt compressor running 40 per cent of the day. It consumes:
70 watts multiplied by 24 hours multiplied by 0.40, or 672 watt-hours per day.
That equals 56 amp-hours at 12 volts or 28 amp-hours at 24 volts.
If tropical heat, poor ventilation or frequent use pushes the duty cycle to 60 per cent, consumption rises to just over 1 kilowatt-hour. The same refrigerator now needs about 84 amp-hours at 12 volts.
A large cruising catamaran may carry a refrigerator, freezer, cockpit drinks fridge, wine cabinet and ice maker. Refrigeration can then require between 2 and 5 kilowatt-hours per day.
The equipment choice matters. So does installation. Indel Webasto Marine states that efficient compressor ventilation is central to performance. Its Smart Energy Control can reduce consumption substantially by adapting compressor speed and using the thermal mass of food and drinks.
A premium refrigerator fitted in a badly ventilated cabinet is not an efficient system. It is simply an expensive appliance working too hard.
The Air-Conditioning System Changes the Entire Calculation
Refrigeration attracts attention because it operates continuously. In reality, The air-conditioning load dominates most luxury-catamaran energy budgets.
A Dometic MCS T16 marine unit provides approximately 4.69 kilowatts of cooling capacity (16,000 BTU per hour) while consuming about 1.32 kilowatts in cooling mode.
If one unit runs for eight hours at a 50 per cent duty cycle, it consumes roughly 5.3 kilowatt-hours. Two equivalent zones consume more than 10 kilowatt-hours.
That is before refrigeration, cooking, navigation, communications, water production or battery losses are considered.
Whole-yacht air conditioning on a large catamaran can therefore add 20 to 40 kilowatt-hours to the daily audit in hot and humid conditions. Large windows, open doors, inadequate shading and low thermostat settings increase the load further.
This is where simplistic battery calculations fail. A 20-kilowatt-hour lithium bank may support normal domestic life comfortably. It will not provide unlimited silent air conditioning throughout a tropical night while preserving a responsible reserve.
The solution is not automatically a larger generator. Better insulation, zoned cooling, variable-speed equipment, external shading and intelligent operating schedules can reduce demand before more generation is added.
Cooling the owner’s suite at night is one requirement. Maintaining hotel temperatures in every cabin, saloon and circulation area is another.
The owner must decide which experience is expected.
The Watermaker and Communications Loads Are More Predictable
Water production is easier to calculate because consumption can be expressed per litre.
Schenker states that its energy-recovery watermakers typically require about 4 watt-hours per litre of fresh water. Its Modular 100 produces 100 litres per hour while consuming 400 watts.
Producing 200 litres therefore requires about 0.8 kilowatt-hours. At 24 volts, that is roughly 33 amp-hours before minor system losses.
The watermaker is rarely the largest onboard consumer. It becomes inefficient when the system is oversized, poorly maintained or operated in short cycles that require repeated flushing.
Communications can consume more.
Starlink Standard equipment typically averages about 75 to 100 watts. Used continuously, it may consume 1.8 to 2.4 kilowatt-hours per day. At 24 volts, that represents approximately 75 to 100 amp-hours.
Starlink Mini typically draws considerably less. For an owner who needs email, weather files and occasional video calls rather than permanent high-capacity connectivity, the difference can save more energy than replacing every light on the yacht.
This is an example of why An audit must model behaviour rather than simply catalogue equipment.
The Passage Profile Is Different From Life at Anchor
A catamaran energy audit should contain at least three operating scenarios.
The first is the marina profile. Shore power is available, batteries remain charged and high-power appliances can operate without meaningful autonomy constraints.
The second is the anchor profile. Refrigeration, water production, cooking, communications, lighting and domestic pumps dominate.
The third is the passage profile. The autopilot, instruments, radar, navigation displays, navigation lights and communications may operate for 24 hours.
Autopilot consumption is particularly difficult to estimate. Raymarine’s ACU-200 can provide up to 15 amps continuously to a drive unit, but maximum output is not average consumption. Calm conditions with balanced sails require little steering effort. Quartering seas and poor sail trim can make the drive work repeatedly.
A realistic passage audit must therefore include sea-state assumptions. It should also treat navigation, bilge pumps, emergency communications and essential lighting as protected loads.
Domestic comfort must never be allowed to exhaust the reserve needed for steering and safety.
The Privilège Range Requires Several Different Energy Strategies
Privilège Marine’s published range extends from the Signature 510 to the Signature 650. The Signature 600 is the natural evolution of the Signature 580.
The Signature 510 measures 17.09 metres (56 feet). The Signature 580 and Signature 600 measure around 19.10 metres and 18.28 metres respectively (62 feet 8 inches and 60 feet). The Signature 650 has a published hull length of 19.60 metres (64 feet 3 inches), with an overall dimension reaching 21.25 metres (approximately 70 feet).
Size alone does not determine consumption. Cabin configuration, crew numbers and lifestyle matter more. Nevertheless, larger yachts generally carry more refrigeration, lighting, pumps, electronics and climate-control zones.
The following figures are indicative engineering envelopes. They are not published standard Privilège specifications. Each yacht must be audited against its final equipment list and owner programme.
| Privilège model | At anchor without overnight air conditioning | On passage | Tropical comfort with zoned air conditioning |
|---|---|---|---|
| Signature 510 | 8 to 12 kWh per day | 11 to 16 kWh per day | 20 to 35 kWh per day |
| Signature 580 | 12 to 17 kWh per day | 15 to 21 kWh per day | 28 to 45 kWh per day |
| Signature 600 | 13 to 19 kWh per day | 16 to 23 kWh per day | 30 to 50 kWh per day |
| Signature 650 | 16 to 24 kWh per day | 20 to 28 kWh per day | 40 to 65 kWh per day |
The Signature 510 may be configured for a couple undertaking long-distance cruising. Its strongest energy strategy may prioritise low continuous consumption, direct-current refrigeration, efficient water production and enough solar generation to cover normal days at anchor.
