International Energy Agency (IEA), 2016.
https://www.iea.org/publications/freepublications/publication/energy-efficiency-market-report-2016.html

"Often called the "first fuel" of the global energy system, energy efficiency is one of the most important steps that any government can take to move towards a sustainable energy system.. To check on the progress made on this front, the IEA Energy Efficiency Market Report tracks the core indicators of energy efficiency. This year, the report takes a new approach and expands the scope of analysis by examining the drivers of energy efficiency programmes in emerging economies, as well as the impact of those policies."