1. World consumption of primary energy by energy type and selected country groups. Energy Information Adminstration, U.S. Department of Energy; 2006.
2. http://en.wikipedia.org/wiki/Global_warming.
3. Annual energy outlook. U.S. Department of Energy; 2006.
4. www.eere.energy.gov/consumer/your_home/water_heating/index.cfm/mytopic=12760.
5. www.solarhome.org/newssolarwaterheaters.html.