Wis. Stat. § 196.604

No person may knowingly solicit, accept or receive any rebate, concession or discrimination from a public utility for any service in or affecting or relating to the production, transmission, delivery or furnishing of heat, light, water or power within this state or for any connected service whereby the service is rendered or is to be rendered free or at a rate less than the rate named in the schedules and tariffs in force, or whereby any other service or advantage is received. Any person violating this section shall be fined not less than $50 nor more than $5,000 for each offense.