Propane generators

I'm in process of an install of a 2080w pv system. Everything so far seems to be lined up but the generator. I've gotten opinions from some folks that say "diesel is best", and some that say "gasoline cheaper" and me I thought maybe propane would be a good idea. It gets cold here in winter - nights can get down to the teens sometimes. Days are 30-40's. Why is this important? - because diesels start hard in cold, and propane vapor pressure can fall off (ever try running a space heater when its zero? - they can fall 70% in output if the propane tank gets too cold from the ambient plus the gas flow).

Costco has this one delivered for $5 grand. What say the experts?

Cummins Onan®
RS12000 Natural Gas
or Propane Generator
Fuels: natural gas, liquid propane (vapour withdrawal)
Rated power: Natural Gas; 9 kW, LP Vapour 11 kW
Rated amps: 75.0/37.5 Natural Gas; 91.7/45.8 LP
Engine: Honda 670 cc, 6 AMP battery charger
Fuel consumption: NG 5.2 m3//hr (184 ft3/hr) at full load, LP 2 m3/hr (73 ft3/hr) at full load
Circuit breaker: 50 amp, 2 pole
Voltage: 120/240 VAC, single phase, 1.0 pf.
Temperature range of operation: -17°C to 50°C (0° F - 120° F)
Battery: requires (1) 12 volt, group 26, 230 CCA, (not included)
Very quiet operation: 71 dB(A) at 7 meters
Pre-mounted on a moulded polymer installation base
Fully automatic operation when used with RSZ Automatic Transfer Panel
Attractive and discreet heavy duty, sound insulated, protective outdoor enclosure, corrosion resistant aluminium with stainless steel fasteners
Lockable access doors
Control features:
Lighted run/off/auto switch
Running time meter
50 amp UL listed circuit breaker
DC control fuse
Microprocessor control manages generator set operation, fault detection and service diagnostics
CSA certified


  • BB.BB. Posts: 27,348Super Moderators admin
    Re: Propane generators

    From what I understand, the Onan is a standby generator (3,600 rpm instead of 1,800 rpm) and probably not the best choice for a generator that may be intended for continuous operations...

    So, that really gets back to questioning what your power system is intended to be... If you are off grid and will have lots of need of solar/wind/generator power--then I would suggest that you look at an 1,800 rpm diesel (or even an 1,800 propane/etc.) instead... I believe that diesel will tend to burn less fuel, especially at the lower power ratings (like 25% load). Gasoline, propane, natural gas, etc. generators tend to draw substantial amounts of fuel even when running at part load...

    For example, the Onan R1200 takes 184ft3/hr at full load (from web site). My natural gas rate is $1.16 to $1.37 per therm (basically 100 CF)... That generator, running at full load will cost about $2.13 (4.2kWhr/$1.00 or $0.24/kWhr) to $2.52 per hour to run...

    From here: PDF spec sheet for the R1200 generator

    at 25% load (2.25 kW instead of of 9kW full load), the unit will still draw ~72 CF/Hr or $0.84 per hour (at baseline gas price $1.16/therm: 2.7kWhr/$1.00 or $0.37/kWhr)...

    The gasoline Propane version of the R1200 uses: 1.1 gal/hr (25% load) to 2.0 gal/hr (100% load)... At $2.50 per gallon (my average prices around here in California NOTE: the guess at $2.50 per gallon for propane in volume is just a guess--I do not use propane much here, so check your prices):

    100% load = $5.00 per hour to operate or 1.8 kWhr/$1.00 of gas -- $0.556/kWhr on gasoline Propane)
    25% (2.25 kW) load = $2.75 per hour to operate or 0.82 kWhrs/$1.00 of gas -- $1.22/kWhr on gasoline propane)

    At least, around here, natural gas is going to be much cheaper to run than gasoline Propane... And notice, how the amount of fuel used does not go down in proportion of load with gasoline/propane/natural gas fuel--reduce load by a factor of 4, reduce fuel flow only by a factor of 2... I believe diesel's fuel usage curves will not be near as bad as the other type of engines--but you should check.

