Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 13-06-2012, 16:14   #1
Registered User
 
callistov42's Avatar

Join Date: Jan 2011
Location: Seabrook, Texas
Boat: Vagabond 42
Posts: 169
Calculating size generator I need (in Kw)

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?
__________________

__________________
callistov42 is offline   Reply With Quote
Old 13-06-2012, 16:28   #2
Moderator
 
sailorchic34's Avatar

Cruisers Forum Supporter

Join Date: Feb 2011
Location: SF Bay Area
Boat: Islander 34
Posts: 4,815
Re: Calculating size generator I need (in Kw)

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.
__________________

__________________
sailorchic34 is offline   Reply With Quote
Old 13-06-2012, 16:28   #3
Senior Cruiser
 
Captain Bill's Avatar

Cruisers Forum Supporter

Join Date: Apr 2008
Location: On the boat
Boat: Endeavourcat Sailcat 44
Posts: 2,313
Re: Calculating size generator I need (in Kw)

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.
__________________
Captain Bill is online now   Reply With Quote
Old 13-06-2012, 16:34   #4
Senior Cruiser
 
bstreep's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: San Antonio, TX/Port Aransas, TX
Boat: 1990 Macintosh 47, "Merlin"
Posts: 2,274
Re: Calculating size generator I need (in Kw)

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
__________________
Bill Streep
San Antonio/Port Aransas, TX
bstreep is offline   Reply With Quote
Old 13-06-2012, 16:49   #5
Senior Cruiser
 
skipmac's Avatar

Cruisers Forum Supporter

Join Date: Oct 2007
Location: 29 49.16 N 82 25.82 W
Boat: Pearson 422
Posts: 12,388
Re: Calculating size generator I need (in Kw)

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.
__________________
The water is always bluer on the other side of the ocean.
skipmac is offline   Reply With Quote
Old 14-06-2012, 04:37   #6
Registered User

Join Date: Dec 2008
Location: Oriental, NC
Boat: Mainship Pilot 34
Posts: 1,429
Re: Calculating size generator I need (in Kw)

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
__________________
djmarchand is offline   Reply With Quote
Old 14-06-2012, 05:20   #7
Senior Cruiser
 
Blue Stocking's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: St. Georges, Bda
Boat: Rhodes Reliant 41ft
Posts: 4,117
Re: Calculating size generator I need (in Kw)

Quote:
Originally Posted by skipmac View Post
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.
__________________
so many projects--so little time !!
Blue Stocking is offline   Reply With Quote
Old 14-06-2012, 07:16   #8
Moderator
 
Dockhead's Avatar

Cruisers Forum Supporter

Join Date: Mar 2009
Location: Cowes (Winter), Baltic (Summer) (the boat!); somewhere in the air (me!)
Boat: Cutter-Rigged Moody 54
Posts: 19,758
Re: Calculating size generator I need (in Kw)

Quote:
Originally Posted by skipmac View Post
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.
__________________
Dockhead is online now   Reply With Quote
Old 18-07-2012, 18:39   #9
Senior Cruiser
 
S/V Antares's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: Annapolis, Bahamas
Boat: 1983 Gulfstar 36
Posts: 1,249
Images: 1
Thumbs up Re: Calculating size generator I need (in Kw)

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.
__________________
Will & Muffin
Lucy the dog

"Yes, well.. perhaps some more wine" (Julia Child)
S/V Antares is offline   Reply With Quote
Old 18-07-2012, 19:02   #10
Registered User

Join Date: Sep 2009
Location: Mackay,QLD, Australia
Boat: planning a approx 45ft cat
Posts: 3,651
Images: 3
Re: Calculating size generator I need (in Kw)

Quote:
Originally Posted by S/V Antares View Post
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/
__________________
downunder is offline   Reply With Quote
Old 18-07-2012, 19:59   #11
Senior Cruiser
 
S/V Antares's Avatar

Cruisers Forum Supporter

Join Date: Sep 2007
Location: Annapolis, Bahamas
Boat: 1983 Gulfstar 36
Posts: 1,249
Images: 1
Re: Calculating size generator I need (in Kw)

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.
__________________
Will & Muffin
Lucy the dog

"Yes, well.. perhaps some more wine" (Julia Child)
S/V Antares is offline   Reply With Quote
Old 22-07-2012, 20:29   #12
Commercial Member
 
CharlieJ's Avatar

Cruisers Forum Supporter

Join Date: Dec 2008
Location: St. Petersburg, FL
Boat: Gulfstar Long Range Trawler; 53'; BearBoat
Posts: 837
Re: Calculating size generator I need (in Kw)

Quote:
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.
__________________
Charlie Johnson
JTB Marine Corporation
"The Devil is in the details and so is salvation."
CharlieJ is offline   Reply With Quote
Old 22-07-2012, 20:41   #13
Registered User

Join Date: Nov 2011
Location: Montegut LA.
Boat: Now we need to get her to Louisiana !! she's ours
Posts: 3,421
Re: Calculating size generator I need (in Kw)

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
__________________
Bob and Connie
bobconnie is offline   Reply With Quote
Old 23-07-2012, 03:05   #14
Marine Service Provider

Join Date: Oct 2011
Posts: 1,205
pirate Re: Calculating size generator I need (in Kw)

Quote:
Originally Posted by CharlieJ View Post
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?
__________________
FlyingCloud1937 is offline   Reply With Quote
Old 23-07-2012, 19:17   #15
Senior Cruiser
 
Captain Bill's Avatar

Cruisers Forum Supporter

Join Date: Apr 2008
Location: On the boat
Boat: Endeavourcat Sailcat 44
Posts: 2,313
Re: Calculating size generator I need (in Kw)

Quote:
Originally Posted by CharlieJ View Post
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.
__________________

__________________
Captain Bill is online now   Reply With Quote
Reply

Tags
generator

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 22:26.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.