Energy efficiency: what does this mean?

A report published by the Authority of the House of Lords in London gives three possible definitions to the term Energy Efficiency:

- The amount of energy consumed in relation to particular outputs or quantities: for instance, terms such as energy intensity and energy ratio are used, though not always consistently, to describe total energy consumption per unit of GDP, or energy consumption per head of population, or per household.
- Impact on absolute energy demand, measured either in terms of delivered energy (the energy provided to end-users) or primary energy (the energy resources feeding into the economy and converted into convenient forms such as electricity).
-Impact upon carbon equivalent emissions, often used as a proxy for changes in energy efficiency. This impact may be measured either in absolute terms, or in terms of carbon intensity—that is, carbon equivalent emissions per unit of GDP.

Traditionally, energy efficiency is regarded as the rate at which delivered energy is converted into useful outputs, concentration on delivered energy alone. But this may be misleading because it can risk overlooking the effects of economic or demographic changes, not to mention the impact of fuel switching (e.g. from coal to gas in electricity generation), which has a profound impact upon primary energy demand and greenhouse gas emissions.