This is a simple but powerful definition of efficiency of a device that converts one energy form into another:

It is a number between 0 and 1, or between 0 and 100%. The key word in the definition is useful (energy output). Were it not there, the efficiency of any device would be 100%, because of the law of conservation of energy (which states that energy cannot be created or destroyed). Instead, we see that quite a few devices used in daily life have very low efficiencies. The purpose of a device determines its useful energy output. For example, we want light from a lamp, but we get mostly heat; only 5% of the energy input (electricity) is converted into light, so the efficiency of a conventional incandescent light bulb is 5%.

It is obvious why fluorescent lights are preferable for heavy-duty use (e.g., basements, kitchens, public buildings). Similarly, diesel engines (the kinds used in most trucks and in some cars, especially the European ones) are typically more efficient than conventional spark-ignition engines.

The familiar term 'fuel economy' or 'gas mileage' (miles traveled per gallon of fuel consumed) is equivalent to the efficiency of an automobile; it is just expressed in different units. Instead of being expressed in units of mechanical energy (work), the useful energy output is given as the average number of miles traveled. And instead of being expressed in units of chemical energy, the energy input is given as the number of gallons of fuel. The following graphs, from a recent article in The New York Times, illustrate the current debate about the efficiency of light trucks in the U.S.

If we want to reduce air pollution and decrease our consumption of gasoline and imported oil, light trucks are the obvious target: they are less efficient than automobiles (20 vs. 28 miles per gallon) and their sales now represent 40 percent of total vehicle sales. (For example, Chevy Suburban from G.M. gets only 13 mpg in the city and 15 mpg on the highway.) The Big Three auto makers are against higher efficiency standards. They are using the same argument that was used when CAFE standards were introduced for automobiles: "Any stiffening in the regulations would hand a chunk of the light-truck market, which they dominate, to Japanese companies, which make more efficient truck engines" (J. Bennet, NYT, 9/5/95, p. D8). It will be interesting to see what Congress will do about this issue; the Clinton administration seems ready to move in the direction of higher standards.

For an update, see "A Car-Based Light Truck Obeys the Fuel Rules, and It Helps Make Money" in NYT of 9/15/99. Apparently, the automobile manufacturers have convinced Congress and the Clinton administration that separate federal fuel economy standards for cars (27.5 miles/gallon) and light trucks (20.7 miles/gallon) should be maintained, even though many current vehicles are car/truck 'hybrids'.

At the macroeconomic level, energy efficiency is called energy intensity. This is among the most important pieces of statistical information about a nation or a region. It is obtained as follows:

So in a sense it is the reciprocal of the efficiency, energy consumption being the “total energy input” and GDP being the “useful energy output.” Low energy intensity implies high efficiency, because a small quantity of energy is consumed to achieve a high standard of living. Here are some recent data from The Economist’s “Pocket World in Figures” (1999).

GDP around the world (1996)

Energy consumption around the world (1996)

Energy intensity around the world (1996)

Energy intensity in Europe, Japan and the USA (1996)

Energy: qualitative definition
Energy: quantitative definition
Energy conversions

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