Beginning January 1, 2009 homeowners and businesses are eligible for a federal income tax credit equal to 30% of the total cost (including equipment and installation) of a qualifying solar energy system. While these tax credits are not new but renewed,there are three big differences this time around;
1)the $2,000 cap on residential credits is removed, and
2)utilities are now eligible for tax credits directly
3)the credits extend to geothermal heat pumps and small wind energy systems.

For a complete breakdown of the credits, go to http://www.dsireusa.org
click on “federal incentives” then “Residential Renewable Energy Tax Credit”

The credits were backdated to the start of January 2008, but the removal of the $2,000 cap on solar systems was not effective until 2009. The cap remains for solar hot water systems.

I hope this helps in some way. If you have more specific questions, do not hesitate to ask.