Calculating size generator I need (in Kw)

[callistov42]

If, with everything running (2 AC units, battery charger, water maker,ice maker) I am pulling 40 amps, does that mean 40amps x 120v = 4800 watts or 4.8 Kw generator? What allowance for starting loads?

[sailorchic34]

inrush current starting a motor is about 2 times starting load, though its just for a second... So add in double your largest motor / compressor load and a bit more, or be safe and include the two largest motors / compressors twice...

There is also a power factor with motors so divide motor loads by .95 also and include an overall safety factor.

[Captain Bill]

First of all you should not plan on running your genset at more than 75% load on a regular basis. If you actually figure you'll have all of that on at once you need to size it accordingly. Motor start loads are significant and could be a couple of times your running load. One alternative to a really big genset is an inverter charger that has a generator helper mode. That is it will phase match it's output with the genset and suck some juice from your batteries to help the generator with motor start loads. I am aware of Xantrex and Victron inverter chargers that have this feature, there may be others. Some of these units can supply 6KW for the few seconds needed to get the motor started. It's a much cheaper solution than moving up to a 12kw genset from a 6 kw.

[bstreep]

Depends upon the starting loads which can be pretty large. You can either consult the manuals or measure the loads when they kick on. Compressors can draw a pretty big load. You'll also want to make sure you leave room for growth.

We have a good friend who does this as a business in Kemah. They also happen to have a Vagabond 42!

Greg Heck, 832.392.8769
WestWindYachtService.com

[skipmac]

You can also add large power capacitors or soft start modules to the AC units to significantly reduce the inrush current to the compressor motor.

[djmarchand]

Don't size you generator for everything running. You don't need to run everything at once. It takes a bit of management but for example you can turn off the ice maker while the water maker is running. And the battery charger only pulls its maximum AC load for about 30 minutes (with 50% discharged batteries) and will taper down after that. You can turn on your hot water heater after these loads drop off for example.

But having said that the smallest practical low speed generator- 1,800 rpm is 5 kw so don't go any smaller than that.

Keeping the generator loaded is important for long life as well as efficiency.

David

[Blue Stocking]

Originally Posted by skipmac

You can also add large power capacitors or soft start modules to the AC units to significantly reduce the inrush current to the compressor motor.

--and if the boat has doors between cooled compartments, keep the doors closed and set the thermostats 3-4 degrees different, so that the compressors dont start up at the same time. World of difference for the generator.

[Dockhead]

Originally Posted by skipmac

You can also add large power capacitors or soft start modules to the AC units to significantly reduce the inrush current to the compressor motor.

But the best thing of all is to run all your AC power through a charger/inverter with power boost function. You can set this to strictly limit the amps which can be drawn from any given AC power source. Beyond the set limit, the battery charge power will be reduced. Once that is not enough, the inverter will kick in to cover any inrush loads or other shortages. It tremendously simplifies power management, and protects your generator against being overloaded, to boot.

Then you won't care about AC inrush loads at all -- easily dealt with with a little push from the inverter.

In general, I have been very happy running all my AC loads off my inverter. Other than AC and immersion heaters, there are no AC loads which I know of which can't be dealt with easily by a good inverter. I understand now that I don't need my 6.5kW AC genset at all -- a silent 1kW or 2kW DC genset keeping the batteries fed would much better suit my needs. I can even wash and dry clothes off the inverter while the main engine is running -- the alternator and inverter easily cover the load.

Of course, AC is a different animal, and for that you do need an AC genset. But no reason at all to size the genset for peak loads if you have a charger/inverter with power boost and a decent battery bank.

[S/V Antares]

Good info here. I have a project boat that is very gen dependent.
4 AC units 12 - 16,000 btu each
2 AC supply pumps for the above
Stove
Icemaker
Watermaker
Large reefer box (12v or 120v)

Anyone doing a similar refit? I would love some suggestions on type and size of gen set. My prefference is Northern lights 12 Kw but it may not fit by a few inches.

[downunder]

Originally Posted by S/V Antares

Good info here. I have a project boat that is very gen dependent.
4 AC units 12 - 16,000 btu each
2 AC supply pumps for the above
Stove
Icemaker
Watermaker
Large reefer box (12v or 120v)

Anyone doing a similar refit? I would love some suggestions on type and size of gen set. My prefference is Northern lights 12 Kw but it may not fit by a few inches.

As others have suggested above a Northern lLghts 6kw with Vitron Quattro inverter/charger with boost would be way to go rather than 12KW.

http://www.victronenergy.com/

Quattro http://www.victronenergy.com/inverte...rgers/quattro/

Check out the generator test.
http://www.victronenergy.com/generatortest/

[S/V Antares]

Downunder, Thanks for the input. The boat has an older freedom 25 (2500 watt) inverter. I do not think it has the "boost " capability. This boat will be on the dock 90% and hook 10% but it must work well as there is a non boater "Admiral" on board. It is a " turn everything on" scenario. And ALL must work without thinking.

[CharlieJ]

The boat has an older freedom 25 (2500 watt) inverter. I do not think it has the "boost " capability.

The freedom absolutely does not have this capability. The only marine inverter/chargers that I am aware of are Victron and Mastervolt.

[bobconnie]

We have had gennys for years ! and the one thing Ive found out in that time is there is no genny to big unless ya can't get it in noway !! Ive reworked things a time or two just to get a larger genny in !! Altho Connies great about turning things off! BUT she loves her wham whams and zoom zooms!! she loves to bake and likes clean clothing, and as a night nurse she sleeps days and air conditioning is important to us !! so if it was I, I would find a way to get the 12 KW genny in wherever ya need it to go !! theres always a way !! Just our 2 cents

[FlyingCloud1937]

Originally Posted by CharlieJ

The freedom absolutely does not have this capability. The only marine inverter/chargers that I am aware of are Victron and Mastervolt.

Did ya forget magnum?

[Captain Bill]

Originally Posted by CharlieJ

The freedom absolutely does not have this capability. The only marine inverter/chargers that I am aware of are Victron and Mastervolt.

You are correct about the older freedom series, but the new Freedom SW series does have what they call Generator Support Mode which provides this functionality. Of course such functionality does not come cheap. I think this functionality is becoming standard on just about everyone's higher end units.