A one-time rebate on the cost of installing a system is not taxable income. If you qualify for the federal solar energy credit, you must reduce the price of the system by the amount of your local rebate so you only claim a credit on the net amount.
However, if you are receiving monthly or yearly checks based on the amount of energy you produce, that is taxable income, and you do not reduce the cost of your system when claiming any federal credit for the original installation.