Net energy is a simple concept really. Once you understand that it takes energy to get energy, the basic math is clear. To calculate the net energy available from an energy resource, you add up the energy used to find, extract, process and deliver that resource and then subtract that amount from the amount of energy the resource contains. But global reserves for finite energy resources such as coal, oil, natural gas and uranium are estimated using measures such as tons, barrels, cubic feet and pounds. These measures tell us little about the ultimate usable energy content of each type of resource.

Nor is it of much use to compare the relative gross energy values of these resources, though such comparisons are readily available. To see some examples, check out this one showing the oil equivalence of nuclear fuel, this one for oil and natural gas, and this table containing a variety of equivalences including two comparing coal and oil. Even conversions into British Thermal Units, or BTUs, don’t really help us.

As the world moves ever closer to the time when vital, finite energy resources begin to decline, we need to know not how much oil, natural gas, coal or uranium is left; rather, we need to know how much usable energy is left in these resources. A recent illustration of the problem we face in understanding usable energy supplies came in the form of a 60 Minutes story on the Canadian oil sands. The program reported that “the reserves are so vast in the province of Alberta that they will help solve America’s energy needs for the next century.”

Nowhere does the reporter explain how much energy it takes to mine and refine the bitumen–it’s not actually oil. In fact, it takes two barrels of oil equivalent to obtain three barrels of usable oil from the oil sands. (This is a far lower return than we get from conventional oil which can provide 20 times the energy consumed for older oil discoveries and eight times the energy consumed for newer oil discoveries.) By this standard we should reduce the generally accepted 180 billion barrels of reserves in the Canadian oil sands by 40 percent. Now, not all of the energy used to mine and process the oil sands comes from petroleum. Of course, the huge mining trucks and other equipment run on diesel fuel. But, the processing plants are heavy users of natural gas, both to heat water for the separation process and to provide a source of hydrogen to transform the bitumen into a flowing, light oil.

But, this shows why we need to know about the total universe of finite fuels since each one increasingly interacts with the others during processing, and one fuel may be called upon to substitute for the another as each resource peaks and then declines in availability. Some say that peak oil production is already upon us. The rate of production for conventional natural gas, which many experts tout as a substitute for declining oil supplies, may peak by mid-century. And, while there are claims that the world has enough coal for 300 years, it is important to note that such figures are always followed by the phrase “at current rates of consumption.” Naturally, if we had to rely more and more on coal, not only for electricity, but also for heat and liquid fuels, its rate of consumption would rise dramatically. Even more worrisome, the net energy of coal is declining. Richard Heinberg reports in his book The Party’s Over that on the current trajectory the net energy from coal could go negative by mid-century as coal grades continue to decline. As for uranium, information on its future supply is sketchy at best.

Oil is facing its own foreshortened depletion trajectory with peak production predictions ranging from last year all the way to 2037 (a date which seems far too optimistic). Increasingly large amounts of energy are needed to find new oil. This is only logical since 1) the easiest oil to find, extract and process has been used first, 2) the new finds tend to be in more remote places such as the Arctic and 3) the new finds tend to be in smaller reservoirs. In addition, new oil is also often more energy intensive to refine because it tends to be of a lower quality. The oil sands are a prime example.

To get the total picture of our finite energy reserves, we need to know at least four important things beyond the raw amounts left: 1) the net energy available from each resource given today’s technology and given projected improvements in that technology over time, 2) the rate at which each resource is likely to be extracted over time, again adjusting for improvements in technology–even a very dense energy resource is of little use if it can only be extracted at a trickle–3) the current and projected interchangibility of finite fuels and their renewable replacements and 4) the time it would take to move toward a new energy infrastructure to accommodate such substitutions. For instance, if coal liquids are going to be substituted for declining supplies of refined oil products, the equation for our energy resources will change dramatically. And, the time it would take to ramp up such production will be an important consideration in its feasibility. (This example does not attempt to address the implications for global warming which need somehow to be considered.)

Modeling these four new pieces of information together with estimates of raw reserves may seem daunting. But, it is actually considerably less daunting than the problems already tackled by those who sought to model future economic constraints in Limits to Growth, the excellent study of resource and pollution constraints on industrial expansion.

Given the gravity of the energy challenges we face, can we afford not to try?