Top Ten Enterprise Customers

3. General Electric

General Electric will buy and install energy efficiency technologies in its own operations, but its real buying power will come in acquisitions. Once the company decides to enter or expand into a segment, one of the first things it does is buy companies or at least invest in them.

In 1999, it entered the water with the acquisition of Glegg. Since then, it's bought other water companies, incubated ideas in its labs and become one of the largest players in the sector. In wind, GE bought Enron's wind group and recently placed a venture investment in Southwest Windpower, which makes small wind turbines. GE now rivals Vestas for leadership in wind.

Arden Realty emerged in the late '90s as the first real estate investment trust with an overt green strategy. GE Real Estate bought it in 2006 for $4.8 billion.

The company in the past several months has talked up the potential of smart grid. Are acquisitions in the offing?

Honorable mentions: Panasonic, Cisco Systems and Siemens. Three other conglomerates that like to shop.