    So--sizing your load to your generator is also going to be very important... If you plan on running your R1200 near full load (such as charging battery banks, running AC, etc... it is not bad. On propane Natural Gas, the costs are almost the same as electricity from a utility (running at 100% load of 9kW). If, however, you are going to run the generator for your average loads (say 1-2 kWatt loads), your fuel usage costs per kWhr generated will be through the roof...

    As an example of using a smaller generator to run my average household loads during a power outage, the Honda portable generator eu2000i (1.6 kW generator) uses about 1.1 gallons for 4 hours of usage at 1.6 kW rating (full load). Using the above numbers ($2.50 per gallon for gasoline)... we get 2.3 kWhr/$1.00 of gasoline, or $0.43/kWhr on gasoline)... Not as nice as running on propaneNatural Gas--but certainly much cheaper than the R1200 running 1.22 kWhr/$1.00 of gasoline propane (at 2.25 kW load or 25% of rated power)

    I am sure that others here can recommend brands/models of generators that will last for your application...

    So try and get the fuel usage specifications for all of the generators you are considering... and run cost tables out on them (kWhrs per $1.00 of fuel, cents per kWhr--or whatever makes sense in your area). And look at really how much power you will need to generate during an outage and/or while charging your batteries--and try not to over-size the generator by too much.

    Have fun,

    PS: Edited to change mix-ups of when Gasoline was mentioned, it should have been propane... And in some cases, I typed propane instead of natural gas... I made the mistake of not realizing that the R1200 was only available in Nat Gas or Propane... There was no gasoline version available on that spec. sheet (or at all?).

    Regarding the price per gallon of Propane--I instead used the price of gasoline... At least in small quantities, propane around here is even more expensive than gasoline--so the places where I used $2.50/gallon for propane, is probably still accurate enough for a first discussion point... -BB
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mike95490mike95490 Posts: 7,611Solar Expert ✭✭✭✭
    Re: Propane generators

    What fuel is easier for you to store/use. Where is your gen shed? Attached to house?
    Can you park a propane tank in a sheltered/warm area?

    I think propane or nat gas is easiest, no stale fuel storage problems.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    gen: ,

  • System2System2 Posts: 6,290 admin
    Re: Propane generators

    I'm amazed every time I post up a question here. BB really impressed me with the tech depth knowledge of LPG. I learn so much here!
    OK - so here are some answers and my "new direction":

    The Utilities-Gen shed is about 150ft away from my homesite. The propane tank is by the homesite. Propane looks to be lesser efficient and greater cost (my duh, I KNOW the energy per unit volume of LPG is lower than Diesel!), so propane in this application is a bad idea.

    Diesel is a good idea. I have diesel equipment (truck) and plans for a tractor, a diesel fuel tank will go by the shed. I got some (good I think) advice about either a 9 or a 15 kW Perkins diesel generator that I think will do what we need - auto start, liquid cooled, etc. 9 KW is big enough I think, but I'm planning for a barn, so maybe a 15kw would be better to carry power for "big stuff" in there that I might use occaisionally.

    Any inputs on a Perkins diesel?

    Tanks again

  • BB.BB. Posts: 27,348Super Moderators admin
    Re: Propane generators


    Thank you for the high praise... And after I re-read my post I realized that I had made a few different errors (i.e., most of the post) :|... One is that where I typed gasoline for the R1200, it really is only available in propane (or natural gas)--so I had to fix those posts (I left price of propane at $2.50 per gallon as a guesstimate--I don't use bulk propane at all).

    Also, I had swapped natural gas and propane in a couple of places...

    So, I think I have it mostly fixed now--The conclusions are still roughly the same (don't over size a generator, and check the price of fuel vs power generated at your expected usage levels)...

