Over the past years, human energy consumption has risen dramatically.  The rise in use of energy resources has meant increasing costs and the depletion of non-renewable supplies.  These and other factors have led people to try to use energy in a more efficient manner.

The First Law of Thermodynamics says that energy can be transferred between forms, but not created or destroyed.  The Second Law of Thermodynamics says that usage of heat to perform work inevitably causes some heat to be lost to the surrounding environment.  As a result, perfect efficiency in energy use is impossible.

For many things that use energy, a maximum efficiency limit exists.  A four-cycle internal combustion engine can achieve at most 54% efficiency, while a diesel engine can reach 56%.  Steam engines only perform at 32% efficiency.  In reality, engines are considerably worse than this due to designs that prevent achievement of maximum efficiency.

If one includes all devices that use energy, about 85% of energy in the US is lost due to inefficient usage.

Many low-efficiency devices exist because when they were developed, energy costs were very low.  Only later, when energy costs rose, did it make sense to be more energy efficient.

Devices that increase efficiency often are priced higher than those that perform inefficiently.  However, because less energy is used and the devices often last longer (such as fluorescent light bulbs), the efficient devices are sometimes cheaper in the long run.

Governments are trying to promote efficient energy use for the sake of the future.  Laws such as the Clean Air Act in the US and other laws elsewhere encourage companies to develop more efficient methods of energy consumption.