Luckily, at least for homeowners in Illinois, this should not be the case. Article 10, Division 1 of the Illinois Property Tax Code says:
It is the policy of this State that the use of solar energy systems should be encouraged because they conserve nonrenewable resources, reduce pollution and promote the health and well-being of the people of this State, and should be valued in relation to these benefits.
The code then goes on to say that homeowners are entitled to claim an “alternate valuation” when they have solar improvements to their property. That alternate valuation is the valuation that would apply if the property did not have a solar system. In other words, a solar system should not cause your property tax to increase.
One less-than-perfect part of the law, however, is that you must file with your county property tax assessor to ensure the alternate valuation methodology is applied to your property. The good news is you only have to do it once and it is a simple one-page form, PTAX-330: