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Old 23-04-2017, 14:03   #1
SFH
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Sizing of generator

Hi

I am about to order a generator for our Jeanneau 53. According to Jeanneau standard inventory the default generator i an ONAN MDKUB 5.5kW

Our boat has a CRUISAIR 40,000btu aircondition (standard option) split in three units with 16000 in the main cabin, 12000btu in the fwd cabin and 12000btu in the aft cabin

If I run a BTU to kW conversion, I get that 1kW equals 3412btu which means that I am quite far off with the ONAN if I wants to run the A/C

Am I wrong in the way of reasoning?

Is the ONAN adequate?

Best regards
Steen
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Old 23-04-2017, 15:12   #2
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Re: Sizing of generator

How many amps of shore power cord do you have?

Generally if you are happy with your shorepower, add 10-20% and size your generator accordingly.

Or, use a smaller generator and ration the power when in use.
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Old 23-04-2017, 15:13   #3
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Re: Sizing of generator

16amps on one shore cable and 32amps on the other one
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Old 23-04-2017, 15:17   #4
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Sizing of generator

Ok, so 220 50hz?

To replicate your shore cord available power you'll need roughly 8.5 kw.

This assumes a .8 power factor (common with A/C). But no cushion above your available shore power capacity.
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Old 23-04-2017, 15:42   #5
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Re: Sizing of generator

I've got the 10Kw generator on a Jeanneau 57 and that is more than sufficient for my needs, but I don't run my generator for air conditioning here in the Caribbean; the A/C gets run when on shore power and the standard Jeanneau connections are more than sufficient for that.
The generator should be run under load, but not close to maximum and it needs to be de-rated for ambient temperatures as well. Once you fire up the generator the batteries will get charged, depending upon your chargers that will take 1-3kW right off the top. Note that the BTU/Watt values for air conditioning are maximum values; once the boat is cooled down the air conditioning won't run close to those numbers.
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Old 23-04-2017, 16:24   #6
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Re: Sizing of generator

That BTU to kw conversion doesn't work. Have a look at the specs on the 16000btu unit. I think it's max 5 amps at 230v so 1.1kw.


Quote:
Originally Posted by SFH View Post
Hi

I am about to order a generator for our Jeanneau 53. According to Jeanneau standard inventory the default generator i an ONAN MDKUB 5.5kW

Our boat has a CRUISAIR 40,000btu aircondition (standard option) split in three units with 16000 in the main cabin, 12000btu in the fwd cabin and 12000btu in the aft cabin

If I run a BTU to kW conversion, I get that 1kW equals 3412btu which means that I am quite far off with the ONAN if I wants to run the A/C

Am I wrong in the way of reasoning?

Is the ONAN adequate?

Best regards
Steen
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Old 23-04-2017, 16:42   #7
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Re: Sizing of generator

Quote:
Originally Posted by SFH View Post
Hi

I am about to order a generator for our Jeanneau 53. According to Jeanneau standard inventory the default generator i an ONAN MDKUB 5.5kW

Our boat has a CRUISAIR 40,000btu aircondition (standard option) split in three units with 16000 in the main cabin, 12000btu in the fwd cabin and 12000btu in the aft cabin

If I run a BTU to kW conversion, I get that 1kW equals 3412btu which means that I am quite far off with the ONAN if I wants to run the A/C

Am I wrong in the way of reasoning?

Is the ONAN adequate?

Best regards
Steen
Just to clear up the units first:
BTU (British Thermal Unit) is an amount of heat. BTU per hour (BTU/h or "BTUH") is a rate of heating or cooling and is what your aircons are rated in.
BTU converts to Watt hours (both measure of energy)
BTU/h (or BTUH) converts to Watts. (both measures of power)

Now to the issue:

BTU/h to Watts isn't an appropriate calculation for air conditioners and similar appliances. You are not converting electrical energy to heat energy, you are using electrical energy to move heat from where you don't want it. It takes a lot less than a Watt hour of energy to move a Watt hour of heat from inside to outside your boat.

Airconditioning is frequently rated by "Energy Efficiency Rating "EER" which is the ratio of heat (BTU) moved for power consumed (Wh) or "Coefficient of Performance (COP) and that ratio is frequently 5-10

When you take into account the other requirements, (water pump, fans etc) you are probably looking at a requirement of a bit less than 100 Watts per 1000 BTU/h. (Call it 100 for planning purposes)

So your 40,000 BTU/h of airconditioning at full use is likely to need about 4kW. In which case, it looks as though your genset will be working hard, but should do the job.
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Old 23-04-2017, 17:03   #8
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Re: Sizing of generator

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Originally Posted by Zanshin View Post
Note that the BTU/Watt values for air conditioning are maximum values; once the boat is cooled down the air conditioning won't run close to those numbers.
The "BTU/Watt values" (?) are the numbers which apply when the compressor is running. The system won't run continuously once the boat is cooled down, so the BTU moved and the Watt hours consumed per hour will not be as high, but the Watts required while the compressor is running will be same - so that consideration has no effect on the required generator size.
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Old 23-04-2017, 17:06   #9
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Re: Sizing of generator

StuM - yes, but there are multiple compressors and they won't be running as often or for as long once the boat is coole down.
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Old 23-04-2017, 17:41   #10
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Re: Sizing of generator

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Originally Posted by Zanshin View Post
StuM - yes, but there are multiple compressors and they won't be running as often or for as long once the boat is coole down.
The OP was asking about the size of his genset.