    As a point of comparison, I looked up an Onan Marine 5 kW diesel generator set to see what it costs to operate... From: PDF file

    Fuel Consumption
    No. 2 diesel fuel, Gal/hr (L/hr)

    5.0 kW MDKBH 60 Hz
    No Load 0.20 (0.7)
    1/4 Load 0.25 (1.0)
    1/2 Load 0.35 (1.3)
    3/4 Load 0.44 (1.7)
    Full Load 0.60 (2.3)

    Don't know the price of diesel in your area (it is still over $2.75 at some places around here--or what diesel without road taxes costs--so use $2.50 per gallon as a starting point:

    No Load: = 0.2 gph * $2.50 = $0.50 per hour (no AC load, 0 kW power)
    1/4 load: = [(5kW * 1/4) / 0.25 gph] / $2.50 per gallon = 2 kWhr / $1.00 of Diesel or $0.50/kWhr (1.25 kW power)
    full load: = [(5kW) / 0.6 gph] / $2.50 per gallon = 3.3 kWhr / $1.00 of Diesel or $0.30/kWhr (5 kW power)

    Try the 11.5 kW diesel version. Looks that it might be much more efficient (at larger loads)...: PDF of 11.5kW Onan diesel marine gen-set

    Fuel Consumption
    No. 2 diesel fuel, Gal/hr (L/hr)

    60 Hz

    No Load 0.3 ( 1.1 )
    1/4 Load 0.4 ( 1.5 )
    Full Load 1.0 ( 3.8 )

    No Load: = 0.3 gph * $2.50 = $0.75 per hour (no AC load, 0 kW power)
    1/4 load: = [(11.5kW * 1/4) / 0.4 gph] / $2.50 per gallon = 2.3 kWhr / $1.00 of Diesel or $0.43/kWhr (2.88 kW power)
    full load: = [(11.5kW) / 1.0 gph] / $2.50 per gallon = 4.6 kWhr / $1.00 of Diesel or $0.22/kWhr (11.5 kW power)

    Hmmm... here the diesel does scale a bit better at lower power levels... But it is still does cost more fuel to generate power at 25% levels even for diesel... And the larger diesel is a bit cheaper to run

    Regarding the Perkins Diesel, one person has a Perkins and appears to love it... This is at the bottom of a long-long story about first trying to use a Chinese Diesel gen-set:

    From his notes, the 11.5 kW Onan Marine Diesel system almost exactly the same amount of fuel at full load as the original Chinese made 12 kW gen-set... 0.08696 gallons/kWhr of diesel (Onan) vs 0.086 gallons/kWhr for the Chinese gen-set. So--we seem to have pretty close agreements between a wide range of vendors (very simple Chinese vs the complex Onan Marine set).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Propane generators

    if your shed is 150 feet from the house and away from the neighbors and have a shed to install the genset in then  look into the 7kw and up onan diesel but buy it without the housing you will find out the main cost is the housing and you can set it up in the shed with the radiator mounted against the wall and install  some gable vents from home depot on the other wall for cooling can order a tank to set the genset on or find a semi truck junk yard and pick up a fuel tank cheap.

    our you can go with a commercial moble and mount it on a stand and just duct the exhaust and discharge out the wall of the shed and your done and with the rv type gensets you can add transfer switches and or a power comand 30 with key fob start like a car remote

    you can also get the lean burn versions for natural gas
  • arcandsparkarcandspark Posts: 63Solar Expert ✭✭✭✭
    Re: Propane generators

    Best advice I can give is never use a 3600 RPM generator that is going to run more than a couple hours per day and no more than a couple times each week. The life span is less than half that of an 1800 RPM unit. Also very important, do not use a STAND-BY generator for any solar alternative energy system that is off grid, if you want reliabilty. You must use a PRIME POWER unit, they are built to run 24/7 I have seen people go through more than one generator because of the reasons I have said above. Pay the extra money and get the proper unit to start with. When you need it the most it will be ready to run. Propane systems can be hard to start some times in very cold weather from personal experiance. Arcandspark
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