MADISON - Alliant Energy's average residential customers will be paying about $8 more for electricity and natural gas beginning next month, according to an order the Public Service Commission issued Thursday. Average electric rates will increase by $4.38, boosting a residential customer's monthly electric bill for 660 kilowatt hours from $87.61 to $91.98. Natural gas rates of 990 therms will be $56.54, up $3.50 per month over current rates.