LP generator propane usage

[MikeFromMesa1]

We are buying a new RV that has a propane generator and I have seen comments on this forum about LP generators being high usage devices. I did some research to try to find out how high the usage might be and found the following:

It requires 2 horsepower to produce 1000 watts of energy per hour under load
Under load each horsepower consumes 10000 BTU per hour
Propane contains 92,000 BTU per gallon
Propane weights 4.2 pounds per gallon

Based on these figures, according to my calculations, using a 2KW generator at 50% for 30 minutes should use about 1/10 of a gallon.

2KW = 4 HP, at 50% for 30 minutes = 1 HP,
1 HP = 10,000 BTU/hour
10,000 BTU = 1/9 gallon

That does not seem excessive to me, but then I have no idea if the calculations are anywhere near reality when it comes to an LP generator in an RV boon-docking somewhere. Perhaps someone can let me know if these figures approach reality. I am just curious as we have already bought the RV, just not yet picked it up.

Thanks.

 

[Back2PA]

I think 1/10 gal is optimistic. A rule of thumb is closer to 1/2 gal/hr at a light load

 

[Lou Schneider]

You're forgetting about the efficiency of the engine at converting the fuel's heat potential into useable power.  It doesn't convert anywhere near 100% of the fuels energy into power, a large part is lost as engine heat and out the exhaust.

Figure on getting no more than 20% of the fuel's energy out of the generator, i.e. multiply that 1/10th of a gallon X 5 and you'll be in the ballpark.

Another way to look at it is propane contains 20% less BTUs per gallon than gasoline, so you'll need 20% more propane than gasoline to produce the same amount of power.

By way of comparison, a Honda EU2000i will run 3.1 hours on it's 0.9 gallon tank producing it's rated 1800 watts.  That's 0.5 gallons per hour of gasoline.

 

[MikeFromMesa1]

You're forgetting about the efficiency of the engine at producing heat input into useable power.  It doesn't convert anywhere near 100% of the fuels energy into power, a large part is lost as heat from the engine and out the exhaust.

Figure on getting about 20% of the fuel's energy out of the generator, i.e. multiply that 1/10th of a gallon X 5 and you'll be in the ballpark.

OK. Thank you. That makes good sense and 1/2 gallon for 30 minutes of LP generator usage gives me a ballpark figure about how long a tank of LP gas would last us.

With our current RV (2005 Pleasure-Way Class B with gasoline generator) a tank of LP gas lasts us more than a year since all we use it for is cooking. The new RV (2018 Winnebago Era with LP generator) will use a significantly larger amount of propane, but perhaps it will last us long enough that we will only need one or two refills per year since we do not use the generator for anything other than short enough periods to use the microwave or toaster if we are boon-docking.

 

[Lou Schneider]

If you're only using the generator for short periods of time, beware of water buildup in the crankcase and change the oil frequently.

Propane creates water vapor when it burns and water will condense inside the engine and contaminate the oil, unless the engine gets hot enough to evaporate it out.

I once changed the oil on a propane powered standby generator that had been run without a thermostat, so it never warmed up.  The first third of the oil that came out was nothing but water.

 

[MikeFromMesa1]

If you're only using the generator for short periods of time, beware of water buildup in the crankcase and change the oil frequently.

Could you be a bit clearer on what you mean by "frequently"? Once every 3 months? Once every 6 months? More? Less? Will this issue be solved if the generator is run long enough to get hot? And, if so, how long would that be?

If our use of the generator in our old RV is any guide, we put about 50 hours on it in 4 1/2 years of use. We use the RV frequently, probably 10-12 times per year, and drive about 11,000 miles in it, usually for relatively short (2-3 day) trips. We typically make one long trip (perhaps 2500 miles) per year, this year being a trip from Arizona to South Padre Island, Texas.

 

[Optimistic Paranoid]

I believe the short answer is that the temperature of the oil in the sump - or crankcase, if you prefer - must rise above 212 degrees in order for any water in the oil to boil off.  Otherwise, the water will stay in the crankcase and accumulate.  Just starting your generator and running it for 5 minutes to power a microwave is unlikely to get it hot enough. 

 

[SargeW]

The other big user of propane is if you use the propane heater. They work well, but suck a bunch of propane in the process. That's why if you drive through a RV park you can tell the long term folks by the 50 gallon propane bottles sitting in front of the rigs. And they get filled regularly in cold weather.

 

[MikeFromMesa1]

I believe the short answer is that the temperature of the oil in the sump - or crankcase, if you prefer - must rise above 212 degrees in order for any water in the oil to boil off.  Otherwise, the water will stay in the crankcase and accumulate.  Just starting your generator and running it for 5 minutes to power a microwave is unlikely to get it hot enough.

I guess I will ask how long to run the generator at the dealer "walk through" when we pick up the RV.

I was always told that I needed to run the generator to prevent problems. The length and time interval varied with most people telling me to run it once a month for 30 minutes under load and others telling me to run it every week for 10 minutes under load. For me "under load" usually meant running the AC while theRV was in storage.

I never knew which was correct or why it needed to be run at all, so perhaps the best thing to do is to continue running it under load regularly.

 

[MikeFromMesa1]

The other big user of propane is if you use the propane heater. They work well, but suck a bunch of propane in the process. That's why if you drive through a RV park you can tell the long term folks by the 50 gallon propane bottles sitting in front of the rigs. And they get filled regularly in cold weather.

We used the heat in our current RV only once in the 4 1/2 years we have owned it. We were in a cold area and turned the heat on, but the fan made so much noise that we turned it off and never used it again. Up until now the only propane we have used was for cooking and for the refrigerator when we were traveling or boon-docking, and we used very little propane that way. I expect my propane usage to go way up and I can see that getting propane may be more of an issue than finding diesel.

 

[Isaac-1]

Keep in mind that they typical Onan RV generator is no where near as fuel efficient as the Honda EU series inverter generators are.

 

[Gary RV_Wizard]

A gallon of LP gas has 80% of the energy in a gallon of gasoline, so LP use compared to gasoline is 1.25:1, i.e. the propane genset uses 25% more LP than the same genset running on gas.

The primary reason for running a genset under load is so that the heat generated internally drives moisture out of the electrical system that actually does the generating.  A secondary benefit with a gasoline generator is to run fresh gas through its carburetor to prevent varnish from forming in it.  Varnish prevention is not a factor for an LP gas or diesel genset, however.

Traditional wisdom is to run once a month for 30 minutes at 50% load (50% of the genset max load wattage). In recent years, Onan has recommended running even longer and at higher load, but nobody knows if they are just being cautious or  they have learned something the rest of us don't know.  They don't have to buy fuel or listen to it run, so they can afford to be extra-cautious.