The installments of assessments remaining unpaid bear simple interest at an annual rate equal to: (1) the sum of: (a) the average interest rate payable on the outstanding bonds issued to finance the improvement in respect of which the special assessments are levied; plus (b) additional interest at a rate to be determined by the board of directors, not exceeding 2% a year; or (2) if bonds are not issued to finance the improvements, at an interest rate to be determined by the board of directors, but not to exceed 10% a year.