Efficiency is a measure of the percentage of the energy that
is converted by a system. It is defined as the work output / work
input x 100%.
For a heat engine, efficiency equals the amount of mechanical work output divided by the heat energy input converted to a percentage. Sadi Carnot (pictured left), a French engineer and scientist, tried to improve the efficiency of steam engines by developing a theory about how they work. He was able to calculate how efficient a given engine could be by looking at the temperature difference between the input and output of the system. He found that the bigger the difference in temperature, the more efficient the system.
It would be nice if all machines were 100 percent efficientable to convert all of the input energy into useful workbut in practice, most are much less efficient. You can, however, save money and conserve energy resources if you select cars, appliances, heating and cooling systems, and light bulbs based on their efficiency ratings.