• If Chicago shops its residents' electric business to a supplier other than Com Ed, remaining customers who pay Com Ed to buy their power from the market could be hit with significant bill increases.
• Municipalities in Illinois can cut deals to buy power for their residents. Com Ed delivers power from any supplier, but it also has contracts in place to buy power for customers who live in towns that have not left the fold or who opt out of municipal deals.
• Experts told the Chicago Tribune that because Com Ed has buying contracts signed in 2007 and contracts for more-expensive renewable power in the future, a customer who stays with Com Ed might wind up paying twice the bill of a neighbor who opted into a municipal power-buying arrangement with another company.

From the article:

The city of Chicago is considering leaving ComEd for another electricity supplier, a move that could prove costly for towns and customers that stick with the utility.