There will be times when all three compressors are running concurrently and that is what determines his power requirement.

Sure, the genset won't be loaded as long/often once the temperature is down but peak power requirement is unchanged.
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Old 23-04-2017, 19:52   #11
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Re: Sizing of generator

A 16 and 32a cord at 230v would be 11kw. (Assuming you plug both in. That is common here )
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Old 23-04-2017, 21:23   #12
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Re: Sizing of generator

Just one note that you have to factor starting inrush current when sizing generators. So if you have three a/c units that each pull 1.5 kW then you will want to add at least 2 kW to the capacity ( I would add 3kW) to allow for inrush of starting the largest motor. In the case of three 1.5kW a/c units your looking at a 7kW (ish) genset (I would go to 8kW).

Actually most generators are sized for continuous load at 80% of name plate rating. That is for continuous running load a generator should not exceed 80% as it's nameplate rated for continuous 24/7 operations. There are gensets rated for 100% load but generally not found at the consumer level.

Of course it depends on the motor size. Gets really important when starting 1000 hp motors. A soft start device wired with the compressor motor might eliminate much of the inrush surge as the fields start the rotor turning. In the case of a 1000 hp motor a VFD (4160v/3ph and fair size) is a must have.
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Old 24-04-2017, 08:25   #13
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Re: Sizing of generator

Glad to see sailorchic chime in.

First, this statement is not germane to your situation:

"If I run a BTU to kW conversion, I get that 1kW equals 3412btu..."

Second, most of the (primarily high speed) light duty generators marketed for recreational use are not designed for continuous duty. I am not sure how frequently, and for what period of time, you intend to operate air conditioning while aboard, but if planning to operate operate for extended periods you will need to be more discerning regarding your choices.

Operating any generator at over 80% of name plate capacity will be stressful, and likely shorten the service life of the unit. At the same time, operating at low loads - less than say 30% of capacity - will also risk damage to the generator.

I would proceed by having an energy assessment done by three experienced vendors (those who service and sell generators) and see what their size recommendations are.

In my professional career I would occasionally specify a generator, though more frequently be responsible for the fuel tank or gas piping for the unit. That experience had only slight relevance to owning and operating my own 4.4kw generator.

The challenges I face are keeping a sufficient load on the generator when operating the unit to charge the house batteries. As the battery charge rate lowers, the load on the generator also lowers. Our solution is to time battery charging to coincide with making coffee, operating the microwave, vacuum, power tools, etc. - anything to prevent "underloading" the unit.

There are downsides to oversizing a generator. More cost upfront to you. More weight and space requirements. The challenge of keeping a sufficient load on the generator (what if you decide to just run one AC unit?).

Operating requirements for the AC units will be listed in the Owners Manual and/or the name plate on the unit: RLA (rated load amps), FLA (full load amps), etc. Use these for calculating electrical load, not what the rated cooling capacity of the unit might be. Add the sea water pumps also.

These are expensive units. Don't screw up the selection. I know of one Nordhavn owner who has a 12kw generator that he never uses, instead using his Yanmar 3GM30 wing engine as an alternative energy source.

By the way, since Cummins took over Onan, my understanding is that they have made changes to their units, particularly the countries of origin (COO) for the components. I am not certain whether the units you are considering are utilizing Chinese engines, but I have heard reports from users complaining about Taiwanese carburetors, for example, on gasoline powered units.

Good luck with your search.

P.S. You have recommendations in posts above ranging from 4 kw to 11 kw. That alone should send you to an experienced vendor for assistance determining required size
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Old 24-04-2017, 09:10   #14
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Re: Sizing of generator

Hi SFH how are you going with this? You can test the amp load of your AC and other appliances by turning each on individually. I assume you have an amp meter for shore power already onboard. Check the start up amps and then the running amps. Make a list. If there is a spike in amps on startup consider fitting a soft start unit to each AC unit. There is a thread on soft starts on this forum. Priced from about$20 each I think.

You can then decide how many amps you need and how you juggle. For example three AC units vs two AC units and the Watermaker vs one AC unit and the Watermaker plus battery charge, etc.

Genset should be loaded continuously at max 80% so add a factor of 1.25 to get there.

Do you have a load sharing inverter / charger like a Victron?

You also need to consider 1500 vs 3000rpm gensets. The faster ones are smaller but noisier. Onan makes both.
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Old 24-04-2017, 16:40   #15
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Re: Sizing of generator

Thanks to all for great inputs and leanings. I have decided to listen to a fellow Jeanneau 53 owner who chipped in. He has an Onan QD MDKBJ . He is happy with it and can run all A/C.
I ordered it this morning
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