"In Massachusetts, businesses may take a credit against the state excise tax for the purchase and installation of a solar water-heating system. The system must be installed at a commercial building, and the credit may not exceed $300 or 15% of the system cost, whichever is less. This credit expires December 31, 2007. A business may carry over unused portions of the credit for use in taxable year 2007" (DSIRE site). Check out Corporate Credit for more information.