1. International Energy Agency (IEA). Energy efficiency market report 2015, market trends and medium-term prospects. https://www.iea.org/publications/freepublications/publication/energy-efficiency-market-report-2015-.html .
2. International Energy Agency (IEA). World energy outlook 2015. Paris: International Energy Agency; 2015. This is an in-depth analysis that looks at potential savings from energy efficiency policies and additional cost-effective energy efficiency measures in countries accounting for nearly half of global energy demand.
3. Energy Information Administration. International Energy Statistics. https://www.eia.gov/cfapps/ipdbproject/IEDIndex3.cfm?tid=44&pid=44&aid=2 . This percentage is for 2012, the last year for which data are available.
4. Nadel S, Elliott N and Langer T. Energy efficiency in the United States: 35 years and counting. American Council for an Energy-Efficient Economy, Washington, DC. 2015. http://aceee.org/research-report/e1502 . Examines many energy efficiency trends in the USA from 1980 to 2014.
5. Schlomann B and Eichhammer W. Energy efficiency policies and measures in Germany. Fraunhofer Institute for Systems and Innovation Research ISI, Karlsruhe, Germany. 2012 http://www.isi.fraunhofer.de/isi-wAssets/docs/x/de/publikationen/National-Report_Germany_November-2012.pdf .