Sizing a Propane Generator

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DaveLeeNC

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Is there a good rule of thumb to use for propane consumption when running a generator? I was thinking of something that is within a factor of 1.5 and might take the form of X lbs/hour (or gal/hr) at idle and Y lbs/KwHr at load. In my case we are dealing with smallish loads in the 1-2KW range (typical) and a generator rating of 5-7 KW. Thanks.

dave
 
The comparison of gasoline and propane consumption should be pretty simple based on the specific power of a pound of propane and a gallon of gasoline.
One small complication is that the output power of a propane-fed IC engine will be less than the same engine or one with the same displacement and RPM running on gasoline. The thermodyamic efficiency for a given load will still be closely comparable.
 
GoldDigger - thanks for the comment. But I am not sure how to translate that into a rough sizing for the required propane capacity given a specific load. I guess I could do it with a decent estimate of the efficiency of a propane powered generator (or gas and then map it over to the relative energy content of propane), but I really don't know what to assume here. 30% efficency maybe? And idle (no load) consumption might (or might not) be a significant factor depending on the load profile and the generator. But I just have to believe that the guys who install these things for a living have some useful sizing guidelines here.
 
You may need to look at propane tank sizing based on how the tank is hoping to be refilled. The standard propane barbeque tank is a 25 lb cylinder. It will have 3 to 5 gallons of propane in it depending on where it is filled. The tanks an RV's are usually 30 to 40 lbs cylinders. The largest DOT approved cylinders that can be transported to be refilled are 100 lb cylinders.

On the other hand, if you want a tank that is a permanent installation and the propane company refills on site. Your going to get an ASME tank that holds 100 gallons or more. These tanks have to transported empty, and filled on site. These tanks are typically rented for a nominal annual fee from the propane company. However then you are then locked into that propane company for refilling the tank. You can buy your own tank, but expect to pay $5-10 per gallon of capacity. Above a certain size, the tank will have to be located at least a certain distance from buildings.
 
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Well, here is one place to start. A gallon of gas has 33.7 KWHrs energy. Propane is about 71% of that so 23.9 KWHrs per gallon of propane. So if you assume 30% net generator efficiency (?????) then you start with 7.1 KWHrs out of a gallon of propane. But for a load that is quite cyclical (maybe 30% for fridge, microwave or single burner on the stove on/off, etc) you probably have a bunch of mostly idle time burning propane (and not showing up in your KwHr estimate). So (I suspect) that idle consumption is required to do any kind of estimate. These considerations drive my question.
 
You are probably in a warm enough area that you don't have to oversize the tank or provide tank heating to get enough vaporized propane to supply the generator. That is a concern in my area and often requires a very oversized tank or heated vaporizers. This does not always apply even here for small generators.
 
DaveLeeNC said:
Well, here is one place to start. A gallon of gas has 33.7 KWHrs energy. Propane is about 71% of that so 23.9 KWHrs per gallon of propane. So if you assume 30% net generator efficiency (?????) then you start with 7.1 KWHrs out of a gallon of propane. But for a load that is quite cyclical (maybe 30% for fridge, microwave or single burner on the stove on/off, etc) you probably have a bunch of mostly idle time burning propane (and not showing up in your KwHr estimate). So (I suspect) that idle consumption is required to do any kind of estimate. These considerations drive my question.
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I’ve been told rule of thumb for idle fuel consumption of a gasoline engine (in a vehicle) is about 0.6 liter per hour per liter of engine displacement. I have no idea if this would apply to a genny or could be translated to equivalent L.P.
 
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