How much energy is there in a gallon of gasoline? The short answer is, a lot. One gallon of gas has the equivalent energy of a man working for 600 hours; that would be 75 eight-hour days. Not bad for one little jug of liquid.

Because we are able to get the energy we need to do work for us out of a nozzle or through a wire, we take energy for granted. We might, for example, forget to turn off an overhead light for a day. If human beings were producing the electricity to run that lightbulb with muscle-power, it would take 5 people working continuously to keep it glowing. They would be mightily upset if you had simply forgotten to throw the switch, thereby condemning them to turning a crank for eight hours.

There are two ways to measure the value of any particular fuel. One is its monetary value. A gallon of gas, which cost about a penny in the United States in 1926, and 99 cents in 1970, costs about $2.50 today. Obviously the monetary value, which changes almost daily, must not be based on the energy content of the gasoline, which does not change.

The second way to measure the value of a fuel is to base it on the actual physical energy content of the substance in question. Regular gasoline, for example, contains about 125,000 BTUs of energy per gallon (BTU stands for British Thermal Unit; one BTU is the amount of energy needed to raise one pound of water one degree Fahrenheit). Gasoline contained 125,000 BTUs of energy when gas cost a penny a gallon, and it has the same amount today. So there is no connection between price and energy content. (1)

There is though a factor in calculating the energy content of any fuel that is hidden from the eyes, and that is the amount of energy that went into obtaining it. In the case of gasoline that would include prospecting for oil, drilling, pumping, transporting, and refining that oil, and then transporting and storing the resulting fuel.

If processing the petroleum was quite difficult, such that it took 125,000 BTUs of energy to get it out of the ground, refined, and to the gas station, then the energy returned by the gasoline (also 125,000 BTUs) is exactly equal to the energy invested, a ratio of 1:1.

If on the other hand you poked a stick in your front yard and petroleum started squirting out, so you built a little refinery on the spot and sold gasoline out of a spigot in the driveway, you might (hypothetically) invest only 1000 BTUs in producing it. You would now have an energy return on the energy invested per gallon of 125:1.

People in the energy world have formalized this measure of “energy return on energy invested” with an acronym, EROEI, made up of the first letter of each word in the phrase. So the EROEI of the gas made from the oil squirting out of the ground in our front yard is 125:1.

EROEI turns out to be one of the most important measurements in the history of humanity.

The EROEI for oil from the 1950s, when it was very easy to find, pump and refine, was as high as 100:1. Remember that each gallon of gasoline is like having a slave work for you for 600 hours — it contains a lot of energy. The tremendous energy return of early oil partly explains why it was possible to rebuild Europe and Japan so quickly after World War II. It also explains why the global population has leapt from about 1.5 billion people when the first oil well was drilled in the United States in 1859, to 6.5 billion today. The high EROEI of petroleum has made it possible to grow enormous amounts of food, transport raw materials and goods all over the world, and create dense urban communities across the globe.

For better and for worse, the EROEI for fuels in the future will not be as high as it has been in the past. The liquid petroleum that we been pumping from the ground now for 150 years (one trillion barrels has been pumped, one trillion barrels remains; we are half-way through the original supply) was a one-time inheritance of concentrated energy; when it’s gone it’s gone forever.

In fact the EROEI for gasoline has already dropped precipitously, from the previously mentioned initial high of 100:1. It fell to 25:1 by 1970, and stands at about 10:1 today. This is because the size of the oil fields is shrinking, the depth at which oil is being found is growing deeper, and the quality of the oil that is being pumped is decreasing.

Not only are we at the halfway point in the consumption of the earth’s liquid petroleum reserves, but in addition the second half of the petroleum produced will not return as much energy profit.

The measurement of EROEI is a valuable tool for assessing the potential of other types of fuel to replace our diminishing energy supplies. Here is a table that shows the EROEI value for many of the energy sources and fuels currently used or being considered for the future:

EROEI (Energy Return on Energy Invested) for Various Fuels

Biodiesel- 3:1
Coal- 1:1 to 10:1
Ethanol- 1.2:1
Natural Gas- 1:1 to 10:1
Hydropower- 10:1
Hydrogen- 0.5:1
Nuclear- 4:1
Oil- 1:1 to 100:1
Oil Sands- 2:1
Solar PV (2) – 1:1 to 10:1
Wind (2) – 3:1 to 20:1

The first thing that must be said about these energy-return ratios is that they are rough estimates. There are many variables involved in producing any energy product. In addition, even the scientific studies done on EROEI for specific energy sources vary widely in their results. But the imprecision notwithstanding, the general ratios are highly informative.

Look, for example, at the energy return for hydrogen. It is negative, less than one. Hydrogen is not an energy source, it is an energy sink, a carrier, like a battery. It takes more energy to produce and store free hydrogen than one gets back when it is utilized as a fuel.

Also consider biodiesel and ethanol; their energy profit ratios are very low. In spite of this you will hear everyone from the president of the United States to the governor of Washington State touting them as the fuels of the future. It will not be possible to run society as we know it today, which is driven by the very high energy profit ratio of petroleum, on the low EROEI offered by biodiesel and ethanol. Not that these fuels might not be useful, but they will be useful only to a society that has adapted to living on a lower energy budget. If you understand EROEI, you know more about energy than either President Bush or Governor Gregoire. Perhaps you will be able to teach them a few things.


1 The fact that the monetary value place on energy resources bears little or no relationship to the energy content of a given resource shows how perverse modern economics has become. Energy writer Hazel Henderson has called modern economics “a form of brain disease” because it is completely disconnected from the physical realities of the earth.

2 Wind and solar power have the potential to offer respectable EROEI ratios and should be very helpful in our energy transition. They cannot though help us out of the near-term challenges we face of having agricultural and transport systems that require very large amounts of petroleum, the global production of which is soon to peak, and then decrease annually.

Dana Visalli is an independent consulting field biologist/ecologist living and working in northcentral Washington. He is the vice-president of a watershed-based non-profit organization called The Partnership for a Sustainable Methow that is interested in lifestyles based on ecological reality. He has traveled to Iraq twice in the past 4 years in an effort first to prevent the U.S. invasion of that country, and then to bring to the attention of the American people the nature of the unfolding disaster there.