Tennessee Valley Authority customers could be facing their biggest electric rate increase in decades this fall. TVA CEO and President Tom Kilgore said in a teleconference yesterday that costs for coal and purchased power from others have skyrocketed. He said every utility is affected, not just TVA. But he said the impact on TVA’s more than 8 million consumers could be a jump in their electric bill of 10 percent to 20 percent on Oct. 1. TVA directors will decide later this month. A 15 percent increase would be TVA’s largest rate hike since 1977.