Wis. Stat. § 196.59: Merchandising by utilities.

Each public utility engaged in the production, transmission, delivery or furnishing of heat, light or power either directly or indirectly to or for the use of the public shall keep separate accounts to show any profit or loss resulting from the sale of appliances or other merchandise. The commission may not take the profit or loss into consideration in arriving at any rate to be charged for service by the public utility.