35 ILCS 200/10-720
For the purpose of this Division 20:
"Allowance for physical depreciation" means (i) the actual age in years of the commercial solar energy system on the assessment date divided by 25 years, multiplied by (ii) its trended real property cost basis. The physical depreciation, however, may not reduce the value of the commercial solar energy system to less than 30% of its trended real property cost basis.
"Commercial solar energy system" means any device or assembly of devices that (i) is ground installed and (ii) uses solar energy from the sun for generating electricity for the primary purpose of wholesale or retail sale and not primarily for consumption on the property on which the device or devices reside.
"Commercial solar energy system real property cost basis" means the owner of a commercial solar energy system's interest in the land within the project boundaries and real property improvements and shall be calculated at $218,000 per megawatt of nameplate capacity. For the purposes of this Section, "nameplate capacity" has the same definition as found in Section 1-10 of the Illinois Power Agency Act.
"Ground installed" means the installation of a commercial solar energy system, with the primary purpose of solar energy generation for wholesale or retail sale, on a parcel or tract of land.
"Trended real property cost basis" means the commercial solar energy system real property cost basis multiplied by the trending factor.
"Trending factor" means a number equal to the Consumer Price Index (U.S. city average all items) published by the Bureau of Labor Statistics for the December immediately preceding the assessment date, divided by the Consumer Price Index (U.S. city average all items) published by the Bureau of Labor Statistics for December of 2017.
35 ILCS 200/10-720