The state’s largest utility is asking the Public Service Commision for a rate increase for its electric and natural gas customers. AmerenUE, which primarily services the eastern part of the state, wants an approximately 18-percent increase in both its electricity and natural gas rates. The proposed increase would, in Ameren’s estimation, add about $6 a month, excluding taxes, to the average customer’s bill. This would be Ameren’s first electric rate increase in 20 years.