Briefly, the renewable energy credit is equal to 30% of qualified expenditures for a system that serves a dwelling unit located in the United States and used as a residence by the taxpayer. The credit is not refundable to you if it exceeds your tax liability. However, the credit can be carried forward until tax year 2016. At this time CONgress has not clarified whether you may carry the credit forward beyond 2016.
It is a package deal that includes all expenses incurred in connection with the installation. The "placed in service" date is determined by occupancy date for a newly built home and by the completion date of the installation for existing homes.
The Solar Energy Industries Association (SEIA) answers FAQ regarding the federal tax credits for solar energy.
Tip of the glass to the Database of State Incentives for Renewables & Efficiency