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Old 01-06-2014, 11:48   #1
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How big a generator?

How big a generator do most of you use if you want to run your A/C on the hook?
What brand and size for two a/c units.....?
Thanks!
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Old 01-06-2014, 12:10   #2
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Re: How big a generator?

What make and size are your Air Cons? Are they new gen. or old units?
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Old 01-06-2014, 12:11   #3
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Re: How big a generator?

They are older...maybe 8 to 10 years old....a 12 and a 9 by Mermaid?
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Old 01-06-2014, 12:17   #4
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Re: How big a generator?

My 7.0KW Westerbeke BCGB runs both AC units, the 'fridge, the microwave, the dual burner cook top, plus all the incidentals, all at the same time, w/o issue.
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Old 01-06-2014, 14:01   #5
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Re: How big a generator?

Generators are rated, usually in Kilowatts, and sold by the highest output they achieve. Thus a 7KW generator can produce 7000 Watts - but not continuously. Depending upon the manufacturer you will need to de-rate that by at least 10% for continuous running. Then, if you are in waters or areas over about 75F then you the output is again derated at least 5%. After all of that, you need to plan for some line losses and a "fudge factor". Thus the 7Kw will, the Caribbean, realistically produce only about 5.5KW for long periods of time without undue overheating or problems.

It looks like a Mermaid 9 and Mermaid 12 are BTU model numbers, so you are cooling 21 BTUs. For household devices there's an EER (energy efficiency ratio) which isn't done for boats, but assuming that we use about 1 amp per 1000BTU (including powering the water pumps) then you are going to be looking at 2.1Kw generator output. Unfortunately, the compressors use about 1.8 times their rating when they start. So unless you have a soft-start like Fischer Panda has, or the Victron chargers/inverters which will temporarily invert the missing amps from the battery bank, you will need a generator that produces 3.8KW, meaning one that sells as a 5KW generator. If you are charging batteries off the genset (which makes a lot of sense) at bulk rate and both AC units happen to start at the same time the generator might just shut off, even with a 4-5KW unit. Thus the soft-start or Victron method is worth it so you don't have to upsize the generator just to deal with very temporary spikes in power usage.
These are just rough numbers, but it will give you an idea of what factors you need to think of. I've got a 9KW unit aboard and have managed to make it overload without even turning on my 5 AC units! (electric stove and two burners on, charging the batteries at 120A@24V, water heater on and then I fired up the air compressor. Reset system, turned off oven and charger, fired up the compressor and then turned everything else on; it was the startup compressor overload that did it).
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Old 01-06-2014, 15:58   #6
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Re: How big a generator?

Many people add up the A/C loads, multiply by two for inrush starting current and add to that all of the possible loads that they could run: water heater, microwave, stove, coffee maker, hair dryer, battery charger, etc. Then they end up with a huge generator that normally loafs along at 10-20% of capacity. That is not good for the genset engine to run at such a low load, particularly at night when the A/C load is very light.

It is much better to size the generator for the minimum number of things that you HAVE to run at the same time. In our case it is the A/C at 12 amps running, and one of the foregoing. I can always shut one down for a few minutes to run the other.

We have one of the smallest A/C gensets made, the NextGen 3.5 KW unit. It runs all of the above, but not all at the same time and works fine for us.

David
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Old 02-06-2014, 05:34   #7
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Re: How big a generator?

If you have a Mastervolt or a Victron charger/inverter with 'power assist mode' then the generator size can be very much smaller as the extra power for a fraction of a second at startup comes from the batteries not the generator. It could possibly be cheaper to change your shore power charger than buy a generator that is much larger than you really need.

See the Victron website for some very good documentation.
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Old 02-06-2014, 14:00   #8
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Re: How big a generator?

I agree with this below, with that said I would not go bigger then 6KW which would handle everything you would want in a low RPM unit. Take a look at Northern lights.
Quote:
Originally Posted by djmarchand View Post
Many people add up the A/C loads, multiply by two for inrush starting current and add to that all of the possible loads that they could run: water heater, microwave, stove, coffee maker, hair dryer, battery charger, etc. Then they end up with a huge generator that normally loafs along at 10-20% of capacity. That is not good for the genset engine to run at such a low load, particularly at night when the A/C load is very light.

It is much better to size the generator for the minimum number of things that you HAVE to run at the same time. In our case it is the A/C at 12 amps running, and one of the foregoing. I can always shut one down for a few minutes to run the other.

We have one of the smallest A/C gensets made, the NextGen 3.5 KW unit. It runs all of the above, but not all at the same time and works fine for us.

David
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Old 02-06-2014, 15:03   #9
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Re: How big a generator?

Quote:
Originally Posted by WebWench View Post
How big a generator do most of you use if you want to run your A/C on the hook?
What brand and size for two a/c units.....?
Thanks!

Quote:
Originally Posted by DeepFrz View Post
What make and size are your Air Cons? Are they new gen. or old units?

Our 8Kw genset runs two 16 BTU ACs (12 years old), plus two fridges, combination microwave/convection oven, electric stovetop, water heater, coffee maker, toaster, etc.

