Cool site for state by state tax rebates regarding energy saving efforts: Database for State Incentives for Renewables & Efficiency

http://www.dsireusa.org/

For federal credit allowances and guidelines see IRS form 5695 entitled Residential Energy Efficiency Property Credit

http://www.irs.gov/pub/irs-pdf/f5695.pdf

Of course, the gold mine of IRS forms and info can be found at the IRS Forms and Publications page at

http://www.irs.gov/formspubs/index.html