The Relative Efficiency of Gasoline, Natural Gas, and Propane Fuels For
Back-Up Generators

By Bruce Dishongh

When purchasing a generator as a back-up power supply for your home, one thing to consider is which type of fuel you will be using. While there are several other important factors when buying a generator, this article is only concerned with the method of comparing the relative efficiency between gasoline, natural gas and propane.

The first thing to understand is the equivalent ratio of energy output for the three fuels as expressed in BTUs, the commonly used unit of energy. The following table will demonstrate:

· Gasoline 1 gallon = 125,000 BTUs

· Natural Gas 1 CCF = 100,015 BTUs (CCF=100 cubic feet)

· Propane 1 gallon = 91,700 BTUs

You can see from above that 1 gallon of gasoline is more efficient than 100 cubic feet of natural gas or 1 gallon or propane. In fact, the ratio is approximately:

· 1 gasoline = 1.25 CCF natural gas = 1.36 gallons propane

In other words, you would need 1.36 gallons of propane to produce the equivalent BTUs of 1 gallon of gasoline; or, 1.25 CCF of natural gas.

As a practical example let's say that gasoline is currently $2.85 a gallon, natural gas $.95 per CCF, and propane $4.00 a gallon (these are today's prices where I live). Next, my generator uses 10 gallons of gasoline a day if run continuously for 24 hours. Therefore, for one day's usage I need:

· 10 gallons of gasoline, or

· 10 x 1.25 CCF of gas, or

· 10 x 1.36 gallons of propane

· 10 gallons of gasoline, or

· 12.5 CCF of gas, or

· 13.6 gallons of propane

If we then enter the prices:

· ($2.65) x (10) for gasoline; ($.95) x (12.5) for natural gas; or, ($4.00) x (13.6) for propane

The cost for running the generator 24 hours is:

· $26.50 for gasoline; $11.88 for natural gas; or, $54.40 for propane

As you can see, once you know the relative efficiency of the three fuels you can just plug in the current prices of the fuels to calculate the daily cost of running your generator for each fuel. However, in the case of propane, the cost per gallon can vary widely depending on the number of gallons purchased. In the example above, $4.00 a gallon was for filling up a small 4-5 gallon container; for larger purchases the price can decrease substantially.

If you are thinking of buying a generator it is best to consider more than just the initial purchase price. If you will be using it for extended periods of time it could be cheaper in the long run to buy a generator capable of running on alternative fuels.

Propane approximations

1 gallon = 91,500 BTU

1 cubic foot = 2,500 BTU

1 pound = 21,500 BTU

4.24 lbs = 1 gallon

36.39 cubic feet = 1 gallon

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