The ACs have a controlled soft-start mode, so they don't both switch on at the same time. (Newer models by the same maker would also be even more efficient.) And we don't always have all that stuff on at the same time anyway (example, fridge compressors only run from time to time... and we don't sit around making toast and coffee all day long).

-Chris
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Old 12-07-2014, 07:43   #10
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Re: How big a generator?

Quote:
Originally Posted by djmarchand View Post
Many people add up the A/C loads, multiply by two for inrush starting current and add to that all of the possible loads that they could run: water heater, microwave, stove, coffee maker, hair dryer, battery charger, etc. Then they end up with a huge generator that normally loafs along at 10-20% of capacity. That is not good for the genset engine to run at such a low load, particularly at night when the A/C load is very light.

David
I totally agree. My Entec/Farymann 4.2kva single cyclinder 3,600 rpm can give 110v max 35 amps usually runs the 16,000 btu air cond with about 16 amps load until one fine day the genset just simply quit.

Smaller load were ok but running the air cond would make the genset putter before quiting.

Took out the fuel pump, injector pump and injector for service. The injector were clogged with a ball of carbon.

Can't imagine what the inside the combustion chamber looks like

Eric
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Old 12-07-2014, 08:55   #11
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How big a generator?

We have a 5KW Northern Lights for our Hunter 356. We only have one 16,000 BTU MarineAire unit. We also have 120 volt hot water heater, battery charging, 2 laptops on power, usually charging 2IPhones, 2IPads and a Kindle. We also have a flat screen TV that runs while we are underway, KVH Trackvision running off a 120 to 12v transformer, a Dish receiver and a ROKU box plugged in. We keep our SpotGen3 plugged into trickle charge and a Verizon MYFi plugged in and that is pretty much the extent of our load and we never come close to loading it up. It burns about .25 gallons of diesel per hour and we plan for that useage. We start the generator, unplug at the dock and run until we plug back in at the next docking. We've run continuously for several days several times - longest for 99.6 hours this spring returning in heavy current from Mobile up the Tenn-Tom to Demopolis, AL. My wife cooks with a small CrockPot while cruising and did several times on the 99.6 hour trip above. We run the 16,000 BTU HVAC continuously as it is a water to air heat pump and either supplies heat or cooling. We keep all the cabin areas conditioned while we are on-board.

I don't have a load monitor on the Northern Lights, but we have never had it load to the point we thought it was struggling to keep up. We change oil, oil filters, fuel filters and impellers on a regular basis and have replaced the injectors and adjusted the valves at 2,691 hours along with a new v belt. I had one v belt change prior to the one at 2,691 hours. Otherwise, no other maintenance in 12 seasons except fluid top offs. I am considering changing the vibration isolation mounts soon. It has 3 cylinders, runs at 1800 RPM and is in a lazarette and has a custom sound enclosure. Inside our bedroom next to the lazarette and in the cockpit, the sound level is about 65 decibels, which is not loud. It is much less in the saloon and we notice the hum, but it is a background noise that we don't pay attention to. Our MarineAire fan makes more noise. We have underwater exhaust and 25 feet away from the boat, it is not noticeable.

I would highly recommend the Northern Lights as it is very durable and relatively easy to service and maintain.


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Old 16-07-2014, 19:47   #12
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Re: How big a generator?

Quote:
Originally Posted by sailinglegend View Post
If you have a Mastervolt or a Victron charger/inverter with 'power assist mode' then the generator size can be very much smaller as the extra power for a fraction of a second at startup comes from the batteries not the generator. It could possibly be cheaper to change your shore power charger than buy a generator that is much larger than you really need.

See the Victron website for some very good documentation.
I agree.

The link to the Victron Generator test is here:
Marine Generator Test - Victron Energy

There is also a paper on this subject by Nigel Calder "Taking Charge" in the April/May 2014 issue of Professional Boatbuilder Professional BoatBuilder magazine - Professional BoatBuilder Magazine
but there is a charge to access back issues electronically.
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Old 17-07-2014, 08:22   #13
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Re: How big a generator?

Quote:
Originally Posted by WebWench View Post
How big a generator do most of you use if you want to run your A/C on the hook?
What brand and size for two a/c units.....?
Thanks!
Although I don't have a boat I have done a lot of research of what I want when I get my cat.

Soft starters allow start up current to be reduced for air conditioners, here is a dometic one being demo'd -

Also, the Victron systems look very appealing.

These two systems allow a smaller generator to be used saving money, weight, fuel and letting them run loaded which is better for them.

Lastly, I will be looking at a ~ 1000AH Lifepo4 battery setup so I can minimise the generator run time.

The generator I want is the Kutobo 1800rpm 3 cylinder 5Kw.
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Old 17-07-2014, 08:30   #14
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Re: How big a generator?

Quote:
Originally Posted by J Clark H356 View Post
We have a 5KW Northern Lights for our Hunter 356.
That's an impressive generator to fit in the lazarette of a Hunter 356 .

Any pictures of the installation?
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Old 17-07-2014, 08:41   #15
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How big a generator?

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Here are the photos of my Northern Lights 5kw generator. It is in the port lazarette and I had a custom sound enclosure made.


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