Energy efficiency, is using less energy to provide the same level of energy service.
For example, insulating a home allows a building to use less heating and cooling energy to achieve the same temperature. Another example would be installing fluorescent lights and/or skylights instead of incandescent lights to attain the same level of illumination. A 13 watt fluorescent light bulb outputs the same amount of visible light as a 60 watt incandescent bulb, so you are getting more light for less energy. Efficient energy use is achieved primarily by means of a more efficient technology or process rather than by changes in individual behaviour.
Energy efficient buildings, industrial processes and transportation could reduce the world's energy needs in 2050 by one third, and help controlling global emissions of greenhouse gases, according to the International Energy Agency.
Energy efficiency and renewable energy are said to be the twin pillars of sustainable energy policy.