In a news release, Cooper said a 7.2 percent rate hike and a resulting 10.5 percent profit margin is too much.
"Consumers need to be taken into account when setting utility profit margins and evaluating requests for rate increases," Cooper offered.
Cooper said such a large increase would be a big hit to some families.
"Allowing double-digit profits when families and small businesses are struggling just to keep the lights on is wrong," Cooper said. "The specific impact on consumers must be part of the equation for determining utility profits and rates."
Duke originally asked for a 17 percent hike, but that was cut to 7.2 percent - an amount approved by the state Utilities Commission. Cooper said he thinks that's still too much.
Cooper plans to argue in court that the Utilities Commission failed to properly take into account economic conditions faced by consumers as required by law.
Filings in the case are available online at http://bit.ly/Tz8mtv.
If approved, the rate hike would be the largest increase for residential customers in at least 20 years - raising the bill for a typical household by nearly $20 a month. Duke Energy has said it is necessary to pay for modernizing its plants. They said it would also pay for higher employee benefits and an employee buyout program.
Plus, sales are lagging because of the poor economy.
"We believe that the $4.8 billion that we've invested in the system on new power plants, transmission and distribution infrastructure, and environmental controls, will really benefit our customers for decades to come," Duke Energy Spokesperson Betsy Conway said earlier this month.
Even with the increase, Duke Energy said its rates would remain below the average in the southeast.