Efficiency is defined as the ability to accomplish a job with a minimum expenditure of time and effort. Energy efficiency is the ability to provide a function or service with the minimum expenditure of energy.
In general, something is more energy-efficient if it can get the job done while using less energy to do so. Energy efficient electric lightbulbs, for example, can provide the same level of light as incandescent light bulbs but use a lot less energy in doing so: they are more energy efficient. The same measurement can be applied to energy efficiency in vehicles, buildings and other processes and services.
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