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Old 02-14-2013, 01:46 PM   #1
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Join Date: Oct 2012
Location: Pawleys Island SC
Posts: 339
Gen Set or Propane

It is obvious that when connected to shore power and electric is included in the campground fee, using electric for everything possible is the way to go. But when dry camping, does it take less fuel to use the generator for those things that can use both 120 AC or Propane?

Some things that can use either electricity from the gen set or propane are water heating, stove top cooking, refrigeration, and space heating.

Electric stove top cooking can be done very effectively with a commercial hot plate. Ours has a total output of 1500 watts with two ranges of that heat. Its top is totally enclosed which makes cleaning a snap. It heats fast enough so that isnít an issue. All this verbiage about electric stove top cooking is just to demonstrate that changing fuel sources does not necessarily make the change less convenient.

Obviously water heating and refrigeration can be powered by either source in most modern RVs.

Space heating can be done by either heat source until the outside temp gets pretty cold. We use two 1500 watt space heaters in the motorhome and a 200 watt heater in the basement. This does fine for us until the temp gets below about 25F.

I spent a lot of time with an Excel spreadsheet to get some factual answers about this question. The fuel consumption of the listed generators came from the manufacturerís data sheets. The heat required for refrigeration came from the nameplate of my frig. The water heater watts came from Atwood.

Energy can be expressed in many different ways and can be converted to the format that is useful at the time. Here are some constants that were used in the calculations:

1 watt = 3.41 BTU (absolute)
1 gal of #2 diesel fuel contains 140,000 btu (average)
1 gal of propane contains 91,000 BTU (average)
1 gal of gasoline contains 120,000 BTU (average)
1 hp = 760 watts (absolute)
Propane appliance efficiency = 80% (average)

This study was done for me with my own MH. It has a PowerTech CD-7 diesel generator, 6 gal Atwood gas/elec water heater, and 10 cu. ft Dometic, two door refrigerator.

The study was extended to show the results with the popular Onan 8000 quiet diesel generator and the Onan 5500 watt gasoline generator. The gasoline generator was encluded because my son has one in his MH.

After working with the many entries on the spread sheet, it became obvious that the real short answer to this issue was to compare the total fuel efficiency of the generators with propane. Since the study was started for my own system, my PowerTech CD-7 is the baseline for this study.

The data sheets of each generator provides us with the information we need. The efficiency figure we need is determined by how many BTUs of fuel are required to produce the net BTU output of the generator. Hereís what it looks like:

7,000 watts output = 23,850 BTU/Hr
No 2 diesel gph = 89,600 BTU/Hr
Fuel Efficiency at all loads = 23,850 / 89,600 = 26.64%

Onan 8000 Quiet Diesel
8,000 watts output = 27,280 BTU/Hr
No 2 diesel gph = 142,800 BTU/Hr
Fuel efficiency, Full Load = 27,280 / 142,800 = 19.1%
Fuel Effeciency, Half Load = 13,640 / 85,400 = 15.8%%

Onan 5500 Gasoline
5,500 watts = 18,755 BTU/Hr
Gasoline gph = 106,800 BTU/Hr
Fuel Efficiency, Full Load = 18,755 / 106,800 = 17.56%
Fuel Efficiency, Half Load = 9,378 / 69,600 = 13.47%

Propane appliances are 80% efficient (average)

Here is a comparison of the hourly fuel consumption of the four sources of heat for a Water heater.
Watts = 1,400 / hr

CD-7: 0.128 gal No 2 diesel: all loads

Onan 8000 diesel: 0.175 gph if gen at full load.
0.209 gph if gen at half load

Onan 5500 gas: 0.261 gph if gen at full load
0.334 gph if gen at half load

Propane: 0.0656 gph

That little table is quite revealing. There is no question about which heat source uses the least amount of fuel. But there are other considerations which might determine which source you choose: fuel cost/gal, availability, or convenience.

The table also says a lot the about the differences in fuel efficiency between diesel and gasoline engines. That difference isnít surprising and it does show very well that gasoline engines are most efficient at full load. The actual fuel used may decrease when the load decreases, but not in direct proportion to the reduction of load.

The different fuel efficiencies between the diesel engines is interesting to me. The CD-7 is a very simple constant speed generator and its fuel efficiency remains constant over its load range as is common with diesel engines. Its only overhead are the normal ones of diesel engines and the operation of the 120V alternator.

The Onan 7500 Quiet generator had reduced noise as one major design objective. Most agree that they did very well at meeting that objective. Achieving that objective carries some additional overhead with it Ė reduced fuel efficiency both at full load and at partial loads is part of that. Note that the Onan burns 1.395 times more fuel at full load than the CD-7, but 1.668 times more at 50% load. That fact illustrates that the overhead is not directly load dependent. Since the engines are of the same general design from the same manufacturer, the figures are an indication of the overhead required to meet Onanís objective.

Well I think that is enough about this subject, and if you got this far, I suspect that you agree.

All the Best!

Wil Andrews
2007 Newmar Kountry Star DP
Cummins ISL, Cummins E-Brake
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Old 02-14-2013, 03:31 PM   #2
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Interesting stuff. This what I got from Onan/Cummins site.
Propane 5500 watt 1.1 gal per hour full load @ 2400rpm
Diesel 6000 watt 0.7 gal per hour full load @ 3600 rpm
Diesels as we know are more efficient.


Martin Picke'
1998 Rexhall Anthem 34' DP, Cummins ISB, Allison 5 speed, Spartan Chassis, 300 watts solar.
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Old 02-14-2013, 04:31 PM   #3
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The short answer works for me. The question was Gen set vs propane for power. Apples and oranges I think. If you need 120 volt use your gen set. If all you need is refer and hot water and heat use propane. Turn on gen set to charge house batteries when needed.

Mark & Carole RVM54
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Old 02-14-2013, 04:39 PM   #4
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Originally Posted by Campingman View Post
The short answer works for me. The question was Gen set vs propane for power. Apples and oranges I think. If you need 120 volt use your gen set. If all you need is refer and hot water and heat use propane. Turn on gen set to charge house batteries when needed.

Dido short and to the point.
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Old 02-14-2013, 04:51 PM   #5
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We chose a Onan 5500W gas generator over the more RV common propane generator. We hope to dry camp hopefully up to half time give or take and thought I had done a rough calculation a year ago and concluded that our 80 lbs of propane would run our air conditioners for about 18 hours. We didn't think that was enough and went gas with 30 gal capacity. Did we do something that made sense? Does my 18 hr propane calculations seem reasonable or did I goof it?
2013 New Horizons 37' Majestic, 2013 Ford F-450 Lariat
Full-time RV April 2013, Retired January 2014
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Old 02-14-2013, 05:16 PM   #6
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Join Date: Oct 2012
Location: Pawleys Island SC
Posts: 339
Well I think this is my last post to this forum. An engineering report does take some effort. People don't seem to be interested in engineering facts -- only opinions.

Well it was fun for a little while.

goodby and good luck!
Wil Andrews
2007 Newmar Kountry Star DP
Cummins ISL, Cummins E-Brake
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Old 02-14-2013, 05:31 PM   #7
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Location: Greer, SC
Posts: 670
Well take all you toys and go home then.

People didn't respond to your goofy little exercise because it's just silly. If you have to fret over the expense of a little propane to heat your teapot, then frankly, mr. Engineer, you are in the wrong hobby!

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