You should check with a tax expert to be sure, given your specific utility and state rules. However, according to ENERGY STAR, the usual treatment is that you can only take the tax credit on the cost of system after subtracting the rebate. So the rebate offered by your utility does decrease the size of your federal tax credit. The federal renewable energy tax credit of 30 percent applies to the total cost of your solar panel system after any other rebates are applied -- it's calculated against the total cost of the system and installation, in your case, $27,000 minus $13,500 = 30% of $13,500.

The rules around the interaction of state tax credits and the federal tax credit are more complicated. Depending on whether the state tax credit is taxable, you may or may not have to subtract it before you calculate your federal tax credit.

Also, make sure to check out Arizona’s Residential Solar and Wind Energy Systems Tax Credit. This state income tax credit works like the federal tax credit in that the utility rebate won’t reduce your overall credit. The total state credit is equal to 25 percent of the system cost for up to $1,000.