Under the laws of physics as we understand them, creating a 100 percent efficient power system is impossible. Some energy is always lost in the conversion process. Nevertheless, many people have tried. Although they have not been successful, the process has led to the invention of more and more efficient machines.

The overall efficiency of a power system, large or small, depends on many factors. Sometimes a series of energy conversions occur before the energy is finally put to work. Though only a small amount of energy is lost in each conversion, the losses can add up. Simple systems eliminate some energy conversions, but the energy loss per conversion can sometimes be great. Seeking the most efficient overall way to accomplish tasks is an important goal for scientists and engineers.

Figure 3 Typical efficiencies of engines Source: Bosch Automotive Handbook, 4th ed.