Energy efficiency is one of the easier ways to play the energy sector for investors who aren't sure what the future holds, or who may not want to dig deep to understand the ins and outs of biofuels, solar panel efficiency, or geothermal power works. One of the disadvantages of energy investing, compared with other sectors, has always been how intangible it is. You can't go out and inspect an oil field or a coal mine the way you can hold an iPhone in your hand or hang out at a Buffalo Wild Wings. When it comes to energy efficiency, however, you can walk into a Home Depot (HD 2.21%). In this video, Fool.com contributor Aimee Duffy takes a look at the unique opportunity Home Depot can provide to energy investors.