Chicago aldermen this week will cast a final vote on a municipal electricity aggregation program, the largest of its kind in the country.

Chicago voters already gave City Hall the power to bundle all their electricity bills together so it could negotiate for cheaper prices with suppliers other than ComEd. But an ordinance that officially gives the mayor this authority will still have to survive two more votes - the finance committee and full city council - before it gets the real go ahead.

The mayor’s office announced Friday they’ve picked Integrys Energy Services as the new supplier that will bring cheaper power to around 900,000 small business and residential customers. The city says Chicagoans will save about 20 to 25 percent a month on their bills from February to June. But savings will drop to about 8 to 12 percent after that, as the ComEd price of electricity is also expected to drop. The city says Integrys will work to eliminate coal from its fuel mix.

“By buying electricity in bulk, we have secured an agreement that will put money back into the pockets of Chicago families and small businesses while ensuring that our electricity comes from cleaner sources,” Mayor Emanuel said in a statement. “I look forward to working with the City Council to approve and implement the agreement on an expedited timeframe so we can start delivering savings.”

Chicagoans will continue to get a ComEd bill in the mail, as the company is still responsible for outages and reading meters. But if all this goes through City Council, the actual electricity will be supplied by Integrys.

Consumers who don’t like this idea will be able to opt-out.