The Signature 580 may support more guests, larger domestic systems and separate crew functions. Its audit should examine simultaneous loads, not merely daily totals. Running an induction hob, watermaker, washing machine and battery charger together may exceed inverter capacity even when the daily energy budget appears acceptable.
The Signature 600 creates a further variable. Its aft portside cabin can become an office, cinema, gym, studio or storage area. An office adds communications and computer loads. A cinema adds audiovisual equipment. A gym may introduce ventilation or powered machinery. Storage adds almost nothing.
The owner’s choice can change the energy architecture before a single battery is ordered.
The Signature 650 may operate closer to a small superyacht. Multiple refrigeration units, crew accommodation, extensive air conditioning and larger entertainment systems can justify a higher-voltage domestic network, substantial lithium storage and carefully coordinated generators, alternators and solar arrays.
The Battery Bank Must Be Sized Around Usable Energy
Battery labels show nominal capacity. Owners live with usable battery capacity.
A 25-kilowatt-hour battery bank should not be treated as 25 kilowatt-hours of unrestricted daily energy. A reserve is required for safety, battery longevity, unexpected weather and reduced charging.
A sensible design might use 70 to 80 per cent of nominal capacity for normal operation, depending on battery chemistry, manufacturer limits and the desired service life.
A yacht needing 12 kilowatt-hours between charging opportunities may therefore require 16 to 20 kilowatt-hours of nominal storage. A yacht requiring 25 kilowatt-hours overnight may need 35 kilowatt-hours or more.
Battery sizing should also consider peak power. A bank may hold enough energy for the day but still be unable to supply several air-conditioning compressors, an induction hob and a washing machine simultaneously.
Voltage matters here. Moving major hotel loads to 48 volts reduces current and cable size for the same power. Critical navigation equipment can remain on protected 12-volt or 24-volt circuits through properly engineered conversion and redundancy.
The architecture must comply with the relevant marine electrical standards, including ISO 13297 and, where appropriate, ABYC requirements for electrical systems and lithium-ion batteries.
The Solar Array Must Be Judged by Daily Yield
Solar panels are sold in watts. Owners need kilowatt-hours.
A 2-kilowatt array does not produce 2 kilowatts throughout the day. Output changes with latitude, season, temperature, shading, orientation, salt deposits and sail position.
A practical estimate multiplies array capacity by equivalent full-sun hours and then applies a system-efficiency factor.
A 2-kilowatt array receiving five equivalent sun hours with 75 per cent net system efficiency may produce:
2 multiplied by 5 multiplied by 0.75, or 7.5 kilowatt-hours per day.
A 4-kilowatt array under the same conditions may produce 15 kilowatt-hours.
Boom shadows, rigging and antennas can reduce production materially. Dividing the array across several maximum-power-point trackers can limit the effect of partial shading.
For a Privilège project, realistic solar targets must be developed around available surfaces, aesthetics, tender handling, boom clearance and weight. The objective is not to cover every horizontal surface. It is to obtain predictable production without compromising sailing function or the yacht’s visual balance.
The Privilège Advantage Is Not Simply a Larger Battery
A Privilège is not automatically more energy-efficient because of its name or price. A larger, more luxurious yacht may consume more than a simpler production catamaran.
The meaningful difference is that Privilège’s advantage is integration.
The shipyard can begin with the owner’s actual programme. It can ask whether the yacht will cross oceans, remain in the Mediterranean, spend months in the Caribbean, carry professional crew or operate primarily as a private home.
It can then coordinate refrigeration volume, air-conditioning zones, solar capacity, batteries, inverters, alternators, generator output, cable sizing, ventilation and monitoring as one system.
This matters because equipment is often added to production yachts in isolation. The owner selects a larger fridge, more air conditioning, lithium batteries and additional solar panels. Each option may be legitimate. The combined system may still be poorly balanced.
A bespoke Privilège can instead be designed around an energy objective.
One owner may request several days at anchor without generator use. Another may prioritise silent cooling in the owner’s suite. A third may need continuous Starlink, office equipment and video conferencing. A diving programme may add a compressor. A serious cook may demand induction, ovens and extensive refrigeration.
The owner’s programme comes first. The equipment should follow.
The Better Yacht Makes Its Energy Limits Visible
The final stage of the audit is monitoring.
A shunt-based battery monitor measures current entering and leaving the bank. Systems such as Victron SmartShunt can calculate state of charge, consumed amp-hours, historical discharge and estimated remaining time. Integrated monitoring can also display solar production, generator status, alternator charging and major loads.
Commissioning should compare the predicted audit with measured consumption. Refrigeration should be tested in realistic ambient temperatures. Air-conditioning duty cycles should be recorded. Solar yield should be measured with sails, boom and antennas in their normal positions.
A yacht that requires its generator every evening is not energy autonomous, whatever its brochure claims. A yacht carrying an enormous battery bank but lacking charging capacity is equally poorly designed.
The better system is not the one with the largest numbers. It is the one that delivers the owner’s expected life at sea, preserves safety reserves and makes every energy flow understandable.
For Privilège Marine, this is where customisation becomes engineering rather than decoration. The yard can build the quiet, connected, tropical or expedition yacht the owner actually intends to use. The honest calculation must come first.
