There is a federal investment tax credit (ITC) of 30% on the total cost of the system for residential and commercial installtions. For residential installations in Massachusetts, a state income tax credit is available for primary residences. The new MA SMART program is an additional production based incentive that adds quite a bit to your bottom line for 10 or 20 years. In addition, businesses are able to take advantage of accelerated depreciation, which can be substantial! More on solar incentives here. For detailed incentive information for your state visit The Database for State Incentives for Renewables and Efficiency (DSIRE).