A public utility shall not directly or indirectly include any costs or expenses attributable to providing nonutility service in regulated rates or charges. Except for contracts existing as of July 1, 1996, a public utility or its affiliates shall not use vehicles, service tools and instruments, or employees, the costs, salaries, or benefits of which are recoverable in the regulated rates for electric service or gas service to install, service, or repair residential or commercial gas or electric heating, ventilating, or air conditioning systems, or interior lighting systems and fixtures; or to sell at retail heating, ventilating, air conditioning, or interior lighting equipment. For the purpose of this section, “commercial” means a place of business primarily used for the storage or sale, at wholesale or retail, of goods, wares, services, or merchandise. Nothing in this section shall be construed to prohibit a public utility from using its utility vehicles, service tools and instruments, and employees to market systems, services, and equipment, to light pilots, or to eliminate a customer emergency or threat to public safety.