220 ILCS 5/9-222.2
The additional charge authorized by Section 9-221 or Section 9-222 shall be made (i) in the case of a tax measured by gross receipts or gross revenue, by adding to the customer's bill a uniform percentage to those amounts payable by the customer for intrastate utility service which are includible in the measure of such tax, except, however, such method is not required where practical considerations justify a utility's or telecommunications carrier's use of another just and reasonable method of recovering its entire liability for such tax, and (ii) in the case of a tax measured by the number of therms or kilowatt-hours distributed, supplied, furnished, sold, transported or transmitted, by adding to the customer's bill an amount equal to the number of therms or kilowatt-hours which are includible in the measure of such tax, multiplied by the applicable tax rate.
220 ILCS 5/9-222.2