The 8kw Generator Choice

Time for boating tech talk”¦

When buying the Nordhavn 43 you get to choose the size of diesel powered electric generator. The options are 8 kilowatt (kw) (66 amps at 120v), 9kw (75 amps), 10kw (83 amps), 12kw (100 amps), or even no generator at all. The short answer is we chose the 8kw and are happy with the choice for the way we use the boat, as live aboard cruisers not connected to shore power. The reason takes a bit of explanation. This is specific to the Nordhavn 43 and its typical systems. In fact determining the power system and power system profile for a boat is an important and sometimes tricky thing to do. It is possible to get by without a generator, but if the boat stays anchored for days at a time, and/or you choose to add certain luxuries, then a generator becomes a necessity.

First off, bigger is not always better when dealing with diesel generators. Our primary goal is to run the generator with as high of load as possible, and secondly run it as little as possible. The main reason is for complete combustion a diesel engine needs some load on it. Roughly speaking, running it with low loads for long periods of time clogs the engine with incompletely burnt fuel. Supposedly, some newer engines are less susceptible to this, but generally it is good practice to keep your diesel loaded above 50% of capacity most of the time it is run. The 8kw is smaller and lighter, which opens up some engine room space and makes it easier to work on. It does use a bit less fuel, but the gallon per watt ratio is very close for all the generators involved. Finally, why pay more for a larger generator when a smaller will suffice?

In picking the size for your boat, you have to look at the systems that use power. Electricity is the root of almost all the luxuries aboard. The systems each use electricity differently. And things change when the boat is underway. Let us look how much the systems use and how often they need charging per 24 hours on a relatively normal day.

Power  Approx, Units  Time, System
2-10,  2-4 hours, Washer/Dryer
10, 0.5-1 hour, Water Heater
11, 2-3 hours, Water Maker
15, 1 hour, Dive Compressor
10-13, 1-4 hours, Battery Charger (for everything else: fridge/freezer, lights, etc.)
13 (x 3 units), variable, Air Conditioning and Heat

If we turn everything at once we would consume 94 units (amps). That right away points to the 100 amp generator (12kw). But in normal usage it is unlikely that you will be running everything at once. Let us break down the systems a bit.

One tricky item is the washer and dryer. It is a combo unit so it is either washing (~2 amps) or drying (10 amps). We found it needs extra pre-wash and the longest wash cycles to do an acceptable job of cleaning clothes, so 2 hours for a wash. Drying is usually 50 -90 minutes. Interrupting a wash is a bad idea, so you need a least 2 hour continuous run, and usually you want 3+ hours to also complete the drying. You cannot run the washer from the batteries. This long run time for the washer has a lot to do why we think the 8kw is enough. The washer/dryer can end up being the only load on the system after everything else is “filled up”.

A second tricky item is the battery charger, or rather the batteries themselves. You want to keep them above 50% charge. Also, they charge fastest between 50% and 85%. This means the batteries need about 2 hours of full load charging, then the amount of electricity they absorb goes down to about 5-8 units or so. So, in practice, the batteries hover around 50%-85%, which means if you have a total of 1000 amps available, you really only have about 350 “working amps”.

A big variable is the air conditioning and heat. If you want it, you will run it. This means the majority of the time it will be the only load on the generator, because everything else will be full. This is 36 amps, and is about 50% load on the 8kw generator. Even then insulation aboard is good and they do not run all the time. This is another big reason we think 8kw is enough.

So, at anchor how does a day in the life of Kosmos’s generator and load look like? We start the generator in the morning. First things on are battery charger (13), water heater (10) and water maker (11). Assume the refrigerators and freezer decide to run and we are at about 40 amps, which eventually drops to about 15 amps after 2 hours. What happens next depends on our day. We can run for about short as 30 minutes and head to shore, but usually it is 1 hour or longer. It is much longer if we do a load of laundry. In the evening we run the generator again depending on if we need to do laundry or charge the battery. If it was really hot we run the air conditioning a bit. We sometimes hit about 60 amps in the evening with the air conditioning on.

