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How to choose the right marine generator for your boat

For many boaters, a marine generator is the ultimate tool for convenience and independence. Choosing the right genset for your boat can be a challenging decision. I will do my best to make your decision-making process less of an ordeal.

Fuel Types

The first choice to be made when selecting a marine generator is the fuel type. Gasoline and diesel fuel are the main fuels used to power marine gensets. Most boaters choose a generator that can burn the same fuel that the main propulsion engine uses, which eliminates the need for additional tanks (although a separate tank can be an advantage).


Many boats up to about 40’ long are equipped with gasoline engines for propulsion. For safety, all electrical components in the engine compartment of a gas-powered boat, or any compartment that contains gasoline, propane (LPG) or CNG tanks or connection fittings, are required to be Ignition Protected. This means that every switch, motor, relay, alternator, circuit board, pump, lamp, etc. has to be tested and certifed to industry standards (SAE J1171 or UL 1500) that it will not ignite a fuel/air mixture (gasoline, LPG [propane], or CNG). For this reason, if the generator will be installed in an engine space or a compartment with gasoline/LPG/CNG tanks or connection fittings, the genset must be certified as Ignition Protected regardless of the fuel the generator uses.

Gas generators have the advantage of costing less than a diesel generator of the same electrical power output. This is because gas engines tend to be much less complicated and cheaper to build than diesel engines.

NOTE: It is not recommended to install a gasoline generator aboard a diesel-powered boat. Boats that are built with diesel engines are not required to be equipped with ignition protected electrical components.

Diesel Fuel

Diesel gensets are very popular because they are more fuel-efficient than equivalent-sized gasoline generators. In addition, a good quality diesel engine will last many times longer than even the best gasoline engine. Many diesel generators are ignition protected, which means they can be installed on any boat.

If you choose to install a diesel generator on a gas-powered boat, make sure you verify that it is Ignition Protected. You will also need to properly install a diesel fuel tank. Make sure the fuel filler fittings are clearly marked for both the diesel tank and the gasoline tank. Mixing fuels can cause a very expensive repair bill!

Output Capacity

The main factor in determining the output capacity of your marine generator is the total electrical load you plan to use. You probably don’t need a generator that can simultaneously power every system on your vessel. Look at your AC electrical distribution panel, and decide which loads you plan to operate with your generator. Add up the wattage for all the resistive and inductive loads that you plan to use simultaneously on board your boat. You should plan to install a generator with at least 20% more continuous output capacity than your total expected power demands.

Resistive Loads

Resistive loads are electrical appliances such as toasters, coffee makers, water heaters, incandescent lamps, and other 120/240 volt appliances that don't have motors or transformers. These loads are very predictable and steady, with negligible startup load spikes. Simply add up the wattages for all the simultaneous resistive loads. If you don't know the wattage for a device, multiply the device's current draw by the voltage (120 or 240).

Inductive Loads

Inductive loads are electrical appliances such as battery chargers, air conditioners, refrigerators, washing machines, and other 110/220 volt appliances that have motors or transformers. These devices can have significant startup current spikes. These spikes are not likely to occur all at once, unless they are all turned on when you switch to generator power. Add up the steady-state current draw for each appliance you plan to run at the same time, then add the peak current draw for the appliance with the highest listed peak current. Multiply the total expected current draw by the appropriate voltage (120 or 240) to get the total watts. If your appliances run on different voltages (for example: battery charger uses 120 V, while your air conditioner runs on 240 V) multiply current draw for all the 120 volt appliances by 120, and the current draw for all the 240 volt appliances by 240, then add them together for the total wattage.

Happy cruising!
Related sites: Determine your optimum house battery bank size. How to select the appropriate Marine Alternator How to get water from the sea! Choosing the right DC-to-AC power inverter is critical to your happy cruising How to keep up with the marine weather forecast the easy way.