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Old 07-27-2015, 10:14 AM   #1
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Generac diesel generator, fuel usage question

If running the generac diesel 7500 (50amp) generator with one air conditioner running, let's say it uses 1/2 gallon per hour of fuel. Will my fuel usage go up if I turn on a second air conditioner?

Why I ask, When I run both or only one, I don't hear the RPM's increase or even hear the generator working any harder. So I am wondering if the fuel usage is the same regardless what load.
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Old 07-27-2015, 10:31 AM   #2
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It will increase only slightly. The biggest increase is the load created by the first one.
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Old 07-27-2015, 01:33 PM   #3
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You have a fixed rpm generator, so unless you have a very keen ear, the differences between load levels (unless you catch it just at the moment the load changes) are hard to identify. If you are running both a/c units and the compressors cycle on and off at various times, you should hear the load differences, but once it's going, they are quiet enough that it always sounds the same

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Old 07-27-2015, 01:45 PM   #4
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Same thing as running 1600rpm on a flat highway, and running 1600rpm climbing a grade. Engine spinning same rpm, more fuel and air climbing the hill !
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Old 07-27-2015, 03:11 PM   #5
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The generator consumes a certain amount of fuel just to operate regardless of load so we can call that overhead.

Past that it converts fuel btu into watts by some conversion factor.

So first ac will use the most o to speak then second very little compared to first one as the generator is already running
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Old 07-29-2015, 12:42 AM   #6
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Sounds great, then both airs running is the way to go. Thanks folks.
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Old 07-29-2015, 01:19 AM   #7
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My generator manual says 1/2 GPH at half load and 1 GPH at full load. Mine is a constant speed 10KW so I guess it's somewhere around 0.1 GPH per 1000 watts but as others have said, it takes some fuel to run even with no load. I would think efficiency in converting diesel to electricity would increase with higher loads.


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Old 07-29-2015, 01:45 AM   #8
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Our 8.0 KW Generac ran at a constant 1800 rpm and never really changed pitch when more load was added. As I remember the manual stated that it used 1.32 GPH at full load and .72 at half.
Our present 12.5 Onan is also a constant speed and again I don't notice any difference under more load but it does burn a little more fuel too.
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Old 12-27-2015, 11:32 PM   #9
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Quote:
Originally Posted by Mr_D View Post
Our 8.0 KW Generac ran at a constant 1800 rpm and never really changed pitch when more load was added. As I remember the manual stated that it used 1.32 GPH at full load and .72 at half.
Our present 12.5 Onan is also a constant speed and again I don't notice any difference under more load but it does burn a little more fuel too.
You are right, ours is constant as well, I am a little under your GPh but we have a 7500 kW. So that makes sense. I would have expected some change when the load increased but nope.

Does yours shake and shutter when you turn it off, while it is finishing up its last few rotations.
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Old 12-28-2015, 09:47 AM   #10
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The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.


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Old 12-28-2015, 09:51 PM   #11
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Quote:
Originally Posted by G Schulz View Post
The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.


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There you go. perfectly good explanation! Thanks.
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Old 12-28-2015, 10:24 PM   #12
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Quote:
Originally Posted by G Schulz View Post
The speed of the generator is what gives you the 60 cycles per second sine wave. That's why the speed is constant. As you increase the load it will consume more fuel but it remains at the same rpm.
Onan, as do other manufacturers, makes variable speed units too. In fact the 7.5 Onans are generally variable speed units.
Here is one: http://power.cummins.com/content/rv-qd-6000 and read up on the different units for RV's, where you will see they do indeed make many variable speed units for the RV market.
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Old 12-29-2015, 02:47 PM   #13
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Quote:
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Does yours shake and shutter when you turn it off, while it is finishing up its last few rotations.
I have the 7.5kw but it shakes too on a sudden shutdown.
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