Marine Generator Size Calculator 2027

Choosing the right marine generator is essential for reliable onboard power, comfort and independence when cruising. For yacht owners with vessels between 30ft and 70ft, selecting a generator typically involves balancing electrical demand, available installation space, fuel efficiency and operating requirements.

Whether you own a catamaran or monohull, the correct marine diesel generator will depend on three key considerations.

1. What appliances do you need to run?

Your marine generator must comfortably support the appliances you intend to operate simultaneously.

Common onboard electrical loads include:

• Air conditioning and refrigeration.
• Battery chargers and inverter systems.
• Watermakers and freshwater pumps.
• Microwave ovens and galley appliances.
• Washing machines and entertainment systems.
• Lighting and navigation electronics.

Remember that air-conditioning compressors and electric motors can require considerably more power during startup. Always calculate starting loads as well as normal running consumption.

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2. What type and size of boat

Boat length is a useful starting point, but vessel layout and electrical equipment are equally important.

Larger catamarans, including Leopard, Lagoon, Fountaine Pajot, Bali, Excess and Seawind models, often have multiple cabins, larger accommodation areas and additional onboard electrical requirements.

Monohulls, including Beneteau yachts, can have different generator requirements depending on their configuration and intended use.

For yachts between 30ft and 70ft, marine generators in the 3kW–10kW range can provide a suitable starting point for assessing requirements.

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3. Diesel or Petrol Marine Generator

For larger cruising yachts, diesel marine generators are generally the preferred option.

Diesel generator advantages:

• Better fuel efficiency under typical operating loads.
• Lower fuel volatility than petrol.
• Convenient integration with existing diesel fuel systems.
• Well suited to extended onboard operation.
• Commonly available as purpose-built, enclosed marine generator sets.

Petrol generator advantages:

• Can suit smaller petrol-powered vessels.
• May be appropriate where petrol is already the vessel's primary fuel.

For permanently installed generators aboard larger yachts, diesel is generally the more practical choice.

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Marine Generator Sizing Calculator – 30ft to 60ft Boats

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These are preliminary guidelines, not fixed recommendations. Actual generator sizing must be based on calculated running loads and starting surges.

Making the right choice in 2027

Avoid assuming that a larger generator is always better. Marine diesel generators running consistently under light loads can experience carbon build-up, cylinder glazing and wet stacking. Ideally, your typical operating load should sit around the middle of the generator's rated capacity.

Also consider installation dimensions, ventilation, raw-water cooling, exhaust routing, noise levels and electrical compatibility. Australian yachts generally require 230V/50Hz generators, so check compatibility before purchasing.

The best marine generator is one correctly matched to your yacht, appliances and cruising lifestyle.

Need help selecting a marine diesel generator? A professional electrical load assessment can help determine the correct generator capacity for your vessel.

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Frequently Asked Questions

What uses the most electricity on a boat?

Air Conditioning (HVAC): High BTU units require massive amounts of power and are often the single largest load when running, making them difficult to run on battery power alone.

Refrigeration & Freezers: Fridges, freezers, and ice makers run continuously (sometimes around the clock) to keep food cold, making them the biggest drain over a 24-hour period.