Propane generator efficiency?

nkinnkin Registered Users Posts: 22 ✭✭
Greetings All,

I’ve been trying to estimate the propane savings possible by increasing solar production.  I seem to be finding conflicting Internet information - supporting two very different estimates of what I might save.   I’m stumped .

It seems clear that a 10 hp generator will produce roughly 5,000+ watts.

On one side, I find multiple sources which say each hp requires about 10,000 btuh.  Since each gal propane provides about 92,000 btu, a 10 hp generator uses roughly one gallon per hour?  That seems to mean roughly 5kwh from one gallon.  (Currently about $.55 per kwh).    [Honda doesn’t list consumption for propane generators, now discontinued, but this rate is reasonably close to their prediction for my gasoline backup.]

On the other side, however, I find multiple sources which claim each gallon of propane is “equivalent” to 27 kwh.  This would seem to suggest that one gallon of propane would have to run that same 10 hp generator for 5+ hours - in order to produce the 27 kwh?   Other sources put #2 oil at about 41 kwh, which seems reasonably consistent, based on known higher btu density.  This puts each kwh down closer to $ .10 of propane?     Nice - if true.

Even more sources claim one btu is equivalent to .293 watts.  That seems to agree with the nicer 27kwh per gallon estimate.  Fantasy world?

My resultant 5x difference seems crazy - much more than some sort of conversion/efficiency factor.  What am I missing in my old age?  I’d appreciate someone setting me straight, please.  [Hard to measure usage off a1,000 gal tank.]



  • nkinnkin Registered Users Posts: 22 ✭✭
    edited January 14 #2
    GOLLY,!  I remembered how much heat my generator produces and just found that internal combustion engines run at abot 20% efficiency!  There’s my 5x.  Wow, had no idea so poor.

    WOW!  Room for improvement for sure!  No wonder we’re trying to get away from our old car engines.
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    edited January 15 #3
    Propane has around 92,000 BTU or 27,000 Watt*Hours per gallon of "Heat" energy...

    And the amount of kWatt*hours per gallon of propane using a 5,500 Watt Cummins/Onan RV genset specification:
    Consumption at No load0.4 GPH
    Consumption at 3/4 load0.8 GPH
    Consumption at full load1.1 GPH

    I suggest that most people use less than 50% of genset capacity for "normal" household/RV loads on average... If you are charging a battery bank, suggest around 80% loading for non-commercial gensets (derating for continuous running at "full load").

    Using 3/4 load at 0.8 GPH:
    • 5,500 Watts * 0.75 loading * 1 hour runtime * 1/0.8 GPH = 5,156 Watt*Hours per gallon of propane
    • 5,156 WH = 17,593 BTU of heat (for 1 gallon of propane)
    • 17,593 BTU of electrical "work" / 92,000 BTU of heat (all per 1 gallon of propane) = 0.19 = 19% propane genset efficiency (75% loading)
    • 1/0.19 ~ 5.3 times more "heat" in propane vs genset driving electric (resistance) heater
    A large utility power station running Natural gas with very efficient gas turbines can hit around 50% (roughly) thermal efficiency...

    Propane has less "thermal energy" per gallon vs gasoline (or oil, diesel, etc.). Propane does have a higher "octane rating" that typical gasoline--So if you upped the compression ratio, you could probably get better thermal efficiency from propane vs a mixed propane/gasoline genset.

    Inverter-Generators tend to be more fuel efficient at less than 50% loading (and fairly fuel efficient down to 25% loading) vs "standard" gasoline/propane/natural gas gensets--Whose GPH fuel consumption tends to plateau at 50% fuel flow from 50% loading down towards 0% loading.

    Diesel (non-spark) engines have better thermal efficiency at 100% power and less than 50% power (although, diesels can have issues with running at 50-40% or less loading--carbon buildup, wet stack, glazing cylindars at low pressure, etc.) (and diesel more BTU per gallon) than spark ignition/throttle plate regulated engines ("pumping" losses as throttle plates close for lower power needs).

    Generally, running genset in morning at 80% loading (charging battery in "bulk" mode--towards 80% or so state of charge)--Lets genset run in more efficient region (>50% loading). And shut down to let solar take care of "absorb" charging where current slowly tappers off to less than 1% rate of charge as battery reaches 100% State of charge.

    You can look around and find other charts for fuel usage on a genset:

    Larger engines tend to be more fuel efficient than smaller engines at >50% loading.... Running a large genset at low output loading (much less than 50% loading)--Generally is a waste of fuel... Run large genset for "bulk charging" a battery bank. And a smaller genset (or solar) for absorb charging battery bank/running random daytime loads.

    I run a Honda eu2000i inverter-genset on 1 gallon of fuel for ~4-9 hours per gallon (1,800-450 Watt loads) (a couple fridge+freezers, koi pond pump, some lighting, etc. during power failures. A larger 3,500 Watt standard genset may use 2-4x more fuel vs inverter-genset.

    Perfectly good Champion 3,500 Watt genset will run 12 hours on 4.7 gallon fuel tank at 50% (or less) loading (at almost 1/3rd the purchase price of a Honda eu2x00i genset).

    Numbers above are very rough, and not that accurate--But close enough for planning.

    Make sense?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nkinnkin Registered Users Posts: 22 ✭✭
    edited January 14 #4
    Thanks Bill.  No wonder there’s such great need to reduce dependence on internal combustion engines!     20%
  • BB.BB. Super Moderators, Administrators Posts: 32,315 admin
    50% thermal efficiency for a utility power plant sounds good... Then add something like 50% distrbution losses (loss as heat, theft, transformer losses, etc.)... And the "thermal efficiency can drop to: (nice overview from Indian writer--Agrees with what I have read over the years from elsewhere)
    • 50% thermal efficiency * 50% "transmission/distribution/etc. efficiencies" = ~25% end to end thermal efficiency to the home
    Vs 20% gasoline/propane local genset efficiency... Under (roughly) optimal conditions... Run your genset at 10% load--Not near as good efficiency... Some places I have read about (in Africa) have gone with "village scale" "hybrid generator systems"... Basically the same as a Prius. Genset to supply peak loads (dinner time, cooking, lights, etc.) charge battery bank for use during off peak times night--morning--With an AC inverter. Run the genset at optimum fuel flow/AC output. Run AC inverter+battery bank during off peak times. Save fuel and runtime on genset.

    So we are back to utility scale where they can (or at least used to be able) buy huge amounts of fuel at low costs... Vs $3-5+ gallon per gallon (gasoline and Diesel) as we have recently seen in California...

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nkinnkin Registered Users Posts: 22 ✭✭
    That’s interesting and somewhat surprising.  
    Complex issues.

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