How to Choose a Genset


An energy audit will tell you how much power you really need  (November 2012)

Nearly since its invention, the reciprocating piston engine has been used aboard almost all offshore-capable vessels as a double-duty workhorse, both powering the vessel via propeller and generating electricity to recharge the battery banks. It’s become common for sailors to run the diesel a few hours a day to “top off the battery bank.” For vessels with well-trimmed electrical loads, modern solar panels and wind generators are often sufficient, but if your boat needs more power than solar and wind can easily provide, there may be a more efficient way to keep those electricity-hungry devices functioning: a dedicated, high-efficiency diesel generator.

The advantages are fairly straightforward: diesel engines prefer to be run under load, and the alternator on most main engines is insufficient to load the engine for efficient running. As a result, both the engine’s longevity and fuel efficiency are sufficiently reduced, sometimes greatly. By running a dedicated engine with a much larger and more capable generator attached, far more power can be produced while keeping the engine properly loaded and at an efficient rpm. You save fuel and your batteries charge much faster, reducing the necessary running time (and associated noise and heat). This combination of generator and dedicated engine is referred to in common parlance as a “genset.”

Before you run out and buy a genset, realize that its installation will be every bit as complex and detailed as that of your main engine. It pays to do a little homework. The biggest question is one of size: how big of a genset do you need? Gensets are typically rated in maximum kilowatts produced, which is a measure of how much power they can generate. It’s important not to buy too big of a genset, because under normal loads it, too, will be inefficient on your vessel. But buying too small of a genset can be frustrating as you need to run it frequently and upgrading is both expensive and a serious hassle.

To properly size a genset to your boat, you need to know approximately how much power your boat will need. The process is not difficult, but it can be tedious. I’ll use some common examples to help clarify the tricky points. Don’t worry—if you can add, subtract, multiply and divide, that’s all the math you’ll need for this process. A common spreadsheet program such as Excel or Numbers can do all the hard work.

If you have an intelligent battery monitor, you may be able to determine how many kilowatt-hours you use in a given day just from the monitor itself. A watt-hour is simply the number of amp-hours you use in a day multiplied by your battery bank voltage. For instance, if you use 70 amp-hours a day and your battery bank is 12 volts, you’ve used 70×12=840 watt hours, or 0.84 kilowatt hours. It’s best to keep track of your usage for a month or two before making a determination, taking the average of the highest 20% of the readings as your typical value.

If you do not have an intelligent battery monitor, you can estimate the figure as follows:
1) Write down a summary of all electrical devices on your boat, along with their power rating—e.g. freshwater pump, 5 amps, steaming light, 12 watts, etc.
2) Convert all power ratings to the same unit. In this case, we will use watts. If you have devices rated in amps, multiply the rating in amps times the voltage the device uses.
3) At 12 volts, that freshwater pump would pull 60 watts (12 x 5 = 60).
Estimate how many hours a day you run the item—if your freshwater pump runs five minutes every half hour, that is the same electrical load as four hours a day.
4) Multiply each device’s wattage by the hours you run it per day to get watt-hours for each device. Our freshwater pump would be 60 x 4 = 240 watt-hours per day.
5) Add all of the watt-hours together. For instance, if you get a total of 6,500 watt-hours for your boat, divide that number by 1000 to get 6.5 kw-hours.

On top of this value, which represents your DC load, you may have AC devices you want to run on a regular basis, such as air conditioning units, vacuum cleaners or microwaves. Determine the power rating of these devices using the same method, but this time pay attention to the peak power the devices use and choose the largest device you’re likely to run at any given time. If you want to run more than one at once, you need to add the power for all of those devices.

For instance, you may have a 1000-watt microwave, an AC unit that pulls 950 watts, and a vacuum cleaner that pulls 1200 watts. If you decide that you’re most likely to run the AC and either the microwave or the vacuum cleaner at the same time, but not both, the appropriate figure to use here would be 950 + 1200 = 2150 watts, or 2.15 kilowatts. Note that we don’t really care about how much per day these run as they need to be powered directly by the generator and thus it needs to match their exact draw at minimum.

Once you have a good feel for how many kilowatt-hours of DC power and AC power you need, it’s fairly straightforward to determine what size of genset you need. To recharge the DC power you use in a given day, you typically want to minimize your generator runtime. A good figure for a runtime target is anywhere from half an hour (if you have AGM batteries) to about two hours (if you have standard flooded lead-acid or gel batteries). For instance, if you determine that you need to replace 6.5 kw of DC power every day and you want to do that in two hours, you need a genset that can produce at least 3.25 kw of power (3.25 kw times 2 hours = 6.5 kw hours).

But there’s one more factor to consider: if you use your AC devices at the same time you charge your batteries (and most genset owners do!), you’ll want to ensure you have enough capacity to run them both at the same time. So add the AC load to the DC load (in the case of this example, you would have the 3.25 kw + the 2.15 kw = 5.4 kw).

Last but not least, let’s factor in a little growth and extra capacity. We don’t want to overload the genset, but we also don’t want to run it inefficiently. A good rule of thumb is to size the genset about 20-30% over the total estimated demand, as running it at 80% of rated load is about the maximum you want to do on a regular basis and running it at less than 50% is inefficient. So let’s go with the rule of thumb and add 25% to our 5.4 kw to get a final value of 6.75 kw. There’s a decent amount of wiggle room here. A genset between 6 and 7.5 kw would be a good match for our theoretical situation.

Now that we have the electrical size of the genset worked out, don’t forget the physical space requirements, tankage for additional fuel, locations of plumbing and thru-hulls for fuel, cooling, and raw water, and the exhaust plumbing. You may want to consider an air-cooled genset for simplicity and weight savings, but these have special installation requirements for heat, exhaust and airflow. Many gensets can produce more power than your battery chargers can handle, so ensure that you size your battery chargers in accordance with your calculated DC load. And of course, always consult with the manufacturer and a quality installer before you make the final decision, even if you think it’s “a really great deal.” It may be a lot harder to get that really great deal in your boat than something new that fits better!

Daniel Collins, an ASA certified sailing and navigation instructor, amateur extra class radio operator and small boat racer, enjoys experimenting with marine electronics. He is also actively involved in community-driven social change. Email him at, or read his blog at

Author: Daniel Collins