The unit of Atlanta-based Southern Co. didn't yet have a cost update in a Wednesday statement, after Mississippi Power pushed back Kemper's completion date for the 10th time in 18 months. The utility is struggling to reliably run the plant.
Customers could be asked to pay more than $4.3 billion. Southern shareholders have lost $2.9 billion.
Mississippi Power will file with the Public Service Commission, whose three elected members ultimately decide how much customers pay.
The company has said it will file two plans— one requiring a sharp, immediate rate increase, and a "rate mitigation plan" that might cushion increases.