The US Department of Energy supplies this useful tool!
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http://www.garlandpower-light.org/energysaver.html
January 2008- Federal Tax Credits EXTENDED For Energy Efficiency!
http://www.energystar.gov/index.cfm?c=products.pr_tax_credits
January 2005 - The U.S. Department of Energy says in its Study that shading the outside of exposed windows or doors is the best way to stop heat build up (http://www.eere.energy.gov). The Energy Efficiency and Renewable Energy Clearinghouse says that roughly 40% of heat build up and heat loss is through windows and doors.