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Old 30-11-2009, 08:54   #1
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Is 3.5kw Generator OK for a 16,000btu AC?

I have a Hunter 356 that has 200 amp-hr battery capacity (2-100 amp-hr Oddessy 2150's). We are planning to sail to the Florida Keys from Mobile, AL next summer. We definately want to use our 16,000btu AC at night. We plan to avoid marinas and be on the hook 90% of the time.

If we are running a generator every night, will 3.5kw be enough to power the AC and bring the batteries back to full each night from a 50% draw down on the battery? We only plan on using power for minimal lights, stereo, fridg/freezer and C120W Raymarine chartplotter.

Currently looking at NextGen 3.5kw but not tied to that. I'm trying to stay low cost since we plan to sell the boat in 3 years.

One of the things confusing me is that I have seen people comment that they have success with small generators and AC but my iphone conversion application says that 16,000btu = 4.69 kw. Maybe those people have smaller AC's!?! The boat show salesman said it would be fine for AC but I didn't know my btu size when I asked him.
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Old 30-11-2009, 08:59   #2
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You may or may not be able to run the ac and the battery charger. Startup load is the killer here. A fischer panda 4kw will start and run a 16k and a 12k with a capacitor mod done to the generator control box (i am not recomending this machine) but the same thing is not able to be done for the nextgen.
In short I can't say if the generator will run the A/C, but I am failrly certain the gen won't be able to run a sizeable charger and start the A/C at the same time......I could be wrong though.
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Old 30-11-2009, 09:01   #3
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I would expect it to work fine for the AC but perhaps not so well if you are trying to charge the batteries at the same time. Why not run AC at night and charge during the day?

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Old 30-11-2009, 09:17   #4
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HOw are you charging the batteries?
My victron inverter charger will power share with the generator.
So on start up of the AC when it uses the most power, the victron will pull power from the battery bank to help, then once the load goes down, the charger will put the power back in plus. So in that way, you will be ok.
But you need to know your power factor of the ac and the genset and the start up load of the ac.
The new ones have a soft start using capacitors to ramp up the load i believe.
Older ones do not.
So depending on your situation, you can make a system that will do what you want, but if requires redoing your installation, it will be very expensive.
For that reason, I went with 1 10K and 1 12K btu ac's and got rid of the single 16k older unit. Also changed to the victron unit, added 600 ah of agm Odyssey batteries, and one day will add the genset. Not sure if it will be 2 honda 2000i's in parallel or a diesel set.

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Old 30-11-2009, 10:47   #5
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A/c systems are rated in ability to move heat from the inside of the cabin to the outside. A 16000 BTU unit will move 16000 BTU from the inside, to the outside. They are NOT rated in power requirements.

There are many an sundry efficency requirements for a/c systems. Since you made the above mistake, I don't think we want to go down that road, as I don't have it straight.

A 16000 BTU until will require MUCH less than 16000 BTU of electrical power to run it. A Marine Air Vector Compact for 16000 BTU capasity should require somewhere around 1000W continous and 2.3W at start up. (At least according to the internet! LOL) I'm working on the assumption that your unit should be similar.

The second problem is your specific battery chargers. How many watts will they take? They should have a start up requirement.

Finally, there is the rating on the generator. There are two commonly used ratings a surge, and a continous rateing. A surge rating can be used to start up an a/c system, but you'll need plenty of continous power to keep it running, and keep the battery chargers working. Looking nextgen's web site the 3.5k is a continous rating, so we'll run with that. If your using 3.5kW of continous rating, and your a/c system uses 1500W, you have 2000W left for the battery chargers etc. That is possibly workable depending on your battery charger. (A standard house hold electrical plug is rates at 15 amps max. 80% is "safe" so a standard 120V plug is good for around 1440W.)

So from there you need to look up the specific requirements for your boats a/c system, and it's power consumption, as well as your specific battery chargers, and then see if the Generator fits.

If you ahve a super inefficent a/c set, and a super inefficent battery charger, it won't be enough power. Then you have a choice. Price out a larger gennny, or the smaller genny, and a more efficent system.
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Old 30-11-2009, 11:12   #6
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You will find that a smart charger/invertor like a Victron will help you a lot. The Victrons (probably some others too) completely solve the startup surge problem by automatically powering up the invertor and using your batteries for a second to assist the generator and cover the surge. They will also automatically cut down the charge current to suit available power when there are other loads. Best thing since sliced bread.

If you have decent ventilation you will not be terribly uncomfortable not using your a/c at anchor. I have been cruising SW Fl for a decade in a boat which has a/c but no generator. We were never tempted to put in a generator, because at anchor where it's quiet (as long as you don't have motor yachts with you running their generators) and you're always head into the wind, you can open the boat up, and it's not usually hot at night. In marinas, it's nice to have the a/c because you may not head into the wind, and there may be noise which makes you want to close up the boat. You might be ok without a generator for the cruise you are planning (I would increase that battery bank, though).
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Old 30-11-2009, 12:26   #7
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I have a 3.5KW NextGen and two A/Cs: a 16,500btu and a 12,000btu unit (both Flagship Marine). Installed these several years ago. I also have a Victron MultiPlus inverter/charger.

