35 ILCS 200/10-10
When a solar energy system has been installed in improvements on any property, the owner of that property is entitled to claim, by filing with the chief county assessment officer, an alternate valuation of those improvements. When a claim for alternate valuation is filed, the chief county assessment officer shall ascertain the value of the improvements as if equipped with a conventional heating or cooling system and the value of the improvements as equipped with the solar energy system. So long as the solar energy system is used in total or part as the means of utilizing solar energy improvements, the alternate valuation computed as the lesser of the two values ascertained under this paragraph shall be applied. When the solar energy system so valued ceases to be used as the means of heating or cooling those improvements, the owner of that property shall within 30 days notify the chief county assessment officer in writing by certified mail.
35 ILCS 200/10-10