How can energy efficiency be measured?

Since energy cannot be created or destroyed—it can only change form—and because entropy, or disorder, is always growing, even changing one form of energy into another also "loses" energy to the environment, our ability to choose processes that use the least amount of source energy to achieve the desired final form or use depends on our understanding of how "efficient" an energy conversion process is.

An engine in a car, for instance, converts energy in two ways: first, during fuel combustion, chemical (potential) energy is transformed into thermal energy (heat); second, engine design transforms that thermal energy into mechanical energy.

From the first pistons to the last drive of the wheel axles, that mechanical energy undergoes multiple mechanical energy changes. Part of the mechanical energy is transformed into electrical energy by generators. Every time the energy is transformed into a different form or application, a portion of it is lost to the environment as thermal energy.

As a result, we can never convert "100%" of the energy available from a source into work that is beneficial; in this case, we might only utilize 15–30% of the fuel's chemical energy! See also:

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This also applies to the production of power, whether it comes from solar, nuclear, hydro, coal, oil, or any other source; in order to truly comprehend the effects on the environment, one must consider the entire cost of energy production, not just the last phase!

Prior to concluding on a "better" energy technology, a thorough evaluation of construction materials, costs, environmental impacts (pollution), land and water use, operating costs, waste materials, AND production and distribution efficiencies is required.

Energy efficiency can be measured using various metrics, including energy intensity, energy efficiency ratio (EER), coefficient of performance (COP), and fuel efficiency.