See the link, above, for descriptions of Energy Tax Credits for energy efficiency improvements
In addition to these homeowner tax credits, a separate tax credit for solar hot water systems is available. These credits are not limited to the $500 cap. Consumers can earn a tax credit of up to 30% of the cost ($2,000 max lifetime) for installing PV or solar hot-water systems used exclusively for purposes other than heating swimming pools and hot tubs. The system must use solar power to provide at least half of a home's hot water. In addition, the water must be used in the dwelling and the system must be certified by the Solar Rating and Certification Corporation (SRCC). There is also a consumer tax credit of up to 30% of the cost (up to $500 per 0.5 kW of capacity maximum) for installing a "qualified" fuel cell unit. The credits are available for 2006, 2007, and 2008.
Go to www.solar-estimate.org