The Flagship Marine A/C units have special circuitry for "soft-starts". They are rated as follows:
.....................................Approx. Start Current Full Load Current
16,500BTU Unit (aft cabin) 21 amps 14.1 amps
12,000BTU Unit (fwd cabin) 19 amps 12.3 amps

Note that these figures do not count the AC-powered water pumps for each unit. I believe they draw about 3 amps each.

The NextGen will power the larger A/C unit just fine. It will also start and run the smaller unit simultaneously (total 28,500btu), but it won't run them both for very long before it cuts back and...if you don't shut one off...the generator will shut down.

While in theory the NextGen puts out 30amps @ 120volts, in practice on a hot day you can't load it that way. Also, the power factor of the load makes a big difference....pure resistive load (like a heater) vs. an inductive load (like motors in A/Cs, frig's, etc.). The NextGen is happy at about 20-22 amps indicated load. Much higher than that it gets finiky.

The Victron doesn't like the NextGen at all, even though it's been set to essentially work with dirty power. The NextGen...even with the special AVR unit...puts out pretty dirty power until it has a substantial load on it. I recently put two oscilloscopes on it to view what is happening at various loads. Bottom line is that I need to put a load on the NextGen before the Victron will say, "OK", and begin to pass power thru and/or do it's charging and charging-inverting thing. This is most annoying, and despite consultations with the NextGen dealer and several highly qualified electrical and marine engineers I don't have a good solution yet.

The NextGen's dirty power output is a function of its design...2 pole. After you put a good load on it, it settles down OK. BTW, the first AVR burned up, and the unit has a new one. Still, the output is dirty without a load.

Since installation I've often wished for a larger generator, but don't have the room for it. The Fisher-Panda's are nice and small, but they are very costly and have had a terrible maintenance reputation. Some of my clients (not for F-P) have been tearing their hair out for years with their F-Ps.

Hope this helps a bit.

Bill
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Old 30-11-2009, 12:26   #8
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If we are running a generator every night, will 3.5kw be enough to power the AC and bring the batteries back to full each night from a 50% draw down on the battery?
Do you have access to 30 A shore power? It provides approximately 3.5 kW, so if you can run your AC and a charger and something else at the same time without tripping a circuit breaker, you should be fine.

I suppose, the 16,000 BTU AC needs somewhere around 1.8-2.5 kW, so you should be able to use an
Odyssey 50 A charger at the same time. If not, you may switch to a different charger, such as 20A, which should still recharge your 50% discharged batteries overnight.
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Old 30-11-2009, 12:46   #9
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If you're going to run a generator all night long, please anchor a long way from me.
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Old 30-11-2009, 18:33   #10
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If you plan to run a generator while at anchor, you will make many enemies and not many freinds.
At anchor most night opn the hatches and turn on the fans, don't go spoiling an anchorage for everyone else.
I have a 16,000 BTU unit but only for when I'm stuck at a dock.
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Old 30-11-2009, 18:41   #11
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If you plan to run a generator while at anchor, you will make many enemies and not many freinds.
At anchor most night opn the hatches and turn on the fans, don't go spoiling an anchorage for everyone else.
I have a 16,000 BTU unit but only for when I'm stuck at a dock.
Have you heard one of the honda genies running?

Probably not
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Old 30-11-2009, 18:49   #12
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I have heard the Honda

They're only quiet to the boat they are running on because they've made the decision to put up with it for their AC or whatever. To everyone else wishing to enjoy the quiet, we hear them. These AC threads always get me, if you're that tied to the comforts of land living why not just stay there?
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Old 30-11-2009, 19:23   #13
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They're only quiet to the boat they are running on because they've made the decision to put up with it for their AC or whatever. To everyone else wishing to enjoy the quiet, we hear them.
I dont have one, but I have anchored near boats with them and boats with airx wind gens

I know which one I prefer.

Quote:
These AC threads always get me, if you're that tied to the comforts of land living why not just stay there?
Have you been in hot humid areas before with zero wind and swarms of mosquito's and sandflies?

Cold air and comfort for a few grand starts looking very good and as you get older, you realise that a few grand for these comforts occasionally is something that we should not deny ourselves while life is so short, especially if you can afford to.
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Old 30-11-2009, 21:16   #14
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Have to go with Cat Man on this one. 90% of the time, good ventilation at night is all you need. I spent a few years cruising with no AC. However, one or two windless nights with the noseeums and mosquitoes and you will start thinking about hijacking the big motor yacht across the harbor with the genset and AC.
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Old 01-12-2009, 06:30   #15
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Have you been in hot humid areas before with zero wind and swarms of mosquito's and sandflies?

Cold air and comfort for a few grand starts looking very good and as you get older, you realise that a few grand for these comforts occasionally is something that we should not deny ourselves while life is so short, especially if you can afford to.
Absolutely have done the hot,humid,bugs. We have lots of hatches, screens on all the hatches, and lots of cabin fans that draw diddly for power. It works for us. The bigger issue for us is the never-ending racket created by other people. If I had to listen to noise, I'd rather hear a wind gen than a dino-juice gen of any kind.

The problem is that when we take the attitude that we should not deny ourselves, we end up denying that our planet has finite resources. While I have no children, I would like to leave something for those who follow us. I cannot in all conscience do things that I know are greedy and selfish and ignore the realities of the planet and the needs/wants of the masses of other people living on it and yet to come.

When we go sailing, I want to feel part of the world and its natural environments, not walled off in some icebox cocoon. For that, I would stay home. Getting off soapbox now.
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