If you purchase an energy-efficient product or renewable energy system for your home, you may be eligible for a federal tax credit.  Existing homes and new construction for primary AND secondary homes are eligible for a 30% return on cost with no upper limit.

What the American Recovery and Reinvestment Act Means to You:

The American Recovery and Reinvestment Act of 2009 extended many consumer tax incentives originally introduced in the Energy Policy Act of 2005 (EPACT) and amended in the Emergency Economic Stabilization Act of 2008 (P.L. 110-343).

Department of Energy web site with a full description of the tax credits

Energy Star web site