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Old 10-09-2013, 02:17 PM   #1
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Generator Fuel Consumption

Often there are threads concerning fuel consumption and fuel efficiency of generators used in or for RVs. The responses include facts, opinions, and guesses. While many of the responses are correct, many are not. This is an updated and more complete thread of the one I provided earlier.

Attached is a PDF table comparing many generators used for RVs. The base information comes from published data from the manufacturer. Some manufacturers provide more data than others, and the table takes those differences into account. No matter how much or how little data the manufacturer gives, there is enough data to indicate the general efficiency of the generator and to make comparisons to other generators. The table does not address the quality of the generator.

The table completely dispels one myth and complicates some other assumptions.

1. It is generally assumed that variable speed, inverter based, generators provide better fuel efficiency than constant speed generators.

The facts are that the variable speed generators, Onan, Honda, and Yamaha, have the lowest overall efficiency when compared to any constant speed generator using the same fuel in this group. This really shouldn’t be surprising since the variable speed inverter generators must first generate AC at some frequency. Then they must convert that current to DC and then invert it to 120V, 60 Hz, AC. Quite a feat, but it has its price in fuel efficiency.

The fuel efficiency leader of all generators in this table is the PowerTech PT-8KSI-2 with a fuel efficiency of 30.82% at full and half load. It operates at a constant 1800 RPM.

The least fuel efficiency diesel generator in this table is the Onan 7.5 HDKAT/41934 with a fuel efficiency of 26.87% at full load and 21.39% at half load. Its RPM is variable up to 3600 RPM.

The variable speed Honda and Yamaha gas generators also show the same fuel reduced fuel efficiency when compared to constant speed gas generators.


2. It is assumed that the variable speed “Quiet” diesel generators produce less noise than constant speed diesel generators. Noise specifications are subject to many more variables than fuel efficiency specifications and extreme judgment must be used for any conclusions about that data, and with that in mind:

The constant speed PowerTech PT-8KSI-2 noise spec at full load is 65 db(A).

The variable speed Onan 8.0 HDKA/1046 noise spec at full load is 72 db(A) and 66 db(A) at half load.

That data is a shocking surprise to me.


3. Many claim it takes less fuel to run a Honda 2000i for light loads than the “big” diesel generator. The manufacturers don’t give data to compare these generators on a load basis at the Honda 2000i capacity (1600 watts), but they give us enough data to compare them on GPH per KW, and that will serve very well here.

The efficiency leader PowerTech PT-8KSI-2 uses 0.0800 GPH per KW @ 4000 watts.
The fuel efficiency of this constant speed generator remains the same over its operating range so its GPH per KW should remain constant.

The variable speed Onan 8.0 HDKA/1046 uses 0.1225 GPH @ 4000 watts.
The variable speed Onan 7.5 HDKAT/41934 uses 0.1493 GPH @ 3750 watts
Since these generator fuel efficiencies decrease as load decreases, we don’t know what the GPH per KW is at the 1600 watt rating of the Honda.

The Honda 2001 variable speed generator uses 0.1484 GPH @ 1600 watts.
Its fuel efficiency also decreases as the load decreases.

As this data shows, decreased fuel consumption is not a reason for using a Honda 20001 (or equivalent Yamaha) for light loads when compared to the PowerTech or Onan 8.0, but may be a consideration when comparing the Honda to the Onan 7.5. Reduced noise (58 db(A)) may be a valid reason in all cases.

Fuel consumption differences for light loads between the Honda 2000i and other gas or propane generators is left as an exercise for the reader.

Remember that this data is compiled directly from manufacturers published data and actual real world results may differ. Also remember that the generator you prefer is the very best one for you.

Good Luck!
Wil
Attached Files
File Type: pdf New Gen Compare 003.pdf (247.7 KB, 356 views)
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Old 10-09-2013, 06:46 PM   #2
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Thank you for taking the time to create the pdf Will. It is quite enlightening and informative.
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Old 10-09-2013, 06:59 PM   #3
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I never thought about fuel consumption at reduced load or even worried about it knowing it is part of the trip. Thanks as it is enlightening and must've taken awhile. Kinda like Figuring (SPFC)fuel consumption for a flight of 350 miles with 850 lb of weight and a 180 HP engine.
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Old 10-10-2013, 07:06 AM   #4
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What I do know is that I get better fuel milage running the generator and the house air than I do running the dash air.
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Old 10-10-2013, 08:32 AM   #5
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That's interesting.
Which MH engine?
Which generator?
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Old 10-10-2013, 08:44 AM   #6
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I have a generator thingy. I use it when needed. That is why I have it.

If I were so worried about fuel efficiency, the generator is the least part of that issue...
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Old 10-10-2013, 08:45 AM   #7
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My generator is an older Power Tech. 7KW. so at half load or so it would use approx 1/8 gal. per hr at 60 mph that would be 320 mpg. With coach getting 7-8 mpg,I wouldn't notice if the generator was running or not. Might as well be comfortable.
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Old 10-10-2013, 08:46 AM   #8
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Here are the consumption facts taken from my Onan 5500 Marquis Gold manual.

Gasoline 5500 watts

No Load 0.3 gal/h

Half Load 2750w 0.6 gal/h

Full Load 5500w 0.9 gal/h

Sound Level dB(A) Half Load 67db

Engine Horsepower 12.9hp
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Old 10-10-2013, 12:58 PM   #9
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Quote:
Originally Posted by wil01 View Post
That's interesting.
Which MH engine?
Which generator?
I have an 05 Dolphin with basement air on an 04 WH chassis with an 8.1 gas engine and a 7.5 ONAN generator. It's not a big improvment(about .5MPG) but every little bit helps and the house air does a better job of cooling the MH going down the road. The salesman told me this when I bought the MH but I didn't beleive him. About a year ago the chassis air quit working so I had to drive home to St.Louis (1300 miles) With the gen. and house air. It really surprized me.
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Old 10-10-2013, 03:38 PM   #10
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That's all very interesting.

Cummins no longer lists a 7.5KW gas generator, but they do list a 7.0 KW gas generator. It uses 0.7 gph at half load and 1.2 gph at full load. Yours must be about the same.

If we assume about half load on the generator working the chassis AC, it surprises me that the dash AC would use more than 0.7 gph. But the important issue is that the generator produces better comfort in the MH than the dash unit. The difference in fuel consumption should not be sn issue.

I wonder how it would come out with a diesel engine and diesel generator. I'll give it a try when I need AC going doen the road again.
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