Illinois uses a specific formula to value commercial solar energy systems for property tax purposes in counties with fewer than 3 million inhabitants. "Commercial" in this sense means that the system is used primarily to generate electricity for wholesale or retail sale, rather than referring to the customer class.
The valuation formula is calculated by substracting allowance for physical depreciation from the trended real property cost basis (set at $218,000 per MW of nameplate capacity in 2018, with adjustments for CPI inflation). More information is available here.