staples. Pre-Hispanic inhabitants of the Basin of Mexico, for example, always ate
plenty of prickly pears, the fruit of the Opuntia cactus, which were easy to gather
from wild plants (Sanders et al. 1979). But in order for even a small woman to sat-
isfy most of her food energy needs on such a diet, she would have to eat nearly 5
kilograms of the fruit every day. She could get the same amount of energy from
just about half a kilogram of tortillas. Conversely, charcoal's high energy den-
sity-about twice that of air-dried wood-made it the best fuel for cooking and
metallurgy in traditional societies (see A1.4).
Power density, the rate at which energies are produced or consumed per unit
of area, constitutes a critical structural determinant of energy systems. For exam-
ple, in all traditional societies dependent on fuelwood and charcoal, city size was
clearly limited by the inherently low power density of biomass production (see
A1.5). The power density of sustainable annual tree growth in temperate climates
is at best equal to just 1 or 2 percent of the power density of energy consumption
required for traditional urban heating, cooking, and manufactures. Conse-
quently, cities had to draw on large areas of the surrounding land-at least fifty to
more than one hundred times their size-in order to obtain an adequate fuel sup-
ply. This need for fuel restricted their growth even where other resources, like
food and water, were abundant.
Yet another rate, one that has assumed much importance with advancing
industrialization, is the efficiency of energy conversions. This ratio of output to
input is used most commonly when describing the performance of energy con-
verters, be they stoves, engines, or lights. Although people cannot do anything
about the entropic dissipation of the energy they use, they can try to improve the
efficiency of conversions by lowering the amount of energy required to perform
specific tasks (see A1.6). Obviously, there are physical limits to these improve-
ments, but in most instances there is still much room for improvement.
When efficiencies are calculated for production of foodstuffs, fuels, or electric-
ity, they are usually called energy ratios. Obviously, energy ratios in every pros-
perous traditional agricultural system had to be greater than one. Edible harvests
had to contain more energy than the amount that humans and animals con-
sumed in producing those crops. However, there was no simple relationship be-
tween food energy ratios and the social complexity of old high cultures. In con-
trast, industrial societies prefer to develop the fossil fuel resources with the
highest net energy ratios. This is why they favor crude oil in general, and the rich
Middle Eastern fields in particular (see Aq).
Finally, energy intensity measures the cost of products or services in standard
energy units. Among the commonly used materials, aluminum and silicon are
highly energy-intensive, whereas glass and paper are fairly cheap (see A1.8). The
technical advances of the past two centuries have brought many substantial de-
clines in energy intensities. One notable example is the coke-fueled smelting of
pig iron in large blast furnaces, which needs less than one-tenth the energy per
unit mass of finished product that charcoal-based production requires.