General rule. — Except as otherwise expressly provided for in this chapter, on and after May 1, 2006, the generation, supply and sale of electricity, including all related facilities and assets, used to serve standard offer service and returning customer service, shall be treated as a public utility service or function. Customers of electric distribution companies in this State shall continue to have the opportunity, but not the obligation, to purchase electricity from their choice of electric suppliers as expressly provided for in this chapter.