Illinois has enacted legislation that allows the fair cash value of wind energy devices to be further reduced by functional obsolescence and external obsolescence, to the extent they are proven by the taxpayer by clear and convincing evidence. Prior to the enactment of S.B. 2612, state law provided that the fair cash value of wind energy devices was determined by subtracting the allowance for physical depreciation from the trended real property cost basis.

This special assessment methodology for wind energy devices was set to expire on December 31, 2016. S.B. 2612 extended the expiration date by five years, to December 31, 2021.