Some variables are if we need to fill the SCUBA tanks, or for some reason used a lot of water. Or if we just want to have more air conditioning. Also if we used the computers a lot or watch a movie it consumes power. But in just about all cases we use less than 60 amps at a time, which means 8kw is enough.

Overall we run the generator between 2 and 4 hours a day. In some cases we can get by with 1 to 1.5 hours.

Underway the generator has less to do, but almost paradoxically is when we may run it the most hours per day. The main engine alternator charges the batteries, and the coolant heats the water in the hot water heater. We have a DC water maker so we do not need to run the generator for water. The main reason to run the generator is for air conditioning, and when we do we will usually leave the genset running 12-24 hours a day. It is interesting to note here that some boats do not have generators at all and just use the main engine to charge the batteries. That, unfortunately, is the undesirable low loading situation on the main diesel engine.

We do have some additional choices and practices with regards to our electric system that help us get the most of it. We have a Xantrex battery monitor, which counts battery amps usage accurately. We added one extra 8D batteries for a total of four 8D house batteries (~1000 total amps). We have a second battery charger (50 amps) which helps charge the batteries faster. It is a Mastervolt that accepts different voltages, which we’ll likely use in Europe. We have 250 watts of solar panels, which add about 30-50 amps per day with a Blue Sky Solar Boost unit (keeps voltage up, if we get shadows on the panels). We installed a switch to turn off the 2 amp draw on the power panel’s LED voltage and current indicator lights. Seems small, but that is 48 amps over 24 hours. We did not get the ice maker in the fridge/freezer, since the model with it uses up about 25% more power than without it (this also gives us more food storage space). When we are low enough on refrigerated food, we turn off one of the refrigerators. That really helps increase battery life. We also run the inverter in “Search” mode, and turn off the 120 volt outlets when the generator is off. This means when the refrigerators are not running their compressors, the invert goes to “sleep” and burns almost zero amps instead of the 2 standby amps.

This may all sound complicated, but it doesn’t take long to get it all down pat. Also the numbers do not need to be exact. There is some time for generator warm up, loading up time, and cool down. You do not want to turn on all air conditioners and the water maker at the same moment because their startup usage is much higher than their continuous run time usage. You have to stagger the start times. You turn a few switches here and there, and it all becomes second nature to life aboard. We are thankful we have an abundance of electricity aboard and enjoy all the luxuries having plenty of electricity provides. Our generator is very quiet, and unless you are on the boat you cannot even hear it.

5 thoughts on “The 8kw Generator Choice”

  1. Greetings again from Florida.

    Thanks for a very insightful explanation of your electrical system. I enjoy hearing about life aboard along with stories from your experiences ashore.

    I don’t know what your capactiy for web surfing is, but it seems you’ve had some difficulties identifying the marine creatures you see on your dives and snorkels. Although I don’t own this particular volume (I do most of my diving in the Caribbean or US Pacific Coast), these folks put out the best fish ID books around.

    Best wishes for continued success in your voyage.

  2. Eric,

    Glad to read all is well. (in case you are not aware of this) On our N43, 8kw genset you must change the impeller every 150 hours of use or impeller blades will start disintegrating and you will have to fish out the rubber bits from the genset. We lost all impeller blades at 350 hours and turned a 30 minute exercise into a 3 hour labor of joy.


  3. Eric,

    You mentioned that you’re powering your dive compressor from your 8kw generator. Is this your hookah (Sea Breathe type) compressor or an air tank compressor? We want to install a dive tank compressor on our boat but have been told by several dive shops that that we need a minimum 10kw genset to do this and that our 8kw set will not be powerful enough.

    Best wishes


  4. Thank you for your explanation which confirms my calculations. I am building a 46′ Trawler in Brazil for retirement cruising purposes.
    In the tropics, most of the days, cruising without AC is like inflicting self punishment. So, we are planning to connect a 6 KVA generator to our ZF gear PTO with independent clutch, and turn it on when the engine is driving within 1600-1800 RPM, which will probably be 85% of the time. Hence, we will have only one source burning fuel.
    My boat is being equiped with a 8 KVA genset.



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