More than a half-million Iowans are getting something unexpected in their gas and electric bills this month — a credit. The bills from MidAmerican Energy contain the line “credit from success in managing operations efficiently in 1999.” MidAmerican spokesman Kevin Waetke says the credit is based on the Des Moines-based utility having sold surplus electricity to other utilities during the warmer months. He says each customer will get a different amount of credit on their bill this month.The credit is based on a customer’s total payments made during all of last year. The average credit for residential customers is about 12 dollars. MidAmerican has 550-thousand electric customers in Iowa. The credits total 15-million dollars.