Energy efficiency is a ratio of energy input to useful energy output, often expressed as a percentage. It measures how much energy of one kind is converted into usable energy of another kind. Incandescent light bulbs convert just 5 percent of the electrical energy they use to light, whereas the energy efficiency of compact fluorescent bulbs is between 15 and 20 percent. Power plants fueled by natural gas convert up to 50 percent of their heat energy to electrical energy, compared to about 38 percent for coal-burning power plants. Energy efficiency is never 100 percent, because some energy is always lost as heat, either directly or as a result of friction in between moving parts in such equipment as motors and generators.

Energy Efficiency in Industry and Transportation

Increasing energy efficiency conserves fossil fuels, cuts down on pollution, and saves money. Steam turbine power plants reduce their energy or heat loss by...