I.How the formula is built
Illinois sets what the statute calls a real property cost basis: a fixed dollar value per megawatt of nameplate capacity (the maximum output the equipment is rated to produce). For commercial wind devices, that basis has been roughly $360,000 per megawatt since the law took shape in the late 2000s, per the state statute. For commercial solar, a later law set a separate basis, on the order of $218,000 per megawatt at last public report. Both are starting points, not final values.
Each year the Department of Revenue publishes a trending factor, a single multiplier that adjusts the basis for inflation and changing construction costs. The county assessor multiplies the per megawatt basis by the project's megawatts, applies the trending factor, then assesses the result at the Illinois standard of one-third of fair cash value, roughly 33 percent, the same share applied to a house or a barn. An allowance for physical depreciation lowers the value as the equipment ages.
II.The honest math, and where it bends
Work a round example. A wind project of, say, 100 megawatts, at a basis near $360,000 per megawatt, starts around $36 million in cost basis before trending. After the trending factor and the one-third assessment level, the taxable assessed value lands far below that headline figure, and depreciation trims it further each year. The exact dollars depend on the current trending factor and the local tax rate, both of which move, so treat any single number as an estimate, not a promise.
The formula trades local upside for predictability. A county cannot assess a wind farm at whatever it wishes, and a developer cannot argue the value down to near nothing. Both give something up. Because the value depreciates on a schedule, a district that leans hard on turbine revenue in year one should plan for less in year ten, which is why many downstate boards pair the tax with a separate road use or host agreement negotiated directly with the company.
III.Who reads the table
The county assessor applies the number but does not set it. The Department of Revenue owns the basis and the yearly trending factor. The county clerk and the local taxing districts, schools, townships, fire protection, and community colleges, divide the resulting tax among themselves by their levies, the same way they split the tax on every other parcel. A landowner who leases ground to the turbines reads the same table to understand what the county will collect.
This matters most where one project is a large share of a small tax base. In a rural township near Pontiac or Paxton, a single wind or solar farm can be the biggest taxpayer on the roll. That is a reason the statewide formula exists: to keep that outsized parcel from turning every assessment cycle into a lawsuit. It also means a reader who wants the real number should pull two documents, the Department of Revenue guidance for the current year's factor and the county's own tax rate, and do the multiplication.
IV.Worth watching this month
1. The Illinois Department of Revenue typically issues the updated trending factor for the coming assessment year in late winter or early spring, so watch for the current figure before trusting any older estimate.
2. County board budget hearings this fall will show how much downstate districts are counting on wind and solar revenue, a routine step that matters more where one farm dominates the roll.
3. Any new commercial solar project reaching commercial operation locks in its megawatts for assessment, so check your county assessor's roll for parcels that came online this year.
4. Proposals in Springfield to adjust the per megawatt basis or the depreciation schedule surface from time to time, and most stall, so track the General Assembly's energy and revenue committees rather than assuming change is near.
5. The physical depreciation allowance steps the value of older wind farms down each year, so a district relying on a project built years ago should confirm this year's depreciated figure, not the original.