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capta 03-13-2019 12:31 AM

eight kilowatts
 
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?

CVAT 03-13-2019 01:08 AM

Re: eight kilowatts
 
Quote:

Originally Posted by capta (Post 2051588712)
If a generator produces 4.4 kilowatts @ 238 volts, 60 Hz, will it produce 8 kW @ 120 volts, 60 Hz?
I was having a discussion with an electrician today who says not. He also said that the Onan 8 kW Genset rated @ 8 kW driven by a 10.5 hp diesel engine cannot possibly produce more than 6 kW.
All MDKD Onan gensets are rated as 8 kW units @ 1800 rpm and it seems as though there must be something wrong with either this gentleman's math or Onan's rating of 8 kW.
Any thoughts?

HP and wattage are both measures of power, one electrical and one physical but can be directly converted.

Your electrician friend is correct 1 hp is 745.69 watts so at 10.5 hp the max wattage it can produce is 7829.745 that is assuming 100% conversion of power between mechanical power, hp, and electrical power, kw. Onan is over stating the generators output by rounding to the nearest thousand, at 10.5 hp the maximum it could produce, without loses and assuming 100% conversion is the above stated 7829.745 watts. No diesel generator can convert 100% efficiently from mechanical power to electrical, so in my opinion this is just Onan overstating the engines ability if it is true. When I did a check of Onan generators online the 8 KW models had 22.3 hp engines attached.

By the way for your first example, assuming the generator is capable of multiple voltages it would produce 4.4 KW, half the voltage but twice the amperage, as wattage = voltage X amperage.

Minnewaska 03-13-2019 05:01 AM

Re: eight kilowatts
 
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

outbound 03-13-2019 06:54 AM

Re: eight kilowatts
 
On the current boat first had 4 kw lombardini. It was a total piece of junk. When I replaced it an new 8 kw Northern Lights was available at a extremely good price so installed that.
I never maxed out the output of the 4 kw unit. The current genset goes on rarely (has 17hours since Oct of 2018). When it goes on I charge batteries, vacuum, turn on the AC, watermaker, charge all electronics and tools etc. but obviously still don’t max out the 8kw unit.
I knew I wanted a low rpm unit. Smallest I could find was a 6kw and it was considerably more expensive than the 8kw.
Did I make a mistake in buying the 8kw? How important is it that a genset be run under full load occasionally?

Minnewaska 03-13-2019 07:10 AM

Re: eight kilowatts
 
I'm not sure, but I doubt it's a problem to run a genset below full load. They run at constant rpm anyway. I suspect it may be poor for the diesel to run it at zero load for lengthy periods of time, but no one would do that. Curious too.

CVAT 03-13-2019 07:28 AM

Re: eight kilowatts
 
Quote:

Originally Posted by Minnewaska (Post 2051588736)
I'm not sure, but I doubt it's a problem to run a genset below full load. They run at constant rpm anyway. I suspect it may be poor for the diesel to run it at zero load for lengthy periods of time, but no one would do that. Curious too.

Found this https://boatmags.com/choosing-the-right-generator/ interesting and informative read.

Don0190 03-13-2019 07:58 AM

Re: eight kilowatts
 
Quote:

Originally Posted by outbound (Post 2051588734)
Did I make a mistake in buying the 8kw? How important is it that a genset be run under full load occasionally?

you are happy with it so, No you didn't make a mistake

JimsCAL 03-13-2019 08:15 AM

Re: eight kilowatts
 
CVAT's post is spot on.

colemj 03-13-2019 08:49 AM

Re: eight kilowatts
 
Quote:

Originally Posted by Minnewaska (Post 2051588726)
I always understood genset wattage ratings to be their max spike load capacity, but continuous load was notably lower.

Our 8kW Kohler genset has a 14hp engine and is continuously rated for 8kW at 77F. It derates 1% for every 10F above 77F.

Our 5.5kW Nextgen genset has a 11hp engine and is continuously rated for 5.0kW at 77F. I don't know its derate factor for temperature, but know that it doesn't like running higher than ~4.5kW at typical hotter temps.

Mark

Minnewaska 03-13-2019 09:13 AM

Re: eight kilowatts
 
Quote:

Originally Posted by CVAT (Post 2051588740)
Found this https://boatmags.com/choosing-the-right-generator/ interesting and informative read.

That was a good read. This was the answer and it would seem unlikely many genset run with less than 25% load or one probably wouldn't be running them at all. I got a chuckle out of the recommendation that one consider having two different size generators to properly match loads.

Quote:

A generator should never run continually with less than a 25% load. 35% to 70% is optimal.


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