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Where the boundaries are drawn affects the final net energy ratios.A society that depends on inexpensive energy to maintain a high standard of living and constant growth faces a predicament—it cannot maintain itself over the long run without high net energy fuels.How much energy is available after subtracting the energy costs to extract, process, and deliver the resource?To know how much energy from a particular source can actually be deployed by society, we must factor in both the production costs and the system costs—that is, the energy required to make energy available to the end user.

With coal-derived electricity, the calculation would include the life cycle from mine to power plant to electric grid.Declining oil field productivity has brought the average net energy ratio for conventional oil down to approximately 20:1 globally, with more remote or hard-to-refine oil significantly worse.For fossil energy generally, the trend is downward despite technological advances in exploration and drilling. Some studies suggest that corn-derived ethanol actually has a negative net energy ratio—that is, more energy than a gallon of ethanol can deliver is used to produce a gallon of ethanol.Experts who study this use the terms “net energy ratio” or “energy returned on energy invested” (EROEI).Decades ago when the most accessible reserves were drilled, an oil company might produce 100 barrels of oil or more for each barrel’s worth of energy invested.